ADT7481ARMZ - ON Semiconductor

 Semiconductor Components Industries, LLC, 2012

July, 2012 − Rev. 6

1Publication Order Number:

ADT7481/D

ADT7481

Dual Channel Temperature

Sensor and Overtemperature

Alarm

The ADT7481 is a 3-channel digital thermometer and under/ over

temperature alarm, intended for use in PCs and thermal management

systems. It can measure its own ambient temperature or the

temperature of two remote thermal diodes. These thermal diodes can

be located in a CPU or GPU, or they can be discrete diode connected

transistors. The ambient temperature, or the temperature of the remote

thermal diode, can be accurately measured to 1C. The temperature

measurement range defaults to 0C to +127C, compatible with

ADM1032, but can be switched to a wider measurement range from

−64C to +191C.

The ADT7481 communicates over a 2-wire serial interface

compatible with System Management Bus (SMBus) standards. The

SMBus address of the ADT7481 is 0x4C. An ADT7481−1 with an

SMBus address of 0x4B is also available.

An ALERT output signals when the on-chip or remote temperature

is outside the programmed limits. The THERM output is a comparator

output that allows, for example, on/off control of a cooling fan. The

ALERT output can be reconfigured as a second THERM output if

required.

Features

1 Local and 2 Remote Temperature Sensors

0.25C Resolution/1C Accuracy on Remote Channels

1C Resolution/1C Accuracy on Local Channel

Extended, Switchable Temperature Measurement Range

0C to 127C (Default) or −64C to +191C

2-wire SMBus Serial Interface with SMBus ALERT Support

Programmable Over/Undertemperature Limits

Offset Registers for System Calibration

Up to 2 Overtemperature Fail-Safe THERM Outputs

Small 10-lead MSOP Package

240 mA Operating Current, 5 mA Standby Current

These Devices are Pb-Free, Halogen Free and are RoHS Compliant

Applications

Desktop and Notebook Computers

Industrial Controllers

Smart Batteries

Automotive

Embedded Systems

Burn-In Applications

Instrumentation

MARKING DIAGRAM

http://onsemi.com

See detailed ordering and shipping information in the package

dimensions section on page 19 of this data sheet.

ORDERING INFORMATION

MSOP−10

CASE 846AC

PIN ASSIGNMENT

T0x

AYWG

T0x = Refer to Ordering Info Table

A = Assembly Location

Y = Year

W = Work Week

G= Pb-Free Package

(Note: Microdot may be in either location)

ALERT/THERM2

SCLK

SDATA

D2+

D2−

VDD

D1+

D1−

THERM

GND

ADT7481

http://onsemi.com

Figure 1. Functional Block Diagram

ON-CHIP

TEMPERATURE

SENSOR

ANALOG

MUX BUSY

11-BIT A-TO-D

CONVERTER

LOCAL TEMPERATURE

VALUE REGISTER

REMOTE 1 AND 2 TEMP

OFFSET REGISTER

RUN/STANDBY

EXTERNAL DIODE OPEN-CIRCUIT

STATUS REGISTERS

SMBUS INTERFACE

LIMIT COMPARATOR

DIGITAL MUX

INTERRUPT

MASKING

SDATA SCLK

109

ONE-SHOT

CONVERSION RATE

LOCAL TEMPERATURE

THERM LIMIT REGISTER

LOCAL TEMPERATURE

LOW LIMIT REGISTER

LOCAL TEMPERATURE

HIGH LIMIT REGISTER

REMOTE 1 & 2 TEMP.

THERM LIMIT REG.

REMOTE 1 & 2 TEMP.

LOW LIMIT REGISTERS

REMOTE 1 & 2 TEMP.

HIGH LIMIT REGISTERS

CONFIGURATION

GND

VDD

ADT7481

D1+

ALERT/THERM2THERM

D1−3

D2+ 7

D2−6

REMOTE 1 AND 2 TEMP

VALUE REGISTER

ADDRESS POINTER

Table 1. ABSOLUTE MAXIMUM RATINGS

Parameter Rating Unit

Positive Supply Voltage (VDD) to GND −0.3 to +3.6 V

D+ −0.3 to VDD + 0.3 V

D− to GND −0.3 to +0.6 V

SCLK, SDATA, ALERT, THERM −0.3 to +3.6 V

Input Current, SDATA, THERM −1 to +50 mA

Input Current, D−1 mA

ESD Rating, All Pins (Human Body Model) 1,500 V

Maximum Junction Temperature (TJ MAX) 150 C

Storage Temperature Range −65 to +150 C

IR Reflow Peak Temperature 220 C

IR Reflow Peak Temperature for Pb-Free 260 C

Lead Temperature, Soldering (10 sec) 300 C

Stresses exceeding Maximum Ratings may damage the device. Maximum Ratings are stress ratings only. Functional operation above the

Recommended Operating Conditions is not implied. Extended exposure to stresses above the Recommended Operating Conditions may affect

device reliability.

NOTE: This device is ESD sensitive. Use standard ESD precautions when handling.

Table 2. THERMAL CHARACTERISTICS

Package Type qJA qJC Unit

10-lead MSOP 142 43.74 C/W

ADT7481

http://onsemi.com

Table 3. PIN ASSIGNMENT

Pin No. Mnemonic Description

1 VDD Positive Supply, 3.0 V to 3.6 V.

2 D1+ Positive Connection to the Remote 1 Temperature Sensor.

3 D1−Negative Connection to the Remote 1 Temperature Sensor.

4 THERM Open-Drain Output. Requires pullup resistor. Signals overtemperature events, could be used to turn a

fan on/off, or throttle a CPU clock.

5 GND Supply Ground Connection.

6 D2−Negative Connection to the Remote 2 Temperature Sensor.

7 D2+ Positive Connection to the Remote 2 Temperature Sensor.

8 ALERT/THERM2 Open-Drain Logic Output. Used as interrupt or SMBALERT. This may also be configured as a second

THERM output. Requires pullup resistor.

9 SDATA Logic Input/Output, SMBus Serial Data. Open-Drain Output. Requires pullup resistor.

10 SCLK Logic Input, SMBus Serial Clock. Requires pullup resistor.

Table 4. TIMING SPECIFICATIONS (Note 1)

Parameter Limit at TMIN and TMAX Unit Description

fSCLK 400 kHz max

tLOW 4.7 ms min Clock Low Period, between 10% Points

tHIGH 4.0 ms min Clock High Period, between 90% Points

tR1.0 ms max Clock/data Rise Time

tF300 ns max Clock/data Fall Time

tSU; STA 4.7 ms min Start Condition Setup Time

tHD; STA

(Note 2)

4.0 ms min Start Condition Hold Time

tSU; DAT

(Note 3)

250 ns min Data Setup Time

tSU; STO

(Note 4)

4.0 ms min Stop Condition Setup Time

tBUF 4.7 ms min Bus Free Time between Stop and Start Conditions

1. Guaranteed by design, not production tested.

2. Time from 10% of SDATA to 90% of SCLK.

3. Time for 10% or 90% of SDATA to 10% of SCLK.

4. Time for 90% of SCLK to 10% of SDATA.

Figure 2. Serial Bus Timing

STOPSTART

tSU; DAT

tHIGH

tHD; DAT

tLOW

tSU; STO

STOP START

SCLK

SDATA

tBUF

tHD; STA

tSU; STA

ADT7481

http://onsemi.com

Table 5. ELECTRICAL CHARACTERISTICS (TA= −40C to +120C, VDD = 3.0 V to 3.6 V, unless otherwise noted)

Parameter Conditions Min Typ Max Unit

Power Supply

Supply Voltage, VDD 3.0 3.30 3.6 V

Average Operating Supply Current, IDD 0.0625 Conversions/Sec Rate (Note 1) −3.0 4.0 mA

