General Description
The MAX6661 is a remote temperature sensor and fan-
speed regulator that provides complete closed-loop fan
control. The remote temperature sensor is typically a
common-collector PNP, such as a substrate PNP of a
microprocessor, or a diode-connected transistor, typi-
cally a low-cost, easily mounted 2N3904 NPN type or
2N3906 PNP type.
The device also incorporates a closed-loop fan con-
troller that regulates fan speed with tachometer feed-
back. The MAX6661 compares temperature data to a
fan threshold temperature and gain setting, both pro-
grammed over the SPI™ bus by the user. The result is
automatic fan control that is proportional to the remote-
junction temperature. The temperature feedback loop
can be broken at any time for system control over the
speed of the fan.
Fan speed is voltage controlled as opposed to PWM
controlled, greatly reducing acoustic noise and maxi-
mizing fan reliability. An on-chip power device drives
fans rated up to 250mA.
Temperature data is updated every 500ms and is read-
able at any time over the SPI interface. The MAX6661 is
accurate to 1°C (max) when the remote junction is
between +60°C to +100°C. Data is formatted as a 10-
bit + sign word with 0.125°C resolution.
The MAX6661 is specified between -40°C to +125°C
and is available in a 16-pin QSOP package.
Applications
Telecom Systems
Servers
Workstations
Electronic Instruments
Features
♦Integrated Thermal Measurement and Fan
Regulation
♦Programmable Fan Threshold Temperature
♦Programmable Temperature Range for Full-Scale
Fan Speed
♦Accurate Closed-Loop Fan-Speed Regulation
♦On-Chip Power Device Drives Fans Rated
Up to 250mA
♦Programmable Under/Overtemperature Alarms
♦SPI-Compatible Serial Interface
♦±1°C (+60°C to +100°C) Thermal-Sensing
Accuracy
MAX6661
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
________________________________________________________________ Maxim Integrated Products 1
Ordering Information
1µF
5kΩ
FAN
12V
2200pF
PENTIUM
SDIN
SC
ALERT
OVERT
SPI CLOCK
SPI DATA IN
DOUT SPI DATA OUT
CS SPI CHIP SELECT
INTERRUPT
TO µP
TO SYSTEM
SHUTDOWN
VFAN
PGND
0.1µF
3V TO 5.5V
50Ω
VCC
TACH IN
FAN
DXP
DXN
AGND
10kΩ
EACH
MAX6661
Typical Operating Circuit
19-2337; Rev 0; 1/02
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
PART TEMP RANGE PIN-PACKAGE
MAX6661AEE -40°C to +125°C 16 QSOP
Pin Configuration appears at end of data sheet.
SPI is a trademark of Motorola, Inc.
MAX6661
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VCC = 3V to 5.5V, VFAN = 12V, TA= -40°C to +125°C, unless otherwise specified. Typical values are at VCC = 3.3V and TA=
+25°C.) (Notes 1 and 2)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
VCC, ALERT, OVERT ...............................................-0.3V to +6V
VFAN, TACH IN, FAN .............................................-0.3V to +16V
DXP, CS, SDOUT, GAIN, SCL, SDIN..........-0.3V to (VCC + 0.3V)
DXN ..........................................................................-0.3V to +1V
SDOUT Current ...................................................-1mA to +50mA
DXN Current ......................................................................±1mA
FAN Out Current ..............................................................500mA
Continuous Power Dissipation (TA= +70°C)
16-Pin QSOP (derate 8.3mW/°C above +70°C)...........667mW
Operating Temperature Range ........................ -40°C to +125°C
Junction Temperature .....................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) ................................+300°C
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
ADC AND POWER SUPPLY
0.125 °C
Temperature Resolution
(Note 3) 11 Bits
TRJ = +60°C to +100°C-1 +1°C
TRJ = +25°C to +125°C-3 +3°C
Remote-Junction Temperature
Measurement Error (Note 4) TETA = +85°C,
VCC = 3.3V
TRJ = -40°C to +125°C-5 +5°C
Fan-Speed Measurement
Accuracy ±25 %
VCC Supply Voltage Range VCC 3.0 5.5 V
VFAN Supply Voltage Range VFAN 4.5 13.5 V
Conversion Time 0.25 s
Conversion Rate Timing Error -25 +25 %
Undervoltage Lockout (UVLO)
Threshold VUVLO VCC falling 2.50 2.80 2.95 V
UVLO Threshold Hysteresis VHYST 90 mV
POR Threshold (VCC)V
CC rising 1.4 2.0 2.5 V
POR Threshold Hysteresis 90 mV
Standby Supply Current ISHDN Shutdown, configuration bit 6 = 1 3 20 µA
Operating Supply Current ICC Fan off 450 700 µA
DXN Source Voltage VDXN 0.7 V
TACH Input Transition Level VFAN = 12V 10.5 V
TACH Input Hysteresis VFAN = 12V 190 mV
TACH Input Resistance 250 kΩ
Fan Output Current IF250 mA
Fan Output Current Limit IL(Note 5) 320 410 mA
Fan Output On-Resistance RONF 250mA load 4 Ω
MAX6661
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VCC = 3V to 5.5V, VFAN = 12V, TA= -40°C to +125°C, unless otherwise specified. Typical values are at VCC = 3.3V and TA=
+25°C.) (Notes 1 and 2)
Note 1: TA= TJ. This implies zero dissipation in pass transistor (no load, or fan turned off).
Note 2: All parameters are 100% production tested at a single temperature, unless otherwise indicated. Parameter values through
temperature are guaranteed by design.
Note 3: The fan control section of the MAX6661 and temperature comparisons use only 9 bits of the 11-bit temperature measure-
ment with a 0.5°C LSB.
Note 4: Wide-range accuracy is guaranteed by design, not production tested.
Note 5: Guaranteed by design.