Standby Mode −5.0 30 mA

Undervoltage Lockout Threshold VDD Input, Disables ADC, Rising Edge −2.55 −V

Power-On-Reset Threshold 1.0 −2.5 V

Temperature-to-Digital Converter

Local Sensor Accuracy (Note 2) 0C  TA  +70C

0C  TA  +85C

−40  TA  +100C

−

1

1.5

2.5

C

Resolution −1.0 −C

Remote Diode Sensor Accuracy (Note 2) 0C  TA  +70C, −55C  TD (Note 3) +150C

0C  TA  +85C, −55C  TD (Note 3)  +150C

−40C  TA  +100C, −55C  TD (Note 3)  +150C

−

1

1.5

2.5

C

Resolution −0.25 −C

Remote Sensor Source Current High Level (Note 4) −233 −mA

Low Level (Note 4) −14 −mA

Conversion Time From Stop Bit to Conversion Complete (Both

Channels) One-shot Mode with Averaging

Switched On

−73 94 ms

One-shot Mode with Averaging Off

(Conversion Rate = 16, 32, or 64 Conversions

per Second)

−11 14 ms

Open-Drain Digital Outputs (THERM, ALERT/THERM2)

Output Low Voltage, VOL IOUT = −6.0 mA − − 0.4 V

High Level Output Leakage Current, IOH VOUT = VDD −0.1 1.0 mA

SMBus Interface (Notes 4 and 5)

Logic Input High Voltage, VIH

SCLK, SDATA

2.1 − − V

Logic Input Low Voltage, VIL

SCLK, SDATA

− − 0.8 V

Hysteresis −500 −mV

SDA Output Low Voltage, VOL IOUT = −6.0 mA − − 0.4 V

Logic Input Current, IIH, IIL −1.0 −+1.0 mA

SMBus Input Capacitance,

SCLK, SDATA

−5.0 −pF

SMBus Clock Frequency − − 400 kHz

SMBus Timeout (Note 6) User Programmable −25 32 ms

SCLK Falling Edge to SDATA Valid Time Master Clocking in Data − − 1.0 ms

1. See Table 11 for information on other conversion rates.

2. Averaging enabled.

3. Guaranteed by characterization, not production tested.

4. Guaranteed by design, not production tested.

5. See Timing Specifications section for more information.

6. Disabled by default. See the Serial Bus Interface section for details to enable it.

ADT7481

http://onsemi.com

TYPICAL PERFORMANCE CHARACTERISTICS

Figure 3. Local Temperature Error vs. Temperature Figure 4. Remote 1 Temperature Error

vs. Temperature

Figure 5. Remote 2 Temperature Error

vs. Temperature

Figure 6. Temperature Error vs. D+/D− Leakage

Resistance

Figure 7. Temperature Error vs. D+/D− Capacitance Figure 8. Operating Supply Current

vs. Conversion Rate

DEV 1

DEV 2

DEV 3

DEV 4

DEV 5

DEV 6

DEV 7

DEV 8

DEV 9

DEV 10

DEV 11

DEV 12

DEV 13

DEV 14

DEV 15

DEV 16

MEAN

HIGH 4S

LOW 4S

DEV 1

DEV 2

DEV 3

DEV 4

DEV 5

DEV 6

DEV 7

DEV 8

DEV 9

DEV 10

DEV 11

DEV 12

DEV 13

DEV 14

DEV 15

DEV 16

HIGH 4S

LOW 4S

DEV 1

DEV 2

DEV 3

DEV 4

DEV 5

DEV 6

DEV 7

DEV 8

DEV 9

DEV 10

DEV 11

DEV 12

DEV 13

DEV 14

DEV 15

DEV 16

MEAN

HIGH 4S

LOW 4S

TEMPERATURE (C)

−50

TEMPERATURE ERROR (C)

−1.0

0 50 100 150

−0.5

0.5

1.0

1.5

2.0

2.5

3.0

3.5

TEMPERATURE (C)

−50

TEMPERATURE ERROR (C)

−1.0

0 50 100 150

−0.5

0.5

1.0

1.5

2.0

2.5

3.0

3.5

TEMPERATURE (C)

−50

TEMPERATURE ERROR (C)

−1.0

0 50 100 150

−0.5

0.5

1.0

1.5

2.0

2.5

3.0

3.5

LEAKAGE RESISTANCE (MW)

TEMPERATURE ERROR (C)

−25

D+ To VCC

D+ To GND

10 100

−20

−15

−10

−5

CAPACITANCE (nF)

TEMPERATURE ERROR (C)

−18

5 10152025

−16

−14

−12

−10

−8

−6

−4

−2

DEV 2

DEV 4

DEV 3

CONVERTION RATE (Hz)

0.01

IDD (mA)

0.1 1 10 100

100

200

300

400

500

600

700

1000

900

800

DEV 4BC

DEV 3BC

DEV 2BC

ADT7481

http://onsemi.com

TYPICAL PERFORMANCE CHARACTERISTICS (Cont’d)

Figure 9. Operating Supply Current vs. Voltage Figure 10. Standby Supply Current vs. Voltage

Figure 11. Standby Supply Current vs. SCLK

Frequency

Figure 12. Temperature Error vs. Common-Mode

Noise Frequency

Figure 13. Temperature Error vs. Differential Mode Noise Frequency

VDD (V)

3.0

408

IDD (mA)

3.1 3.2 3.3 3.4 3.5 3.6

410

412

414

416

418

420

422

DEV 4BC

DEV 3BC

DEV 2BC

VDD (V)

3.0

IDD (mA)

DEV 3

DEV 4

DEV 2

3.1 3.2 3.3 3.4 3.5 3.6

3.2

3.4

3.6

3.8

4.0

4.2

4.4

ISTBY (mA)

DEV 2BC

DEV 3BC

DEV 4BC

10 100 1000

FSCL (kHz) NOISE FREQUENCY (MHz)

TEMPERATURE ERROR (C)

100 200 300 400 500 600

50 mV

20 mV

100 mV

NOISE FREQUENCY (MHz)

TEMPERATURE ERROR (C)

−10 100 200 300 400 500 600

50 mV

20 mV

100 mV

ADT7481

http://onsemi.com

Theory of Operation

The ADT7481 is a local and dual remote temperature

sensor and over/under temperature alarm. When the

ADT7481 is operating normally, the on-board ADC

operates in a free-running mode. The analog input

multiplexer alternately selects either the on-chip

temperature sensor to measure its local temperature, or

either of the remote temperature sensors. The ADC digitizes

these signals and the results are stored in the local, Remote 1,

and Remote 2 temperature value registers.

The local and remote measurement results are compared

with the corresponding high, low, and THERM temperature

limits, stored in on-chip registers. Out-of-limit comparisons

generate flags that are stored in the status register. A result that

exceeds the high temperature limit, the low temperature limit,

or remote diode open circuit will cause the ALERT output to

assert low. Exceeding THERM temperature limits causes the

THERM output to assert low. The ALERT output can be

reprogrammed as a second THERM output.

The limit registers can be programmed, and the device

controlled and configured via the serial SMBus. The

contents of any register can also be read back via the SMBus.

Control and configuration functions consist of switching

the device between normal operation and standby mode,

selecting the temperature measurement scale, masking or

enabling the ALERT output, switching Pin 8 between

ALERT and THERM2, and selecting the conversion rate.

Temperature Measurement Method

A simple method of measuring temperature is to exploit

the negative temperature coefficient of a diode, measuring

the base-emitter voltage (VBE) of a transistor operated at

constant current.

This technique requires calibration to null the effect of the

absolute value of VBE, which varies from device to device.

The technique used in the ADT7481 measures the change

in VBE when the device is operated at two different currents.

Figure 14 shows the input signal conditioning used to

measure the output of a remote temperature sensor. This

figure shows the remote sensor as a substrate transistor, but

it could equally be a discrete transistor. If a discrete

transistor is used, the collector is not grounded and is linked

to the base. To prevent ground noise interfering with the

measurement, the more negative terminal of the sensor is not

referenced to ground, but is biased above ground by an

internal diode at the D− input. C1 may optionally be added

as a noise filter with a recommended maximum value of

1,000 pF.

To measure DVBE, the operating current through the

sensor is switched among two related currents. The currents

through the temperature diode are switched between I, and

NI, giving DVBE. The temperature can then be calculated

using the DVBE measurement.

The resulting DVBE waveforms pass through a 65 kHz

low-pass filter to remove noise and then to a

chopper-stabilized amplifier. This amplifies and rectifies the

waveform to produce a dc voltage proportional to DVBE.

The ADC digitizes this voltage producing a temperature

measurement. To reduce the effects of noise, digital filtering

is performed by averaging the results of 16 measurement

cycles for low conversion rates. At rates of 16, 32, and

64 conversions/second, no digital averaging takes place.

Signal conditioning and measurement of the local

temperature sensor is performed in the same manner.

Figure 14. Input Signal Conditioning

LOW-PASS FILTER

fC = 65 kHz

REMOTE

SENSING

TRANSISTOR

BIAS

DIODE

D+

D−

VDD

IBIAS

IN  I

VOUT+

VOUT−

To ADC

C1*

*CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS. C1 = 1000 pF MAX

Temperature Measurement Results

The results of the local and remote temperature

measurements are stored in the local and remote temperature

value registers and are compared with limits programmed

into the local and remote high and low limit registers.

The local temperature measurement is an 8-bit

measurement with 1C resolution. The remote temperature

measurements are 10-bit measurements, with the 8 MSBs

stored in one register and the 2 LSBs stored in another

registers.

ADT7481

http://onsemi.com

Table 6. REGISTER ADDRESS FOR THE

TEMPERATURE VALUES

Temperature

Channel

Address, MSBs

Address, LSBs

Local 0x00 N/A

Remote 1 0x01 0x10 (2 MSBs)

Remote 2 0x30 0x33 (2 MSBs)

If Bit 3 of the Configuration 1 register is set to 1, then the

Remote 2 temperature values can be read from the following

Remote 2, MSBs = 0x01

Remote 2, LSBs = 0x10

The above is true only when Bit 3 of the Configuration 1

needs to be switched back to 0.

Only the two MSBs in the remote temperature low byte

are used. This gives the remote temperature measurement a

resolution of 0.25C. Table 7 shows the data format for the

remote temperature low byte.

Table 7. EXTENDED TEMPERATURE RESOLUTION

(REMOTE TEMPERATURE LOW BYTE)

Extended Resolution Remote Temperature Low Byte

0.00C0 000 0000

0.25C0 100 0000

0.50C1 000 0000

0.75C1 100 0000

When reading the full remote temperature value,

including both the high and low byte, the two registers

should be read LSB first and then the MSB. This is because

reading the LSB will cause the MSB to be locked until it is

read. This is to guarantee that the two values read are derived

from the same temperature measurement. The MSB register

updates only after it has been read. The MSB will not lock

if a SMBus repeat start is used between reading the two

registers. There needs to be a stop between reading the LSB

and MSB.

If the LSB register is read but not the MSB register, then

fail-safe protection is provided by the THERM and ALERT

signals which update with the latest temperature measurements

rather than the register values.

Temperature Measurement Range

The temperature measurement range for both local and

remote measurements is, by default, 0C to +127C.

However, the ADT7481 can be operated using an extended

temperature range. The temperature range in the extended

mode is −64C to +191C. The user can switch between these

two temperature ranges by setting or clearing Bit 2 in the

Configuration 1 register. A valid result is available in the next

measurement cycle after changing the temperature range.

Bit 2 Configuration Register 2 = 0 = 0C to +127C = default

Bit 2 Configuration Register 2 = 1 = −64C to +191C

In extended temperature mode, the upper and lower

temperatures that can be measured by the ADT7481 are

limited by the remote diode selection. While the temperature

registers can have values from −64C to +191C, most

temperature sensing diodes have a maximum temperature

range of −55C to +150C.

Note that while both local and remote temperature

measurements can be made while the part is in extended

temperature mode, the ADT7481 should not be exposed to

temperatures greater than those specified in the Absolute

section. Furthermore, the device is only guaranteed to operate

as specified at ambient temperatures from −40C to +120C.

Temperature Data Format

The ADT7481 has two temperature data formats. When

the temperature measurement range is from 0C to +127C

(default), the temperature data format is binary for both local

and remote temperature results. See the Temperature

Measurement Range section for information on how to

switch between the two data formats.

When the measurement range is in extended mode, an

offset binary data format is used for both local and remote

results. Temperature values in the offset binary data format

are offset by +64. Examples of temperatures in both data

formats are shown in Table 8.

Table 8. TEMPERATURE DATA FORMAT

(LOCAL AND REMOTE TEMPERATURE HIGH BYTE)

Temperature Binary

Offset Binary

(Note 1)

−55C0 000 0000

(Note 2)

0 000 1001

110C0 000 0000 0 100 0000

+1C0 000 0001 0 100 0001

+10C0 000 1010 0 100 1010

+25C0 001 1001 0 101 1001

+50C0 011 0010 0 111 0010

+75C0 100 1011 1 000 1011

+100C0 110 0100 1 010 0100

+125C0 111 1101 1 011 1101

+127C0 111 1111 1 011 1111

+150C0 111 1111

(Note 3)

1 101 0110

1. Offset binary scale temperature values are offset by +64.

2. Binary scale temperature measurement returns 0 for all

temperatures <0C.

3. Binary scale temperature measurement returns 127 for all

temperatures >127C.

The user may switch between measurement ranges at any

time. Switching the range will also switch the data format.

The next temperature result following the switching will be

reported back to the register in the new format. However, the

contents of the limit registers will not change. It is up to the

user to ensure that when the data format changes, the limit

registers are reprogrammed as necessary. More information

on this can be found in the Limit Registers section.

ADT7481

http://onsemi.com

Registers

The registers in the ADT7481 are eight bits wide. These

registers are used to store the results of remote and local

temperature measurements, high and low temperature limits,

and to configure and control the device. A description of these

registers follows.

Address Pointer Register

The address pointer register does not have, nor does it

require, an address because the first byte of every write

operation is automatically written to this register. The data

in this first byte always contains the address of another

pointer register. It is to this register address that the second

byte of a write operation is written to, or to which a

subsequent read operation is performed.

The power-on default value of the address pointer register

is 0x00, so if a read operation is performed immediately after

power-on, without first writing to the address pointer, the

value of the local temperature will be returned since its

Temperature Value Registers

The ADT7481 has five registers to store the results of

local and remote temperature measurements. These

registers can only be written to by the ADC and read by the

user over the SMBus.

The local temperature value register is at Address 0x00.

The Remote 1 temperature value high byte register is at

Address 0x01, with the Remote 1 low byte register at

Address 0x10.

The Remote 2 temperature value high byte register is at

Address 0x30, with the Remote 2 low byte register at

Address 0x33.

The Remote 2 temperature values can also be read from

Address 0x01 for the high byte, and Address 0x10 for the

low byte if Bit 3 of Configuration Register 1 is set to 1.

To read the Remote 1 temperature values, set Bit 3 of

Configuration Register 1 to 0.

The power-on default value for all five registers is 0x00.

Table 9. CONFIGURATION 1 REGISTER (READ ADDRESS 0x03, WRITE ADDRESS 0x09)

Bit Mnemonic Function

7Mask Setting this bit to 1 masks all ALERTs on the ALERT pin. Default = 0 = ALERT enabled. This applies only if Pin 8 is

configured as ALERT, otherwise it has no effect.

6Mon/STBY Setting this bit to 1 places the ADT7481 in standby mode, that is, it suspends all temperature measurements

(ADC). The SMBus remains active and values can be written to, and read from, the registers. However THERM

and ALERT are not active in standby mode, and their states in standby mode are not reliable.

Default = 0 = temperature monitoring enabled.

5AL/TH This bit selects the function of Pin 8. Default = 0 = ALERT. Setting this bit to 1 configures Pin 8 as the THERM2 pin.

4Reserved Reserved for future use.

3Remote

1/2

Setting this bit to 1 enables the user to read the Remote 2 values from the Remote 1 registers. When default = 0,

Remote 1 temperature values and limits are read from these registers.

2Tem p

Range

Setting this bit to 1 enables the extended temperature measurement range of −64C to +191C. When using the

default = 0, the temperature range is 0C to +127C.

1Mask R1 Setting this bit to 1 masks ALERTs due to the Remote 1 temperature exceeding a programmed limit. Default = 0.

0Mask R2 Setting this bit to 1 masks ALERTs due to the Remote 2 temperature exceeding a programmed limit. Default = 0.

Table 10. CONFIGURATION 2 REGISTER (ADDRESS 0x24)

Bit Mnemonic Function

7Lock Bit Setting this bit to 1 locks all lockable registers to their current values. This prevents tampering with settings until

the device is powered down. Default = 0.