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
INTERFACE PINS (SDIN, SC, CS, DOUT, ALERT, OVERT)
Serial Bus Maximum Clock
Frequency (Note 5) SC 2.5 MHz
VCC = 3V 2.2
Logic Input High Voltage VCC = 5.5V 2.4 V
Logic Input Low Voltage VCC = 3V to 5V 0.8 V
Logic Output High-Voltage
DOUT VCC = 3V, ISOURCE = 6mA (Note 5) VCC -
0.4V V
Logic Output Low-Voltage DOUT VCC = 3V, ISINK = 6mA (Note 5) 0.4 V
Logic Output Low-Voltage
ALERT, OVERT VCC = 3V, ISINK = 6mA (Note 5) 0.4 V
ALERT, OVERT Output
High Leakage Current ALERT, OVERT forced to 5.5V 1 µA
Logic Input Current Logic inputs forced to VCC or GND -2 2 µA
SPI AC TIMING (Figure 5)
CS High to DOUT Three-State tTR CLOAD = 100pF, RGS = 10kΩ (Note 5) 200 ns
CS to SC Setup Time tCSS (Note 5) 200 ns
SC Fall to DOUT Valid tDO CLOAD = 100pF 200 ns
DIN to SC Setup Time tDS 200 ns
DIN to SC Hold Time tDH (Note 5) 200 ns
SC Period tCP 400 ns
SC High Time tCH 200 ns
SC Low Time tCL 200 ns
CS High Pulse Width tCSW (Note 5) 400 ns
Output Rise Time tRCLOAD = 100pF 10 ns
Output Fall Time tFCLOAD = 100pF 10 ns
SC Falling Edge to CS Rising tSCS (Note 5) 200 ns
MAX6661
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
4 _______________________________________________________________________________________
Typical Operating Characteristics
(TA = +25°C, unless otherwise noted.)
1 10 100
TEMPERATURE ERROR
vs. PC BOARD RESISTANCE
MAX6661 toc01
LEAKAGE RESISTANCE (MΩ)
TEMPERATURE ERROR (°C)
20
-30
-25
-20
-15
-10
-5
0
15
10
5
PATH = DXP TO GND
PATH = DXP TO VCC (5V)
-5
-2
-3
-4
-1
0
1
2
3
4
5
-50 0 50 100 150
MAX6661 toc02
TEMPERATURE (°C)
TEMPERATURE ERROR (°C)
TEMPERATURE ERROR
vs. REMOTE-DIODE TEMPERATURE
1 100 10k 1M10 1k 100k 10M 100M
TEMPERATURE ERROR
vs. POWER-SUPPLY NOISE FREQUENCY
MAX6661 toc03
FREQUENCY (Hz)
TEMPERATURE ERROR (°C)
20
-30
-25
-20
-15
-10
-5
0
15
10
5
VIN = 100mVP-P
VIN = SQUARE WAVE APPLIED TO VCC
WITH NO 0.1µF VCC CAPACITOR
VIN = 250mVP-P
4.0
-1.5
-1.0
MAX6661 toc04
FREQUENCY (Hz)
TEMPERATURE ERROR (°C)
-0.5
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
1 10 100M1M 10M100 1k 10k 100k
TEMPERATURE ERROR
vs. COMMON-MODE NOISE FREQUENCY
VIN = 50mVP-P
VIN = SQUARE WAVE
AC-COUPLED TO DXN
VIN = 100mVP-P
VIN = 25mVP-P
-8
-6
-7
-4
-5
-3
-2
0
-1
1
0102030405060708090100
MAX6661 toc05
DXP-DXN CAPACITANCE (nF)
TEMPERATURE ERROR (°C)
TEMPERATURE ERROR
vs. DXP-DXN CAPACITANCE
0
1
3
2
4
5
MAX6661 toc06
SUPPLY VOLTAGE (V)
STANDBY SUPPLY CURRENT (µA)
3.0 4.0 4.53.5 5.0 5.5
STANDBY SUPPLY CURRENT
vs. SUPPLY VOLTAGE
CONFIG BIT 6 = 1
3.0 3.9 4.23.3 3.6 4.5 4.8 5.1 5.4
MAX6661 toc07
SUPPLY VOLTAGE (V)
AVERAGE SUPPLY CURRENT (µA)
450
AVERAGE SUPPLY CURRENT
vs. SUPPLY VOLTAGE
Detailed Description
The MAX6661 is a remote temperature sensor and fan
controller with an SPI interface. The MAX6661 converts
the temperature of a remote PN junction to a 10-bit +
sign digital word. The remote PN junction can be a
diode-connected transistor, such as a 2N3906, or the
type normally found on the substrate of many proces-
sors’ ICs. The temperature information is provided to the
fan-speed regulator and is read over the SPI interface.
The temperature data, through the SPI interface, can be
read as a 10-bit + sign two’s complement word with a
0.125°C resolution (LSB) and is updated every 0.5s.
The MAX6661 incorporates a closed-loop fan controller
that regulates the fan speed with tachometer feedback.
The temperature information is compared to a threshold
and range setting, which enables the MAX6661 to auto-
matically set fan speed proportional to temperature.
Full control of the fan is available by being able to open
either the thermal control loop or the fan control loop.
Figure 1 shows a simplified block diagram.
ADC
The ADC is an averaging type that integrates the signal
input over a 125ms period with excellent noise rejec-
tion. A bias current is steered through the remote
diode, where the forward voltage is measured, and the
temperature is computed. The DXN pin is the cathode
of the remote diode and is biased at 0.7V above
ground by an internal diode to set up the ADC inputs
for a differential measurement. The worst-case DXP-
DXN differential input voltage range is 0.25V to 0.95V.
Excess resistance in series with the remote diode caus-
es about 1/2°C error per ohm. Likewise, 200mV of off-
set voltage forced on DXP-DXN causes approximately
1°C error.
A/D Conversion Sequence
A temperature-conversion sequence is initiated every
500ms in the free-running autoconvert mode (bit 6 = 0
in the configuration register) or immediately by writing a
one-shot command. The result of the new measurement
is available after the end of conversion. The results of
the previous conversion sequence are still available
when the ADC is converting.
Remote-Diode Selection
Temperature accuracy depends on having a good-
quality, diode-connected, small-signal transistor.
Accuracy has been experimentally verified for all
devices listed in Table 1. The MAX6661 can also direct-
ly measure the die temperature of CPUs and other ICs
that have on-board temperature-sensing diodes.
MAX6661
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
_______________________________________________________________________________________ 5
Pin Description
PIN NAME FUNCTION
1V
FAN Power Supply for Fan Drive: 4.5V to 13.5V
2V
CC Power Supply: 3V to 5.5V. Bypass with a 0.1µF capacitor to GND.
3 DXP Input: Remote-Junction Anode. Place a 2200pF capacitor between DXP and DXN for noise filtering.
4 DXN Input: Remote-Junction Cathode. DXN is internally biased to a diode voltage above ground.
5 FAN Output to Fan Low Side
6 N.C. No External Connection. Must be left floating.
7 PGND Power Ground
8 AGND Analog Ground
9OVERT Output to System Shutdown. Active-low output, programmable for active high, if desired. Open drain.
10 CS SPI Chip Select. Active low.
11 ALERT Open-Drain Active-Low Output
12 DOUT SPI Data Output. High-Z when not being read.
13 GAIN Leave open if tachometer feedback is being used. Connect an external resistor to VCC to reduce the
gain of the current sense.
14 SCL SPI Clock
15 SDIN SPI Data In
16 TACH IN Fan Tachometer Input. 13.5V tolerant, pullup from VCC to 13.5V is allowed on this line.
MAX6661
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
6 _______________________________________________________________________________________
N
FAN
FAN
TACH IN
VFAN
FAN-SPEED
REGULATOR
MUXDXP
DXN
THIGH
TLOW
CONFIGURATION
FAN TACHOMETER
DIVISOR (FTD)
FAN TACHOMETER
PERIOD (FTP)
FULL SCALE
(FS)
FAN GAIN (FG)
TFAN (FT)
FAN-CONVERSION
RATE (FCR)
MODE (M)
FAN TACHOMETER
PERIOD LIMIT (FTPL)
FAN-SPEED CONTROL
(FSC)
STATUS
THYST COMPARAT0R
TMAX
THERMAL OPEN/
CLOSE LOOP
FAN OPEN/
CLOSE LOOP
FAN
CONTROL
CIRCUIT
REMOTE
TEMPERATURE
DATA
REGISTERS
SC
SDIN
DOUT
CS
ADC
CONTROL
LOGIC
SPI
INTERFACE
OVERT
ALERT
Figure 1. MAX6661 Block Diagram
The transistor must be a small-signal type with a rela-
tively high forward voltage. Otherwise, the A/D input
range could be violated. The forward voltage must be
greater than 0.25V at 10µA. Check to ensure this is true
at the highest expected temperature. The forward volt-
age must be less than 0.95V at 100µA. Check to ensure
that this is true at the lowest expected temperature.
Large power transistors, power diodes, or small-signal
diodes must not be used. Also, ensure that the base
resistance is less than 100Ω. Tight specifications for
forward-current gain (50 < ß <150, for example) indi-
cate that the manufacturer has good process controls
and that the devices have consistent VBE characteris-
tics. Bits 5–2 of the mode register can be used to
adjust the ADC gain to achieve accurate temperature
measurements with diodes not included in the recom-
mended list or to calibrate individually the MAX6661 for
use in specific control systems.
Thermal Mass and Self-Heating
When measuring the temperature of a CPU or other IC
with an on-chip sense junction, the thermal mass of the
sensor has virtually no effect; the measured tempera-
ture of the junction tracks the actual temperature within
a conversion cycle. When measuring temperature with
discrete remote sensors, smaller packages (e.g., a
SOT23) yield the best thermal response times. Take
care to account for thermal gradients between the heat
source and the sensor, and ensure that stray air cur-
rents across the sensor package do not interfere with
measurement accuracy. Sensor self-heating, caused
by the diode current source, is negligible.
ADC Noise Filtering
The ADC is an integrating type with inherently good
noise rejection, especially of low-frequency noise such
as 60Hz line interference. Micropower operation places
constraints on high-frequency noise rejection; there-
fore, careful PC board layout and proper external noise
filtering are required for high-accuracy remote mea-
surements in electrically noisy environments. High-fre-
quency EMI is best filtered at DXP and DXN with an
external 2200pF capacitor. This value can be increased
to about 3300pF (max), including cable capacitance.
Capacitance higher than 3300pF introduces errors due
to the rise time of the switched current source. Nearly
all noise sources tested cause the ADC measurements
to be higher than the actual temperature, typically by
1°C to 10°C, depending on the frequency and ampli-
tude (see Typical Operating Characteristics).
PC Board Layout
Follow these guidelines to reduce the measurement
error of the temperature sensors:
1) Place the MAX6661 as close as practical to the
remote diode. In noisy environments, such as a
computer motherboard, this distance can be 4in to
8in (typ). This length can be increased if the worst
noise sources are avoided. Noise sources include
CRTs, clock generators, memory buses, and
ISA/PCI buses.
2) Do not route the DXP-DXN lines next to the deflec-
tion coils of a CRT. Also, do not route the traces
across fast digital signals, which can easily intro-
duce a 30°C error, even with good filtering.
3) Route the DXP and DXN traces in parallel and in
close proximity to each other, away from any higher
voltage traces, such as 12VDC. Leakage currents
from PC board contamination must be dealt with
carefully since a 20MΩleakage path from DXP to
ground causes about a 1°C error. If high-voltage
traces are unavoidable, connect guard traces to GND
on either side of the DXP-DXN traces (Figure 2).
4) Route through as few vias and crossunders as pos-
sible to minimize copper/solder thermocouple
effects.
MAX6661
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
_______________________________________________________________________________________ 7
Note: Transistors must be diode connected (base shorted to
collector).
MANUFACTURER MODEL NO.
Central Semiconductor (USA) 2N3904, 2N3906
Fairchild Semiconductor (USA) 2N3904, 2N3906
Rohm Semiconductor (Japan) SST3904
Samsung (Korea) KST3904-TF
Siemens (Germany) SMBT3904
Zetex (England) FMMT3904CT-ND
Table 1. Remote-Sensor Transistors
MINIMUM
10mils
10mils
10mils
10mils
GND
DXN
DXP
GND
Figure 2. Recommended DXP-DXN PC Trace
MAX6661
5) When introducing a thermocouple by inserting differ-
ent metals in the connection path, make sure that
both the DXP and the DXN paths have matching
thermocouples, i.e., the connection paths are sym-
metrical. A copper-solder thermocouple exhibits
3µV/°C. Adding a few thermocouples causes a neg-
ligible error.
6) The 10mil widths and spacings that are recommend-
ed in Figure 2 are not absolutely necessary, as they
offer only a minor improvement in leakage and noise
over narrow traces. Use wider traces when practical.
7) Add a 5Ωresistor in series with VCC for best noise
filtering (see Typical Operating Circuit).