<6:0> Res Reserved for future use.

Conversion Rate/Channel Selector Register

The conversion rate/channel selector register for reads is

at Address 0x04, and at Address 0x0A for writes. The four

LSBs of this register are used to program the conversion

times from 15.5 ms (Code 0x0A) to 16 seconds

(Code 0x00). To program the ADT7481 to perform

continuous measurements, set the conversion rate register to

0x0B. For example, a conversion rate of eight

conversions/second means that beginning at 125 ms

intervals, the device performs a conversion on the local and

the remote temperature channels.

This register can be written to, and read back from, the

SMBus. The default value of this register is 0x08, giving a

rate of 16 conversions per second. Using slower conversion

times greatly reduces the device power consumption.

Bit 7 in this register can be used to disable averaging of the

temperature measurements. All temperature channels are

measured by default. It is possible to configure the

ADT7481 to measure the temperature of one channel only.

This can be configured using Bit 4 and Bit 5 (see Table 11).

ADT7481

http://onsemi.com

Table 11. CONVERSION RATE/CHANNEL SELECTOR REGISTER (READ ADDRESS 0x04, WRITE ADDRESS 0x0A)

Bit Mnemonic Function

7 Averaging Setting this bit to 1 disables averaging of the temperature measurements at the slower conversion

rates (averaging cannot take place at the three faster rates, so setting this bit has no effect). When

default = 0, averaging is enabled.

6 Reserved Reserved for future use. Do not write to this bit.

<5:4> Channel Selector These bits are used to select the temperature measurement channels:

00 = Round Robin = Default = All Channels Measured

01 = Local Temperature Only Measured

10 = Remote 1 Temperature Only Measured

11 = Remote 2 Temperature Only Measured

<3:0> Conversion Rates These bits set how often the ADT7481 measures each temperature channel.

Conversion rates are as follows:

Conversions/sec Time (seconds)

0000 = 0.0625 16

0001 = 0.125 8

0010 = 0.25 4

0011 = 0.5 2

0100 = 1 1

0101 = 2 500 m

0110 = 4 250 m

0111 = 8 = Default 125 m

1000 = 16 62.5 m

1001 = 32 31.25 m

1010 = 64 15.5 m

1011 = Continuous Measurements 73 m (Averaging Enabled)

Limit Registers

The ADT7481 has three limits for each temperature

channel: high, low, and THERM temperature limits for

local, Remote 1, and Remote 2 temperature measurements.

The remote temperature high and low limits span two

registers each to contain an upper and lower byte for each

limit. There is also a THERM hysteresis register. All limit

registers can be written to, and read back from, the SMBus.

See Table 16 for details of the limit register addresses and

power-on default values.

C will result in an out-of-limit condition, setting a flag in

the status register.

If the low limit register is programmed with 0C,

measuring 0C or lower will result in an out-of-limit

condition.

Exceeding either the local or remote THERM limit asserts

THERM low. When Pin 8 is configured as THERM2,

exceeding either the local or remote high limit asserts

THERM2 low. A default hysteresis value of 10C is

provided that applies to both THERM channels. This

hysteresis value may be reprogrammed.

It is important to remember that the temperature limits

data format is the same as the temperature measurement data

format. So if the temperature measurement uses the default

binary scale, then the temperature limits also use the binary

scale. If the temperature measurement scale is switched,

however, the temperature limits do not automatically

switch.

The user must reprogram the limit registers to the desired

value in the correct data format. For example, if the remote

low limit is set at 10C and the default binary scale is being

used, the limit register value should be 0000 1010b. If the

scale is switched to offset binary, the value in the low

temperature limit register should be reprogrammed to be

0100 1010b.

Status Registers

The status registers are read-only registers, at

Address 0x02 (Status Register 1) and Address 0x23 (Status

ADT7481.

Table 12. STATUS REGISTER 1 BIT ASSIGNMENTS

Bit Mnemonic Function ALERT

7 BUSY 1 when ADC Converting No

6LHIGH

(Note 1)

1 when Local High

Temperature Limit Tripped

Yes

5LLOW

(Note 1)

1 when Local Low

Temperature Limit Tripped

Yes

4 R1HIGH 1 when Remote 1 High

Temperature Limit Tripped

Yes

3R1LOW

(Note 1)

1 when Remote 1 Low

Temperature Limit Tripped

Yes

2D1 OPEN

(Note 1)

1 when Remote 1 Sensor

Open Circuit

Yes

1 R1THRM1 1 when Remote1 THERM

Limit Tripped

0 LTHRM1 1 when local THERM Limit

Tripped

1. These flags stay high until the status register is read, or they are

reset by POR.

ADT7481

http://onsemi.com

Table 13. STATUS REGISTER 2 BIT ASSIGNMENTS

Bit Mnemonic Function ALERT

7 Res Reserved for Future Use No

6 Res Reserved for Future Use No

5 Res Reserved for Future Use No

4R2HIGH

(Note 1)

1 when Remote 2 High

Temperature Limit Tripped

Yes

3R2LOW

(Note 1)

1 when Remote 2 Low

Temperature Limit Tripped

Yes

2D2 OPEN

(Note 1)

1 when Remote 2 Sensor

Open Circuit

Yes

1 R2THRM1 1 when Remote 2 THERM

Limit Tripped

0 ALERT 1 when ALERT Condition

Exists

1. These flags stay high until the status register is read, or they are

reset by POR.

The eight flags that can generate an ALERT are NOR’d

together. When any flag is high, the ALERT interrupt latch

is set and the ALERT output goes low (provided that the

flag(s) is/are not masked out).

Reading the Status 1 register will clear the five flags (Bit 6

through Bit 2) in Status Register 1, provided the error

conditions that caused the flags to be set have gone away.

Reading the Status 2 register will clear the three flags (Bit 4

through Bit 2) in Status Register 2, provided the error

conditions that caused the flags to be set have gone away. A

flag bit can only be reset if the corresponding value register

contains an in-limit measurement, or if the sensor is good.

The ALERT interrupt latch is not reset by reading the

status register. It will be reset when the ALERT output has

been serviced by the master reading the device address,

provided the error condition has gone away and the status

When Flag 1 and/or Flag 0 of Status Register 1, or Flag 1

of Status Register 2 are set, the THERM output goes low to

indicate that the temperature measurements are outside the

programmed limits. The THERM output does not need to be

reset, unlike the ALERT output. Once the measurements are

within the limits, the corresponding status register bits are

reset automatically, and the THERM output goes high. The

user may add hysteresis by programming Register 0x21.

The THERM output will be reset only when the temperature

falls below the THERM limit minus hysteresis.

When Pin 8 is configured as THERM2, only the high

temperature limits are relevant. If Flag 6 and Flag 4 of Status

THERM2 output goes low to indicate that the temperature

measurements are outside the programmed limits. Flag 5

and Flag 3 of Status Register 1, and Flag 3 of Status

THERM2 is otherwise the same as THERM.

Bit 0 of Status Register 2 gets set whenever the ALERT

output is asserted low. Thus, the user need only read Status

the ALERT. This bit gets reset when the ALERT output gets

reset. If the ALERT output is masked, then this bit is not set.

Offset Register

Offset errors may be introduced into the remote

temperature measurement by clock noise or by the thermal

diode being located away from the hot spot. To achieve the

specified accuracy on this channel, these offsets must be

removed.

The offset values are stored as 10-bit, twos complement

values.

The Remote 1 offset MSBs are stored in Register 0x11

and the LSBs are stored in Register 0x12 (low byte, left

justified).

The Remote 2 offset MSBs are stored in Register 0x34

and the LSBs are stored in Register 0x35 (low byte, left

justified). The Remote 2 offset can be written to, or

read from, the Remote 1 offset registers if Bit 3 of the

Configuration 1 register is set to 1. This bit should be

set to 0 (default) to read the Remote 1 offset values.

Only the upper two bits of the LSB registers are used. The

MSB of the MSB offset register is the sign bit. The minimum

offset that can be programmed is −128C, and the maximum

is +127.75C. The value in the offset register is added to, or

subtracted from, the measured value of the remote

temperature.

The offset register powers up with a default value of 0C

and will have no effect unless the user writes a different

value to it.