PC Board Layout Checklist
•Place the MAX6661 close to the remote-sense junc-
tion.
•Keep traces away from high voltages (12V bus).
•Keep traces away from fast data buses and CRTs.
•Use recommended trace widths and spacings.
•Place a ground plane under the traces.
•Use guard traces connected to GND flanking DXP
and DXN.
•Place the noise filter and the 0.1µF VCC bypass
capacitors close to the MAX6661.
Twisted-Pair and Shielded Cables
Use a twisted-pair cable to connect the remote sensor
for distances longer than 8in or in very noisy environ-
ments. Twisted-pair cable lengths can be between 6ft
and 12ft before noise introduces excessive errors. For
longer distances, the best solution is a shielded twisted
pair like that used for audio microphones. For example,
Belden 8451 works well for distances up to 100ft in a
noisy environment. At the device, connect the twisted
pair to DXP and DXN and the shield to GND. Leave the
shield unconnected at the remote sensor. For very long
cable runs, the cable’s parasitic capacitance often pro-
vides noise filtering, so the 2200pF capacitor can often
be removed or reduced in value. Cable resistance also
affects remote-sensor accuracy. For every ohm of
series resistance, the error is approximately 1/2°C.
Low-Power Standby Mode
Standby mode reduces the supply current to less than
10µA (typ) by disabling the ADC, the control loop, and
the fan driver. Enter standby mode by setting the
RUN/STOP bit to 1 (bit 6) in the configuration byte reg-
ister. In standby mode, all data is retained in memory,
and the SPI interface is alive and listening for SPI com-
mands. In standby mode, the one-shot command initi-
ates a conversion. Activity on the SPI bus causes the
device to draw extra supply current.
If a standby command is received while a conversion is
in progress, the conversion cycle is interrupted, and
the temperature registers are not updated. The previ-
ous data is not changed and remains available.
SPI Interface
The data interface for the MAX6661 is compatible with
SPI, QSPI™, and MICROWIRE™ devices. For SPI/QSPI,
ensure that the CPU serial interface runs in master
mode so that it generates the serial clock signal. Select
a 2.5MHz clock frequency or lower, and set zero values
for clock polarity (CPOL) and phase (CPHA) in the µP
control registers.
Data is clocked into the MAX6661 at SDIN on the rising
edge of SC when CS is low. The first byte is the com-
mand byte and the second byte is the data byte. The
command byte can be either a read byte or a write byte
(Table 2). The last bit READ/WRITE (LSB) of the com-
mand byte tells the MAX6661 whether it is a read or a
write operation, where a high signifies a read, and a
low signifies a write. When CS is high, the MAX6661
does not respond to any activity on the SPI bus. All
valid communications on the SPI should have 16 bits
except for the SPOR and the OSHT.
During a READ operation, the DOUT line goes low on
the falling clock edge after the READ/WRITE bit (8th
bit). The data in the shift register is moved to the DOUT
line during the 8th to 15th falling-clock edges and the
MSB of the data is available to be read at the rising
edge of the 9th clock pulse. The remaining clock puls-
es in the READ operation shift the register contents on
the negative clock edge so that they can be latched
into the master on the positive edge. Any READ opera-
tion with less than 16 bits results in truncated data.
Figure 3 shows the read cycle.
For a WRITE operation, the command byte is decoded
during the 8th clock pulse. Then data is loaded into the
shift register on the positive edges of the 9th to 16th
clock pulses and transferred to the appropriate register
on the negative edge of the 16th clock period. Any
WRITE operation that does not have the 16th clock
edge does not get shifted out of the shift register and
thus is ignored. Since returning CS high resets the SPI
interface at the end of a transfer, this cannot be done
until after the 16th falling clock edge. If CS is returned
high before this 16th falling clock edge, the appropriate
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
8 _______________________________________________________________________________________
QSPI is a trademark of Motorola, Inc.
MICROWIRE is a trademark of National Semiconductor Corp.
register is not loaded. DOUT is high impedance during
a WRITE operation. Figure 4 shows the write cycle.
For single byte commands such as OSHT and SPOR,
the operation need only be 7 bits long where the
READ/WRITE bit is omitted. Here the command is
loaded into the shift register on the rising edge of SC
and the command is decoded during the high period of
the 7th clock pulse. The 7th falling edge of SC shifts the
command from the shift register to the appropriate reg-
ister. CS can then go high after the SC low to CS high
hold time tCSH (see SPI AC Timing, Electrical Char-
acteristics). Figure 5 shows the timing waveforms for
the MAX6661’s SPI interface.
Remote Temperature Data Register
Two registers, at addresses 81h and 83h, store the
measured temperature data from the remote diode. The
data format for the remote-diode temperature is 10 bits
+ sign, with each LSB corresponding to 0.125°C, in
two’s complement format (Table 3). Register 83h con-
tains the sign bit and the first 7 bits. Bits 7, 6, and 5 of
register 81h are the 3LSBs. If the two registers are not
read immediately, one after the other, their contents
may be the result of two different temperature measure-
ments, leading to erroneous temperature data. For this
reason, a parity bit has been added to the 81h register.
Bit 4 of this is zero if the data in 81h and 83h are from
the same temperature conversion and 83h is read first.
Otherwise, bit 4 is one. The remaining bits are don’t
cares. When reading temperature data, register 83h
must be read first.
Alarm Threshold Registers
The MAX6661 provides four alarm threshold registers
that can be programmed with a two’s complement tem-
perature value with each LSB corresponding to 1°C.
The registers are THIGH, TLOW, TMAX, and THYST. If the
measured temperature equals or exceeds THIGH, or is
less than TLOW, an ALERT interrupt is asserted. If the
measured temperature equals or exceeds TMAX, the
OVERT output is asserted (see the Overtemperature
Output
OVERT
section). The POR state for THIGH is
MAX6661
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
_______________________________________________________________________________________ 9
REGISTERS COMMAND POR STATE FUNCTION
RRL 81h 00000000 Read Remote Temperature Low Byte (3MSBs)
RRH 83h 00000000 Read Remote Temperature High Byte (Sign Bit and First 7 Bits)
RSL 85h 00000000 Read Status Byte
RCL/WCL 87h/92h 00000000 Read/Write Configuration Byte
RFCR/WFCR 89h/94h 00000010 Read/Write Fan-Conversion Rate Byte
RTMAX/WTMAX A1h/A4h 01100100 (+100°C) Read/Write Remote TMAX
RTHYST/WTHYST A3h/A6h 01011111 (+95°C) Read/Write Remote THYST
RTHIGH/WTHIGH 8Fh/9Ah 01111111 (+127°C) Read/Write Remote THIGH
RTLOW/WTLOW 91h/9Ch 11001001 (-55°C) Read/Write Remote TLOW
SPOR F8h N/A Write Software POR
OSHT 9Eh N/A Write One-Shot Temperature Conversion
RTFAN/WTFAN A9h/B2h 00111100 (+60°C) Read/Write Fan-Control Threshold Temperature TFAN
RFSC/WFSC ABh/B4h 00000000 Read/Write Fan-Speed Control
RFG/WFG ADh/B6h 10000000 Read/Write Fan Gain
RFTP AFh 00000000 Read Fan Tachometer Period
RFTCL/WFTPLP B1h/B8h 11111111 Read/Write Fan Tachometer Period Limit (Fan-Failure Limit)
RFTD/WFTD BBh/BCh 00000001 Read/Write Fan Tachometer Divisor
RFS/WFS BFh/C0h 11111111 Read/Write Full-Scale Register
RM/WM F5h/F6h 00000000 Read/Write Mode Register
ID CODE FDh 01001101 Read Manufacturer ID Code
ID CODE FFh 00001001 Read Device ID Code
Table 2. MAX6661 Command-Byte Bit Assignments
MAX6661
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
10 ______________________________________________________________________________________
SC
COMMAND BYTE
D7–D0 DATA BYTE
CS
DIN
D15 IS START BIT
ALWAYS HIGH
D15
D8 IS READ/WRITE BIT
HIGH FOR READ
D8
DOUT THREE-STATE D7 D6 THREE-STATE
D0
Figure 3. Read Cycle
DIN
D15 IS START BIT
ALWAYS HIGH
CS
DOUT
SC
THREE-STATE
D15
D8 IS READ/WRITE BIT
LOW FOR WRITE
D15–D8 COMMAND BYTE
D8 D7
D7–D0 DATA BYTE
THREE-STATE
D0
Figure 4. Write Cycle
CS
SC
tTR
tCSW
SDIN
DOUT
tCSS
tCL
tCH
tDS
tDH
tDO
tSCS
tCP
Figure 5. Serial Interface Timing
+127°C, for TLOW is -55°C, for TMAX is +100°C, and for
THYST is +95°C.
Overtemperature Output (
OVERT
)
The MAX6661 has an overtemperature output (OVERT)
that is set when the remote-diode temperature crosses
the limits set in the TMAX register. It is always active if
the remote-diode temperature exceeds TMAX. The
OVERT line clears when the temperature drops below
THYST. Bit 1 of the configuration register can be used to
mask the OVERT output. Typically, the OVERT output is
connected to a power-supply shutdown line to turn sys-
tem power off. At power-up, OVERT defaults to low
when activated but the logic can be reversed by setting
bit 5 of the configuration register. If reversed, OVERT is
a logic one when the tMAX register temperature value is
exceeded. The OVERT line can be taken active, either
by the MAX6661 or driven by an external source.
OVERT also acts as an input when set to go low when
activated (default). If OVERT is driven or forced low
externally, the fan loop forces the fan to full speed and
bit 1 of the status register is set. The OVERT input can
be masked out by bit 2 of the configuration register.
Diode Fault Alarm
A continuity fault detector at DXP detects an open cir-
cuit between DXP and DXN. If an open or short circuit
exists, register 83h is loaded with 1000 0000.
Additionally, if the fault is an open circuit, bit 2 of the
status byte is set to 1 and the ALERT condition is acti-
vated at the end of the conversion. Immediately after
POR, the status register indicates that no fault is pre-
sent until the end of the first conversion.
ALERT
Interrupts
The ALERT interrupt output signal is activated (unless it
is masked by bit 7 in the configuration register) when-
ever the remote-diode’s temperature is below TLOW or
exceeds THIGH. A disconnected remote diode (for con-
tinuity detection), a shorted diode, or an active OVERT
also activates the ALERT signal. The activation of the
ALERT signal sets the corresponding bits in the status
register. Once activated, ALERT is latched until
cleared. To clear the ALERT, read the status register.
The interrupt does not halt automatic conversions. New
temperature data continues to be available over the SPI
interface after ALERT is asserted. ALERT is an active-
low open-drain output so that devices can share a
common interrupt line. The interrupt is updated at the
end of each temperature conversion so, after being
cleared, it reappears after the next temperature conver-
sion if the cause of the fault has not been removed.
By setting bit 0 in the configuration register to 1, the
ALERT line always remains high. Prior to taking correc-
tive action, always check to ensure that an interrupt is
valid by reading the current temperature and the status
register.
Example: The remote temperature reading crosses
THIGH, activating ALERT. The host responds to the
interrupt by reading the status register, clearing the
interrupt. If the condition persists, the interrupt reap-
pears.
One Shot
The one-shot command immediately forces a new con-
version cycle to begin. In software standby mode
(RUN/STOP bit = high), a new conversion is begun by
writing an OSHT (9Eh) command. After the conversion,
the device returns to standby mode. If a conversion is
in progress when a one-shot command is received, the
command is ignored. If a one-shot command is
between conversions in autoconvert mode (RUN/STOP
bit = low), a new conversion begins immediately.
Configuration Register Functions
The configuration register table (Table 4) describes this
register’s bit assignments.
Status Register Functions
The status byte (Table 5) reports several fault condi-
tions. It indicates when the fan driver transistor of the
MAX6661 has overheated and/or in thermal shutdown,
when the temperature thresholds, TLOW and THIGH,
have been exceeded, and whether there is an open cir-
cuit in the DXP-DXN path. The register also reports the
state of the ALERT and OVERT lines and indicates
when the fan driver is fully on. The final bit in the status
register indicates when a fan failure has occurred.