Table 14. SAMPLE OFFSET REGISTER CODES

Offset Value 0x11/0x34 0x12/0x35

−128C1000 0000 00 00 0000

−4C1111 1100 00 00 0000

−1C1111 1111 00 000000

−0.25C1111 1111 10 00 0000

0C0000 0000 00 00 0000

+0.25C0000 0000 01 00 0000

+1C0000 0001 00 00 0000

+4C0000 0100 00 00 0000

+127.75C0111 1111 11 00 0000

One-shot Register

The one-shot register is used to initiate a conversion and

comparison cycle when the ADT7481 is in standby mode,

after which the device returns to standby. Writing to the

one-shot register address (0x0F) causes the ADT7481 to

perform a conversion and comparison on both the local and

the remote temperature channels. This is not a data register

as such, and it is the write operation to Address 0x0F that

causes the one-shot conversion. The data written to this

address is irrelevant and is not stored. However the ALERT

and THERM outputs are not operational in one-shot mode

and should not be used.

ADT7481

http://onsemi.com

Consecutive ALERT Register

The value written to this register determines how many

out-of-limit measurements must occur before an ALERT is

generated. The default value is that one out-of-limit

measurement generates an ALERT. The maximum value

that can be chosen is 4.

The purpose of this register is to allow the user to perform

some filtering of the output. This is particularly useful at the

fastest three conversion rates, where no averaging takes

place. This register is at Address 0x22. This register has

other functions that are listed in Table 15.

Table 15. CONSECUTIVE ALERT REGISTER BIT

Bit Name Description

7 SCL

Timeout

Set to 1, enables the SMBus SCL

timeout bit. Default = 0 = Timeout

disabled. See the Serial Bus Interface

section for more information.

6 SDA

Timeout

Set to 1 to enable the SMBus SDA

Timeout Bit. Default = 0 = Timeout

disabled. See the Serial Bus Interface

section for more information.

5Mask Local Setting this bit to 1 masks ALERTs

due to the local temperature

exceeding a programmed limit.

Default = 0.

4 Res Reserved for future use.

<3:0> Consecutive

ALERT

These bits set the number of

consecutive out-of-limit

measurements that have to occur

before an ALERT is generated.

000x = 1

001x = 2

011x = 3

111x = 4

Table 16. LIST OF REGISTERS

Read

Address

(Hex)

Write

Address

(Hex) Mnemonic Power-On Default Comment Lock

N/A N/A Address Pointer Undefined No

00 N/A Local Temperature Value 0000 0000 (0x00) No

01 N/A Remote 1 Temperature Value High Byte 0000 0000 (0x00) Bit 3 Conf Reg = 0 No

01 N/A Remote 2 Temperature Value High Byte 0000 0000 (0x00) Bit 3 Conf Reg = 1 No

02 N/A Status Register 1 Undefined No

03 09 Configuration Register 1 0000 0000 (0x00) Yes

04 0A Conversion Rate/Channel Selector 0000 0111 (0x07) Yes

05 0B Local Temperature High Limit 0101 0101 (0x55) (85C) Yes

06 0C Local Temperature Low Limit 0000 0000 (0x00) (0C) Yes

07 0D Remote 1 Temp High Limit High Byte 0101 0101 (0x55) (85C) Bit 3 Conf Reg = 0 Yes

07 0D Remote 2 Temp High Limit High Byte 0101 0101 (0x55) (85C) Bit 3 Conf Reg = 1 Yes

08 0E Remote 1 Temp Low Limit High Byte 0000 0000 (0x00) (0C) Bit 3 Conf Reg = 0 Yes

08 0E Remote 2 Temp Low Limit High Byte 0000 0000 (0x00) (0C) Bit 3 Conf Reg = 1 Yes

N/A 0F

(Note 1)

One-Shot N/A

10 N/A Remote 1 Temperature Value Low Byte 0000 0000 Bit 3 Conf Reg = 0 No

10 N/A Remote 2 Temperature Value Low Byte 0000 0000 Bit 3 Conf Reg = 1 No

11 11 Remote 1 Temperature Offset High Byte 0000 0000 Bit 3 Conf Reg = 0 Yes

11 11 Remote 2 Temperature Offset High Byte 0000 0000 Bit 3 Conf Reg = 1 Yes

12 12 Remote 1 Temperature Offset Low Byte 0000 0000 Bit 3 Conf Reg = 0 Yes

12 12 Remote 2 Temperature Offset Low Byte 0000 0000 Bit 3 Conf Reg = 1 Yes

13 13 Remote 1 Temp High Limit Low Byte 0000 0000 Bit 3 Conf Reg = 0 Yes

13 13 Remote 2 Temp High Limit Low Byte 0000 0000 Bit 3 Conf Reg = 1 Yes

14 14 Remote 1 Temp Low Limit Low Byte 0000 0000 Bit 3 Conf Reg = 0 Yes

14 14 Remote 2 Temp Low Limit Low Byte 0000 0000 Bit 3 Conf Reg = 1 Yes

19 19 Remote 1 THERM Limit 0101 0101 (0x55) (85C) Bit 3 Conf Reg = 0 Yes

ADT7481

http://onsemi.com

Table 16. LIST OF REGISTERS (continued)

Read

Address

(Hex) LockCommentPower-On DefaultMnemonic

Write

Address

(Hex)

19 19 Remote 2 THERM Limit 0101 0101 (0x55) (85C) Bit 3 Conf Reg = 1 Yes

20 20 Local THERM Limit 0101 0101 (0x55) (85C) Yes

21 21 THERM Hysteresis 0000 1010 (0x0A) (10C) Yes

22 22 Consecutive ALERT 0000 0001 (0x01) Yes

23 N/A Status Register 2 0000 0000 (0x00) No

24 24 Configuration 2 Register 0000 0000 (0x00) Yes

30 N/A Remote 2 Temperature Value High Byte 0000 0000 (0x00) No

31 31 Remote 2 Temp High Limit High Byte 0101 0101 (0x55) (85C) Yes

32 32 Remote 2 Temp Low Limit High Byte 0000 0000 (0x00) (0C) Yes

33 N/A Remote 2 Temperature Value Low Byte 0000 0000 (0x00) No

34 34 Remote 2 Temperature Offset High Byte 0000 0000 (0x00) Yes

35 35 Remote 2 Temperature Offset Low Byte 0000 0000 (0x00) Ye s

36 36 Remote 2 Temp High Limit Low Byte 0000 0000 (0x00) (0C) Yes

37 37 Remote 2 Temp Low Limit Low Byte 0000 0000 (0x00) (0C) Yes

39 39 Remote 2 THERM Limit 0101 0101 (0x55) (85C) Yes

3D N/A Device ID 1000 0001 (0x81)

3E N/A Manufacturer ID 0100 0001 (0x41) N/A

1. Writing to Address 0F causes the ADT7481 to perform a single measurement. It is not a data register as such, and it does not matter what

data is written to it.

Serial Bus Interface

Control of the ADT7481 is achieved via the serial bus.

The ADT7481 is connected to this bus as a slave device

under the control of a master device.

The ADT7481 has an SMBus timeout feature. When this

is enabled, the SMBus will typically timeout after 25 ms of

no activity. However, this feature is not enabled by default.

Set Bit 7 (SCL timeout bit) of the consecutive alert register

(Address 0x22) to enable the SCL timeout. Set Bit 6 (SDA

timeout bit) of the consecutive alert register (Address 0x22)

to enable the SDA timeout.

The ADT7481 supports packet error checking (PEC) and

its use is optional. It is triggered by supplying the extra clock

for the PEC byte. The PEC byte is calculated using CRC−8.

The frame check sequence (FCS) conforms to CRC−8 by the

polynomial:

C(x) +x8)x2)x1)1(eq. 1)

Consult the SMBus 1.1 specification for more

information (www.smbus.org).

Addressing the Device

In general, every SMBus device has a 7-bit device

address, except for some devices that have extended, 10-bit

addresses. When the master device sends a device address

over the bus, the slave device with that address responds.

The ADT7481 is available with one device address, 0x4C

(1001 100b). An ADT7481−1 is also available. The only

difference between the ADT7481 and the ADT7481−1 is the

SMBus address. The ADT7481−1 has a fixed SMBus

address of 0x4B (1001 011b). The addresses mentioned in

this datasheet are 7-bit addresses. The R/W bit needs to be

added to arrive at an 8-bit address. Other than the different

SMBus addresses, the ADT7481 and the ADT7481−1 are

functionally identical.