After POR, the normal state of the flag bits is zero,
assuming no alert or overtemperature conditions are
MAX6661
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
______________________________________________________________________________________ 11
TEMP (°C) DIGITAL OUTPUT
+127 0111 1111 111
+125.00 0111 1101 000
+25 0001 1001 000
+0.125 0000 0000 001
0 0000 0000 000
-0.125 1111 1111 111
-25 1110 0111 111
-40 1101 1000111
Table 3. Temperature Data Format
(Two’s Complement)
MAX6661
present. Bits 2 through 6 of the status register are
cleared by any successful read of the status register,
unless the fault persists. The ALERT output follows the
status flag bit. Both are cleared when successfully
read, but if the condition still exists, the ALERT and the
corresponding status bit are reasserted at the end of
the next conversion.
When autoconverting, if the THIGH and TLOW limits are
close together, it is possible for both high-temperature
and low-temperature status bits to be set, depending
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
12 ______________________________________________________________________________________
BIT NAME POR STATE DESCRIPTION
7(MSB) ALERT Mask 0 When set to 1, ALERT is masked from internally generated errors.
6 Run/Stop 0 When set to 1, the MAX6661 enters low-power standby.
5OVERT Polarity 0 0 provides active low, 1 provides active high.
4 Write Protect 0
When set to 1, Write Protect is in effect for the following applicable registers:
1. Configuration register bits 6, 5, 4, 3
2. TMAX register
3. THYST register
4. Fan conversion rate register
3Thermal Closed/
Open Loop 0
When set to 1, the thermal loop is open. The fan speed control retains the
last closed-loop value unless overwritten by a bus command (in closed
loop, the fan speed control is read only). If fan mode is set to open loop by
writing a 1 to bit 0 of the fan gain register, then this bit is automatically set.
2OVERT Input Inhibit 0 When set to 1, an external signal on OVERT is masked from bit 1 of the
status register.
1Mask OVERT
Output 0 Mask the OVERT output from an internally generated overtemperature error.
0 N/A 0 Not used.
Table 4. Configuration Register Bit Assignments
BIT NAME POR STATE DESCRIPTION
7(MSB) MAX6661
Overheat 0
When high, indicates that the fan driver transistor of the MAX6661 has
overheated (temperature > +150°C) and is in thermal shutdown. The fan driver
remains disabled until temperature falls below +140°C.
6ALERT 0When high, indicates ALERT has been activated (pulled low), regardless of
cause (internal or external).
5Fan Driver Full
Scale 0When high, indicates the fan driver is at full scale. Only valid in fan
closed-loop mode.
4 Remote High 0 When high, the remote-junction temperature exceeds the temperature in the
remote high register.
3 Remote Low 0 When low, the remote-junction temperature is lower than the temperature in the
remote low register.
2 Diode Open 0 When high, the remote-junction diode is open.
1OVERT 0When active, indicates that OVERT has been activated, regardless of cause
(internal or external).
0 Fan Failure 0 When high, indicates the count in the fan tachometer period register is higher
than the limit set in the fan tachometer period limit register.
Table 5. Status Register Bit Assignments
on the amount of time between status read operations.
In these circumstances, it is best not to rely on the sta-
tus bits to indicate reversals in long-term temperature
changes. Instead, use a current temperature reading to
establish the trend direction.
Manufacturer and Device ID Codes
Two ROM registers provide manufacturer and device
ID codes. Reading the manufacturer ID returns 4D,
which is the ASCII code M (for Maxim). Reading the
device ID returns 09h, indicating the MAX6661 device.
POR and UVLO
The MAX6661 has a volatile memory. To prevent unreli-
able power-supply conditions from corrupting the data
in memory and causing erratic behavior, a POR voltage
detector monitors VCC and clears the memory if VCC
falls below 1.91V (see Electrical Characteristics). When
power is first applied and VCC rises above 2.0V (typ),
the logic blocks begin operating, although reads and
writes at VCC levels below 3.0V are not recommended.
A second VCC comparator, the ADC UVLO comparator
prevents the ADC from converting until there is suffi-
cient headroom (VCC = 2.89V typ).
The software POR (SPOR) command can force a
power-on reset of the MAX6661 registers through the
serial interface. This can be done by writing F8h to the
MAX6661.
Power-up defaults include:
•Interrupt latch is cleared.
•ADC begins autoconverting.
•Command register is set to 00h to facilitate quick-
internal Receive Byte queries.
•THIGH and TLOW registers are set to +127°C and
-55°C, respectively.
•THYST and TMAX are set to +95°C and +100°C,
respectively.
Fan Control
The fan-control function can be divided into the thermal
loop, the fan-speed-regulation loop (fan loop), and the
fan-failure sensor. The thermal loop sets the desired fan
speed based on temperature while the fan-speed-regu-
lation loop uses an internally divided down reference
oscillator to regulate the fan speed. The fan-speed-reg-
ulation loop includes the fan driver and the tachometer
sensor. The fan-failure sensor provides a FAN FAIL
alarm that signals when the value in the fan tachometer
period register is greater than the fan tachometer peri-
od limit register value, which corresponds to a fan
going slower than the limit. The fan driver is an N-chan-
nel, 4ΩMOSFET with a 13.5V maximum VDS whose
drain terminal connects to the low side of the fan. The
tachometer sensor (TACH IN) of the MAX6661 is driven
from the tachometer output of the fan and provides the
feedback signal to the fan-speed regulation loop for
controlling the fan speed. For fans without tachometer
outputs, the MAX6661 can generate its own tachometer
pulses by monitoring the commutating current pulses
(see the Commutating Current Pulses section).
Thermal Loop
Thermal Closed Loop
The MAX6661 can be operated in a complete closed-
loop mode, with both the thermal and fan loops closed,
where the remote-diode sensor temperature directly
controls fan speed. Setting bit 3 of the configuration
register to zero places the MAX6661 in thermal closed
loop (Figure 6). The remote-diode temperature sensor
is updated every 500ms. The value is stored in a tem-
porary register (TEMPDATA) and compared to the pro-
grammed temperature values in the THIGH, TLOW,
THYST, TMAX, and TFAN registers to produce the error
outputs OVERT and ALERT.
The fan conversion rate (FCR) register (Table 6) can be
programmed to update the TEMPDATA register every
0.5s to 32s. This enables control over timing of the ther-
mal feedback loop to optimize stability.
The fan threshold (TFAN) register value is subtracted
from the UPDATE register value. If UPDATE exceeds
TFAN temperature, then the fan-speed control (FSC)
register (Table 7), stores the excess temperature in the
form of a 7-bit word with an LSB of 0.5°C. If the differ-
ence between the TFAN and UPDATE registers is high-
er than 32°C, then bits 6-0 are set to 1. In thermal
closed loop, the FSC register is READ ONLY.
The fan gain (FG) register (Table 8) determines the
number of bits used in the fan-speed control register.
This gain can be set to 4, 5, or 6. If bits 6 and 5 are set
to 10, all 6 bits of TEMPDATA are used directly to pro-
gram the speed of the fan so that the thermal loop has
a control range of 32°C with 64 temperature steps from
fan off to full fan speed. If bits 6 and 5 are set to 01, the
thermal control loop has a control range of 16°C with 32
temperature steps from fan off to full fan speed. If bits 6
and 5 are set to 00, the thermal control loop has a con-
trol range of 8°C with 16 temperature steps from fan off
to full fan speed.
Thermal Open Loop
Setting bit 3 of the configuration register (Table 4) to 1
places the MAX6661 in thermal open loop. In thermal
open-loop mode, the FSC register is read/write.
MAX6661
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
______________________________________________________________________________________ 13
MAX6661
In thermal open-loop mode, the fan loop can operate in
open or closed mode. In fan open loop, the FSC regis-
ter programs fan voltage directly, accepting values
from 0 to 64 (40h). For example, in fan open-loop
mode, zero corresponds to no voltage across the fan
and 40h corresponds to full fan voltage. Proportional
control is available over the 0 to 63 (3Fh) range with 64
(40h) forcing unconditional full speed.
In fan closed-loop mode, zero corresponds to zero fan
speed. When the FG register is set to 4 bits, 10h corre-
sponds to 100% fan speed; 100% fan speed is 20h at 5
bits, and 3Fh at 6 bits.
Fan Loop
The fan loop (Figure 7) is based on an up/down counter
where a reference clock representing the desired fan
speed drives the count up, while tachometer pulses
drive it down. The reference clock frequency is divided
down from the MAX6661 internal clock to a frequency
of 8415Hz. This clock frequency is further divided by
the fan full-scale (FS) register (Table 9), which is limited
to values between 127 to 255, for a range of reference
clock full-scale frequencies from 33Hz to 66Hz. A fur-
ther division is performed to set the actual desired fan
speed. This value appears in the fan-speed control reg-
ister in thermal closed-loop mode. If the thermal loop is
open, but the fan-speed control loop is closed, this
value is programmable in the FSC. When in fan open-
loop mode (which forces the thermal loop to open), the
FSC register becomes a true DAC, programming the
voltage across the fan from zero to nearly VFAN. The
tachometer input (TACH IN) includes a programmable
(1/2/4/8) prescaler. The divider ratio for the (1/2/4/8)
prescaler is stored in the fan tachometer divisor (FTD)
register (Table 10). In general, the values in FTD should
be set such that the full-speed fan frequency divided
by the prescaler fall in the 33Hz to 66Hz range.
The UP/DN counter has six stages that form the input of
a 6-bit resistive ladder DAC whose voltage is divided
down from VFAN. This DAC determines the voltage
applied to the fan. Internal coding is structured such
that when in fan closed-loop mode (which includes
thermal closed loop), higher values in the 0 to 32 range
correspond to higher fan speeds and greater voltage
across the fan. In fan open-loop mode (which forces
thermal open loop), acceptable values range from 0 to
63 (3Fh) for proportional control; a value of 64 (40h)
commands unconditional full speed.
Fan closed-loop mode is selected by setting bit 0 of the
FG to zero; open-loop mode is selected by setting bit 0
to 1. In open-loop mode, the gain block is bypassed
and the FSC register is used to program the fan voltage
rather than the fan speed. When in fan open-loop
mode, both the temperature feedback loop and fan-
speed control loop are broken, which result in the
TACH IN input becoming disabled. A direct voltage
can be applied to the fan after reading the temperature,
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
14 ______________________________________________________________________________________
DATA BINARY FAN UPDATE
RATE (Hz)
SECONDS
BETWEEN
UPDATES
00h 00000000 0.0625 16
01h 00000001 0.125 8
02h 00000010 0.25 4 (POR)
03h 00000011 0.5 2
04h 00000100 1 1
05h 00000101 2 0.5
06h 00000110 4 0.25
Table 6. Fan Conversion Update Rate
UPDATE
FCR
0.25s TO 16s
UPDATE
TEMP DATA FAN
CONVERSION
RATE
TACH IN FAN-SPEED
CONTROL
(FSC)
FAN
THRESHOLD
TEMPERATURE
(TFAN)
TEMPERATURE
CONVERTER
REMOTE
SENSOR
Figure 6. MAX6661 Thermal Loop
MAX6661
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
______________________________________________________________________________________ 15
REGISTER/ADDRESS FS (BFH = READ, C0H = WRITE)
COMMAND READ/WRITE MAXIMUM TEMPERATURE LIMIT BYTE
Label 7
MSB
6
Data Bit
5
Data Bit
4
Data Bit
3
Data Bit
2
Data Bit
1
Data Bit
0
Data Bit
POR State 11111111
Table 9. Fan Full-Scale Register (RFS/WFS)
Note: This register determines the maximum reference frequency at the input of the up/down counter. It controls a programmable
divider that can be set anywhere between 127 and 255. The value in this register must be set in accordance with the procedure
described in the TACH IN section (equivalent 8415/(Max Tachometer Frequency ✕Fan Tachometer Divisor)). Programmed value
below 127 defaults to 127. POR value is 255.
REGISTER/ADDRESS FTD (BBH = READ, BCH = WRITE)
COMMAND READ LIMIT/FAILURE REGISTER
Label 7
Not Used
6
Not Used
5
Not Used
4
Not Used
3
Not Used
2
Not Used
1
Divisor
Bit 1
0
Divisor
Bit 0
POR State 0 0 000001
Table 10. Fan Tachometer Divisor Register (RFTD/WFTD)
Note: This byte sets the prescalar division ratio for tachometer or current-sense feedback. (This register does not apply to the tach
signal used in the fan-speed register.) Select this value such that the fan frequency (rpm / 60s x number of poles) divided by the
FCD falls in the 33Hz to 66Hz range. See TACH IN section:
Bits 1, 0: 00 = divide by 1, 01 = divide by 2, 10 = divide by 4, 11 = divide by 8.