The serial bus protocol operates as follows:

The master initiates data transfer by establishing a start

condition, defined as a high-to-low transition on the serial

data line (SDATA) while the serial clock line (SCLK)

remains high. This indicates that an address/data stream

follows. All slave peripherals connected to the serial bus

respond to the start condition and shift in the next eight bits,

consisting of a 7-bit address (MSB first) plus a R/W bit,

which determines the direction of the data transfer, that is,

whether data will be written to, or read from, the slave

device. The peripheral with the address corresponding to the

transmitted address responds by pulling the data line low

during the low period before the ninth clock pulse, known as

the acknowledge bit. All other devices on the bus remain idle

while the selected device waits for data to be read from or

written to it. If the R/W bit is 0, the master writes to the slave

device. If the R/W bit is 1, the master reads from the slave

device.

Data is sent over the serial bus in a sequence of nine clock

pulses, eight bits of data followed by an acknowledge bit

from the slave device. Transitions on the data line must

occur during the low period of the clock signal and remain

stable during the high period, since a low-to-high transition

when the clock is high may be interpreted as a stop signal.

ADT7481

http://onsemi.com

The number of data bytes that can be transmitted over the

serial bus in a single read or write operation is limited only

by what the master and slave devices can handle.

When all data bytes have been read or written, stop

conditions are established. In write mode, the master will

pull the data line high during the tenth clock pulse to assert

a stop condition. In read mode, the master device will

override the acknowledge bit by pulling the data line high

during the low period before the ninth clock pulse. This is

known as no acknowledge. The master will then take the

data line low during the low period before the tenth clock

pulse, then high during the tenth clock pulse to assert a stop

condition.

Any number of bytes of data may be transferred over the

serial bus in one operation, but it is not possible to mix read

and write in one operation because the type of operation is

determined at the beginning and cannot subsequently be

changed without starting a new operation. In the case of the

ADT7481, write operations contain either one or two bytes,

while read operations contain one byte.

To write data to one of the device data registers or to read

data from it, the address pointer register must be set so that

the correct data register is addressed. The first byte of a write

operation always contains a valid address that is stored in the

address pointer register. If data is to be written to the device,

the write operation contains a second data byte that is written

to the register selected by the address pointer register.

This procedure is illustrated in Figure 15. The device

address is sent over the bus followed by R/W set to 0 and

followed by two data bytes. The first data byte is the address

of the internal data register to be written to, which is stored

in the address pointer register. The second data byte is the

data to be written to the internal data register.

Figure 15. Writing a Register Address to the Address Pointer Register, then Writing Data to the Selected Register

SCLK

SDATA 00 1101D7 D6 D5 D4 D3 D2 D1 D0

ACK. BY

ADT7481

START BY

MASTER

ACK. BY

ADT7481

D7 D6 D5 D4 D3 D2 D1 D0

ACK. BY

ADT7481 STOP BY

MASTER

SCLK (CONTINUED)

SDATA (CONTINUED)

FRAME 1

SERIAL BUS ADDRESS BYTE FRAME 2

ADDRESS POINTER REGISTER BYTE

FRAME 3

DATA BYTE

R/W

Figure 16. Writing to the Address Pointer Register Only

SCLK

SDATA 00

1101D7 D6 D5 D4 D3 D2 D1 D0

ACK. BY

ADT7481

STOP BY

MASTER

START BY

MASTER FRAME 1

SERIAL BUS ADDRESS BYTE

FRAME 2

ADDRESS POINTER REGISTER BYTE

119

ACK. BY

ADT7481

R/W

Figure 17. Reading Data from a Previously Selected Register

SCLK

SDATA 001101D7 D6 D5 D4 D3 D2 D1 D0

ACK. BY

ADT7481 STOP BY

MASTER

START BY

MASTER FRAME 1

SERIAL BUS ADDRESS BYTE

FRAME 2

DATA BYTE FROM ADT7481

119

ACK. BY

ADT7481

R/W

ADT7481

http://onsemi.com

When reading data from a register there are two possible

scenarios:

1. If the address pointer register value of the

ADT7481 is unknown or not the desired value, it

is necessary to set it to the correct value before

data can be read from the desired data register.

This is done by performing a write to the

ADT7481 as before, but only the data byte

containing the register read address is sent, as data

is not to be written to the register. This is shown in

Figure 16.

A read operation is then performed consisting of

the serial bus address, R/W bit set to 1, followed

by the data byte read from the data register (shown

in Figure 17).

2. If the address pointer register is already at the

desired address, data can be read from the

corresponding data register without first writing to

the address pointer register, and the bus transaction

shown in Figure 16 can be omitted.

NOTES:It is possible to read a data byte from a data register without

first writing to the address pointer register. However, if the

address pointer register is already at the correct value, it is

not possible to write data to a register without writing to the

address pointer register. This is because the first data byte

of a write is always written to the address pointer register.

Remember that some of the ADT7481 registers have

different addresses for read and write operations. The write

address of a register must be written to the address pointer

if data is to be written to that register, but it may not be

possible to read data from that address. The read address

of a register must be written to the address pointer before

data can be read from that register.

ALERT Output

Pin 8 can be configured as an ALERT output. The ALERT

output goes low whenever an out-of-limit measurement is

detected, or if the remote temperature sensor is open circuit.

It is an open-drain output and requires a pullup. Several

ALERT outputs can be wire-OR’ed together, so that the

common line will go low if one or more of the ALERT

outputs goes low.

The ALERT output can be used as an interrupt signal to a

processor, or it may be used as an SMBALERT. Slave

devices on the SMBus cannot normally signal to the bus

master that they want to talk, but the SMBALERT function

allows them to do so.

One or more ALERT outputs can be connected to a

common SMBALERT line connected to the master. When

the SMBALERT line is pulled low by one of the devices, the

following procedure occurs as illustrated in Figure 18.

Figure 18. Use of SMBALERT

ALERT RESPONSE

ADDRESS

MASTER SENDS

ARA AND READ

COMMAND DEVICE SENDS

ITS ADDRESS

RDSTART ACK DEVICE

ADDRESS

ACK STOP

MASTER RECEIVES SMBALERT

1. SMBALERT is pulled low.

2. Master initiates a read operation and sends the

alert response address (ARA = 0001 100). This is

a general call address that must not be used as a

specific device address.

3. The device with a low ALERT output responds to

the alert response address, and the master reads the

address from the responding device. An LSB of 1

is added because the device address is comprised

of seven bits. The address of the device is now

known and it can be interrogated in the usual way.

4. If more than one device has a low ALERT output,

the one with the lowest device address will have

priority, in accordance with normal SMBus

arbitration.

5. Once the ADT7481 has responded to the alert

response address, it will reset its ALERT output,

provided that the error condition that caused the

ALERT no longer exists. If the SMBALERT line

remains low, the master sends the ARA again, and

so on until all devices with low ALERT outputs

respond.

Masking the ALERT Output

The ALERT output can be masked for local, Remote 1,

Remote 2 or all three channels. This is done by setting the

appropriate mask bits in either the Configuration 1 register

(read address = 0x03, write address = 0x09) or in the

consecutive ALERT register (address = 0x22)

To mask ALERTs due to local temperature, set Bit 5 of the

consecutive ALERT register to 1. Default = 0.

To mask ALERTs due to Remote 1 temperature, set Bit 1 of

the Configuration 1 register to 1. Default = 0.

To mask ALERTs due to Remote 2 temperature, set Bit 0 of

the Configuration 1 register to 1. Default = 0.

To mask ALERTs due to any channel, set Bit 7 of the

Configuration 1 register to 1. Default = 0.

Low Power Standby Mode

The ADT7481 can be put into low power standby mode

by setting Bit 6 (Mon/STBY bit) of the Configuration 1

The ADT7481 operates normally when Bit 6 is 0. When

Bit 6 is 1, the ADC is inhibited, and any conversion in

progress is terminated without writing the result to the

corresponding value register.

The SMBus is still enabled in low power standby mode.

Power consumption in this standby mode is reduced to a

typical of 5 mA if there is no SMBus activity, or up to 30 mA

if there are clock and data signals on the bus.

When the device is in standby mode, it is still possible to

initiate a one-shot conversion of both channels by writing to

the one-shot register (Address 0x0F), after which the device

will return to standby. It does not matter what is written to

the one-shot register, all data written to it is ignored. It is also

possible to write new values to the limit register while in

ADT7481

http://onsemi.com

standby mode. ALERT and THERM are not available in

standby mode and, therefore, should not be used because the

state of these pins is unreliable.

Sensor Fault Detection

The ADT7481 has internal sensor fault detection circuitry

at its D+ input. This circuit can detect situations where a

remote diode is not connected, or is incorrectly connected,

to the ADT7481. If the voltage at D+ exceeds VDD −1.0 V

(typical), it signifies an open circuit between D+ and D−, and

consequently, trips the simple voltage comparator. The

output of this comparator is checked when a conversion is

initiated. Bit 2 (D1 open flag) of the Status Register 1

(Address 0x02) is set if a fault is detected on the Remote 1

channel. Bit 2 (D2 open flag) of the Status Register 2

(Address 0x23) is set if a fault is detected on the Remote 2

channel. If the ALERT pin is enabled, setting this flag will

cause ALERT to assert low.

If a remote sensor is not used with the ADT7481, then the

D+ and D− inputs of the ADT7481 need to be tied together

to prevent the open flag from being continuously set.

Most temperature sensing diodes have an operating

temperature range of −55C to +150C. Above 150C, they

lose their semiconductor characteristics and approximate

conductors instead. This results in a diode short, setting the

open flag. The remote diode in this case no longer gives an

accurate temperature measurement. A read of the

temperature result register will give the last good

temperature measurement. The user should be aware that

while the diode fault is triggered, the temperature

measurement on the remote channels is likely to be

inaccurate.

Interrupt System

The ADT7481 has two interrupt outputs, ALERT and

THERM. Both outputs have different functions and

behavior. ALERT is maskable and responds to violations of

software-programmed temperature limits or an open-circuit

fault on the remote diode. THERM is intended as a fail-safe

interrupt output that cannot be masked.

If the Remote 1, Remote 2, or local temperature exceeds

the programmed high temperature limits, or equals or

exceeds the low temperature limits, the ALERT output is

asserted low. An open-circuit fault on the remote diode also

causes ALERT to assert. ALERT is reset when serviced by

a master reading its device address, provided the error

condition has gone away, and the status register has been

reset.

The THERM output asserts low if the Remote 1,

Remote 2, or local temperature exceeds the programmed

THERM limits. The THERM temperature limits should

normally be equal to or greater than the high temperature

limits. THERM is automatically reset when the temperature

falls back within the (THERM −hysteresis) limit. The local

and remote THERM limits are set by default to 85C. A

hysteresis value can be programmed, in which case THERM

will reset when the temperature falls to the limit value minus

the hysteresis value. This applies to both local and remote

measurement channels. The power-on hysteresis default

value is 10C, but this may be reprogrammed to any value

after powerup.

The hysteresis loop on the THERM outputs is useful when

THERM is used for on/off control of a fan. The user’s

system can be set up so that when THERM asserts, a fan can

be switched on to cool the system. When THERM goes high

again, the fan can be switched off. Programming a hysteresis

value protects from fan jitter, a condition wherein the

temperature hovers around the THERM limit, and the fan is

constantly being switched on and off.

Table 17. THERM HYSTERESIS

THERM Hysteresis Binary Representation

0C 0 000 0000

1C 0 000 0001

10C 0 000 1010

Figure 19 shows how the THERM and ALERT outputs

operate. A user may wish to use the ALERT output as a

SMBALERT to signal to the host via the SMBus that the

temperature has risen. The user could use the THERM

output to turn on a fan to cool the system, if the temperature

continues to increase. This method would ensure that there

is a fail-safe mechanism to cool the system, without the need

for host intervention.

Figure 19. Operation of the ALERT and THERM

Interrupts

1005C

THERM LIMIT

905C

805C

705C

605C

505C

405C

THERM LIMIT − HYSTERESIS

HIGH TEMP LIMIT

RESET BY MASTER

TEMPERATURE

ALERT

THERM

If the measured temperature exceeds the high

temperature limit, the ALERT output will assert low.

If the temperature continues to increase and exceeds the

THERM limit, the THERM output asserts low. This can

be used to throttle the CPU clock or switch on a fan.

The THERM output de-asserts (goes high) when the

temperature falls to THERM limit minus hysteresis. In

Figure 19, the default hysteresis value of 10C is

shown.

The ALERT output de-asserts only when the

temperature has fallen below the high temperature

limit, and the master has read the device address and

cleared the status register.

ADT7481

http://onsemi.com

Pin 8 on the ADT7481 can be configured as either an

ALERT output or as an additional THERM output.

THERM2 will assert low when the temperature exceeds the

programmed local and/or remote high temperature limits. It

is reset in the same manner as THERM, and it is not

maskable. The programmed hysteresis value also applies to

THERM2.

Figure 20 shows how THERM and THERM2 might

operate together to implement two methods of cooling the

system. In this example, the THERM2 limits are set lower

than the THERM limits. The THERM2 output could be used

to turn on a fan. If the temperature continues to rise and

exceeds the THERM limits, the THERM output could

provide additional cooling by throttling the CPU.

Figure 20. Operation of the THERM and THERM2

Interrupts

THERM2 LIMIT

905C

805C

705C

605C

505C

405C

TEMPERATURE

THERM

305C

THERM LIMIT

THERM2

When the THERM2 limit is exceeded, the THERM2

signal asserts low.

If the temperature continues to increase and exceeds the

THERM limit, the THERM output asserts low.

, there is no hysteresis value shown.

As the system cools further, and the temperature falls

below the THERM2 limit, the THERM2 signal resets.

Again, no hysteresis value is shown for THERM2.

The temperature measurement could be either the local or

the remote temperature measurement.

Applications Information

Noise Filtering

For temperature sensors operating in noisy environments,

previous practice was to place a capacitor across the D+ and

D− pins to help combat the effects of noise. However, large

capacitances affect the accuracy of the temperature

measurement, leading to a recommended maximum

capacitor value of 1,000 pF.

Factors Affecting Diode Accuracy

Remote Sensing Diode

The ADT7481 is designed to work with substrate

transistors built into processors or with discrete transistors.

Substrate transistors will generally be PNP types with the

collector connected to the substrate. Discrete types can be

either a PNP or an NPN transistor connected as a diode (base

shorted to collector). If an NPN transistor is used, the

collector and base are connected to D+ and the emitter to D−.

If a PNP transistor is used, the collector and base are

connected to D− and the emitter to D+.

To reduce the error due to variations in both substrate and

discrete transistors, a number of factors should be taken into

consideration:

The ideality factor, nf, of the transistor is a measure of

the deviation of the thermal diode from ideal behavior.

The ADT7481 is trimmed for an nf value of 1.008. Use

the following equation to calculate the error introduced

at a temperature, T (C), when using a transistor where

nf does not equal 1.008. Consult the processor data

sheet for the nf values.

(eq. 2)

DT+ǒnf*1.008Ǔń1.008 ǒ273.15 Kelvin )TǓ

To factor this in, the user can write the DT value to the

offset register. It will then automatically be added to, or

subtracted from, the temperature measurement by the

ADT7481.

Some CPU manufacturers specify the high and low

current levels of the substrate transistors. The high

current level of the ADT7481, IHIGH, is 233ĂmA. The

low level current, ILOW, is 14ĂmA. If the ADT7481

current levels do not match the current levels specified

by the CPU manufacturer, it may become necessary to

remove an offset. The CPU data sheet will advise

whether this offset needs to be removed and how to

calculate it. This offset may be programmed to the

offset register. It is important to note that if more than

one offset must be considered, the algebraic sum of

these offsets must be programmed to the offset register.

If a discrete transistor is being used with the ADT7481,

the best accuracy is obtained by choosing devices according

to the following criteria:

Base-emitter voltage greater than 0.25 V at 6 mA, at the

highest operating temperature.

Base-emitter voltage less than 0.95 V at 100 mA, at the

lowest operating temperature.

Base resistance less than 100 W.

Small variation in hFE (say 50 to 150) that indicates

tight control of VBE characteristics.

Transistors, such as 2N3904, 2N3906, or equivalents in

SOT−23 packages, are suitable devices to use.

Thermal Inertia and Self-heating

Accuracy depends on the temperature of the remote

sensing diode and/or the local temperature sensor being at

the same temperature as that being measured. A number of

factors can affect this. Ideally, the sensor should be in good

thermal contact with the part of the system being measured;

otherwise, the thermal inertia caused by the sensor’s mass

ADT7481

http://onsemi.com

causes a lag in the response of the sensor to a temperature

change.

In the case of the remote sensor, this should not be a

problem, since it will either be a substrate transistor in the

processor or a small package device, such as an SOT−23,

placed in close proximity to it.

The on-chip sensor, however, will often be remote from

the processor and only monitors the general ambient

temperature around the package. In practice, the ADT7481

package will be in electrical, and hence, thermal contact with

a PCB and may also be in a forced airflow. How accurately

the temperature of the board and/or the forced airflow

reflects the temperature to be measured will also affect the

accuracy of the measurement. Self-heating, due to the power

dissipated in the ADT7481 or the remote sensor, causes the

chip temperature of the device (or remote sensor) to rise

above ambient. However, the current forced through the

remote sensor is so small that self-heating is negligible. The

worst-case condition occurs when the ADT7481 is

converting at 64 conversions per second while sinking the

maximum current of 1 mA at the ALERT and THERM

output. In this case, the total power dissipation in the device

is about 4.5 mW. The thermal resistance, qJA, of the

MSOP−10 package is about 142C/W.

Layout Considerations

Digital boards can be electrically noisy environments, and

the ADT7481 measures very small voltages from the remote

sensor, so care must be taken to minimize noise induced at

the sensor inputs. Take the following precautions:

1. Place the ADT7481 as close as possible to the

remote sensing diode. Provided that the worst

noise sources such as clock generators,

data/address buses, and CRTs are avoided, this

distance can range from 4 to 8 inches.

2. Route the D+ and D− tracks close together, in

parallel, with grounded guard tracks on each side.

To minimize inductance and reduce noise pick up,

a 5 mil track width and spacing is recommended.

Provide a ground plane under the tracks if

possible.

Figure 21. Typical Arrangement of Signal Tracks

5 MIL

GND

D−

D+

GND

3. Try to minimize the number of copper/solder

joints that can cause thermocouple effects. Where

copper/solder joints are used, make sure that they

are in both the D+ and D− path and at the same

temperature.

Thermocouple effects should not be a major

problem as 1C corresponds to about 200 mV, and

thermocouple voltages are about 3 mV/C of

temperature difference.

Unless there are two thermocouples with a large

temperature differential between them,

thermocouple voltages should be much less than

200 mV.

4. Place a 0.1 mF bypass capacitor close to the VDD

pin. In extremely noisy environments, an input

filter capacitor may be placed across D+ and D−

close to the ADT7481. This capacitance can affect

the temperature measurement, so care must be

taken to ensure that any capacitance seen at D+

and D− is a maximum of 1,000 pF. This maximum

value includes the filter capacitance, plus any

cable or stray capacitance between the pins and the

sensor diode.

5. If the distance to the remote sensor is more than 8

inches, the use of twisted pair cable is

recommended. A total of 6 feet to 12 feet of cable

is needed.

For really long distances (up to 100 feet), use

shielded twisted pair, such as Belden No. 8451

microphone cable. Connect the twisted pair to D+

and D− and the shield to GND close to the

ADT7481. Leave the remote end of the shield

unconnected to avoid ground loops.

Because the measurement technique uses switched

current sources, excessive cable or filter capacitance can

affect the measurement. When using long cables, the filter

capacitance can be reduced or removed.

Application Circuit

Figure 22 shows a typical application circuit for the

ADT7481, using discrete sensor transistors. The pullups on

SCLK, SDATA, and ALERT are required only if they are not

already provided elsewhere in the system.

The SCLK and SDATA pins of the ADT7481 can be

interfaced directly to the SMBus of an I/O controller, such

as the Intel820 chipset.

ADT7481

http://onsemi.com

Figure 22. Typical Application Circuit

FAN

ENABLE

VDD

TYP 10 kW

FAN

CONTROL

CIRCUIT

SMBUS

CONTROLLER

5.0 V or 12 V

3.0 V to 3.6 V

TYP 10 kW

0.1 mF

GND

2N3904/06

CPU THERMAL

DIODE

ADT7481

SCLK

SDATA

ALERT

THERM

VDD

D1−

D1+

D2−

D2+

Table 18. ORDERING INFORMATION

Device Order Number* Package Type Shipping†Branding SMBus Address

ADT7481ARMZ−REEL 10-lead MSOP 3,000 Tape & Reel T08 4C

ADT7481ARMZ−1RL 10-lead MSOP 3,000 Tape & Reel T0M 4B

†For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging

Specifications Brochure, BRD8011/D.

*The “Z’’ suffix indicates Pb-Free package available.

ADT7481

http://onsemi.com

PACKAGE DIMENSIONS

MSOP10

CASE 846AC−01

ISSUE O

0.08 (0.003) A S

DIM MIN MAX MIN MAX

INCHESMILLIMETERS

A2.90 3.10 0.114 0.122

B2.90 3.10 0.114 0.122

C0.95 1.10 0.037 0.043

D0.20 0.30 0.008 0.012

G0.50 BSC 0.020 BSC

H0.05 0.15 0.002 0.006

J0.10 0.21 0.004 0.008

K4.75 5.05 0.187 0.199

L0.40 0.70 0.016 0.028

NOTES:

1. DIMENSIONING AND TOLERANCING PER

ANSI Y14.5M, 1982.

2. CONTROLLING DIMENSION: MILLIMETER.

3. DIMENSION “A” DOES NOT INCLUDE MOLD

FLASH, PROTRUSIONS OR GATE BURRS.

MOLD FLASH, PROTRUSIONS OR GATE

BURRS SHALL NOT EXCEED 0.15 (0.006)

PER SIDE.

4. DIMENSION “B” DOES NOT INCLUDE

INTERLEAD FLASH OR PROTRUSION.

INTERLEAD FLASH OR PROTRUSION

SHALL NOT EXCEED 0.25 (0.010) PER SIDE.

5. 846B−01 OBSOLETE. NEW STANDARD

846B−02

−B−

−A−

PIN 1 ID 8 PL

0.038 (0.0015)

−T−SEATING

PLANE

HJL

ǒmm

inchesǓ

SCALE 8:1

10X 10X

1.04

0.041

0.32

0.0126

5.28

0.208

4.24

0.167

3.20

0.126

0.50

0.0196

*For additional information on our Pb-Free strategy and soldering

details, please download the ON Semiconductor Soldering and

Mounting Techniques Reference Manual, SOLDERRM/D.

SOLDERING FOOTPRINT*

ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC owns the rights to a number of patents, trademarks,

copyrights, trade secrets, and other intellectual property. A listing of SCILLC’s product/patent coverage may be accessed at www.onsemi.com/site/pdf/Patent−Marking.pdf. SCILLC

reserves the right to make changes without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any

particular purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without

limitation special, consequential or incidental damages. “Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications

and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC

does not convey any license under its patent rights nor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for

surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where

personal injury or death may occur. Should Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and

its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly,

any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture

of the part. SCILLC is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner.

PUBLICATION ORDERING INFORMATION

N. American Technical Support: 800−282−9855 Toll Free

USA/Canada

Europe, Middle East and Africa Technical Support:

Phone: 421 33 790 2910

Japan Customer Focus Center

Phone: 81−3−5817−1050

ADT7481/D

LITERATURE FULFILLMENT:

Literature Distribution Center for ON Semiconductor

P.O. Box 5163, Denver, Colorado 80217 USA

Phone: 303−675−2175 or 800−344−3860 Toll Free USA/Canada

Fax: 303−675−2176 or 800−344−3867 Toll Free USA/Canada

Email: orderlit@onsemi.com

ON Semiconductor Website: www.onsemi.com

Order Literature: http://www.onsemi.com/orderlit

For additional information, please contact your local

Sales Representative