REGISTER/ADDRESS FSC (ABH = READ, B4H = WRITE)
COMMAND READ/WRITE FAN DAC REGISTER
Label 7
Not Used
6
Overflow Bit
5
(MSB)
4
Data
3
Data
2
Data
1
Data
0
Data
POR State 0 0 0 0 0 0 0 0
Table 7. Fan Speed Control Register (RFSC/WFSC)
REGISTER/ADDRESS FG (ADH = READ, B6H = WRITE)
COMMAND READ/WRITE FAN GAIN REGISTER
Label
7
Always
Write
a 1
6
Fan
Gain
5
Fan Gain
4
Always
Write
a 0
3
Always
Write
a 0
2
Always
Write
a 0
1
Fan
Driver
Mode Bit
0
Fan
Feedback
Mode
POR State 1 0 0 0 0 0 0 0
Table 8. Fan Gain Register (RFG/WFG)
Notes: Bit 0: Fan driver mode. When bit 0 is set to 1, the fan driver is in fan open-loop mode. In this mode, the fan DAC programs the
fan voltage rather than the fan speed. Tachometer feedback is ignored, and the user must consider minimum fan drive and startup
issues. Thermal open loop is automatically set to 1 (see configuration register). Fan Fail (bit 0 of the status register) is set to 1 in this
mode and should be ignored.
Bit 1: Fan feedback mode. When bit 1 is set to 1, the fan loop uses driver current sense rather than tachometer feedback.
Bits 6, 5: Fan gain of the fan loop, where 00 = 8°C with resolution = 4 bits. This means that the fan reaches its full-scale (maximum)
speed when there is an 8°C difference between the remote-diode temperature and the value stored in TFAN, 01 = +16°C, with a 5-bit
resolution and 10 = +32°C with a 6-bit resolution.
Bit 7: Writing a zero to bit 7 forces bits 6 and 5 to their POR values.
MAX6661
using the FSC register. By selecting fan open-loop
mode, the MAX6661 automatically invokes thermal
open-loop mode.
Fan Conversion Rate Register
The FCR register (Table 6) programs the fan’s update
time interval in free-running autonomous mode
(RUN/STOP = 0). The conversion rate byte’s POR state
is 02h (0.25Hz). The MAX6661 uses only the 3LSBs of
this register. The 5MSBs are don’t cares. The update
rate tolerance is ±25% (max) at any rate setting.
Fan Driver
The fan driver consists of an amplifier and low-side
NMOS power device whose drain is connected to FAN
and is the connection for the low side of the fan. There
is an internal connection from the fan to the input of the
amplifier. The FET has 4Ωon-resistance with 320mA
(typ) current limit. The driver has a thermal shutdown
sensor that senses the driver’s temperature. It shuts
down the driver if the temperature exceeds +150°C.
The driver is reactivated once the temperature has
dropped below +140°C.
TACH IN
The TACH IN input connects directly to the tachometer
output of a fan. Most commercially available fans have
two tachometer pulses per revolution. The tachometer
input is fully compatible with tachometer signals, which
are pulled up to VFAN.
Commutating Current Pulses
When a fan does not come equipped with a tachometer
output, the MAX6661 uses commutating generated cur-
rent pulses for speed detection. This mode is entered
by setting the FG register’s bit 1 to 1. An internal pulse
is generated whenever a step increase occurs in the
fan current. Connecting an external resistor between
the GAIN pin and VCC can reduce the sensitivity of
pulses to changes in fan current. In general, the lower
the resistor value, the lower the sensitivity, and the fan
is easier to turn ON and can use a smaller external
capacitor across its terminals. A suitable resistor range
is 1kΩto 5kΩ.
Fan-Failure Detection
The MAX6661 detects fan failure by comparing the
value in the fan tachometer period (FTP) register, a
READ ONLY register, with a limit stored in the fan
tachometer period limit (FTPL) register (Table 11). A
counter counts the number of on-chip oscillator pulses
between successive tachometer pulses and loads its
value into the FTP register every time a tachometer
pulse arrives. If the value in FTP is greater than the
value in FTPL, a failure is indicated. In fan closed loop,
a flag is activated when the fan is at full speed.
Set the fan tachometer period limit byte to:
fTACH = 8415 / [N ✕f]
where N = fan-fail ratio and fTACH = maximum frequen-
cy of the fan tachometer. The factor N is less than 1
and produces a fan-failure indication when the fan
should be running at full speed, but is only reaching a
factor of N of its expected frequency. The factor N is
typically set to 0.75 for all fan speeds except at very
low speeds where a fan failure is indicated by an over-
flow of the fan-speed counter. The overflow flag cannot
be viewed separately in the status byte but is ORed
with bit 0, the fan-fail bit.
Applications Information
Mode Register
Resistance in series with the remote-sensing junction
causes conversion errors on the order of 0.5°C per
ohm.
The MAX6661 mode register gives the ability to elimi-
nate the effects of external series resistance of up to
several hundred ohms on the remote temperature mea-
surement and to adjust the temperature-measuring
ADC to suit different types of remote-diode sensor. For
systems using external switches or long cables to con-
nect to the remote sensor, a parasitic resistance can-
cellation mode can be entered by setting mode register
bit 7 = 1. This mode requires a longer conversion time
and so can only be used for fan conversion rates of
1Hz or slower. Bits 6, 1, and 0 of the mode register are
not used. Use bits 5–2 to adjust the ADC gain to
achieve accurate temperature measurements with
diodes not included in the recommended list or to indi-
vidually calibrate the MAX6661 for use in specific con-
trol systems. These bits adjust gain to set the
temperature reading at +25°C, using two’s complement
format reading. Bit 5 is the sign (1 = increase, 0 =
decrease), bit 4 = 2°C shift, bit 3 = 1°C shift, bit 2 =
1/2°C shift. Origin of gain curve is referred to 0°K. To
use this feature, the sensor must be calibrated by the
user.
General Programming Techniques
The full-scale range of the fan-regulation loop is
designed to accommodate fans operating between the
1000rpm to 8000rpm range of different fans. An on-
chip 8415Hz oscillator is used to generate the 33Hz to
66Hz reference frequency. Choose the FTD value such
that the fan full-speed frequency divided by this value
falls in the 33Hz to 66Hz range. The full-scale reference
frequency is further divided by the value in the FSC
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
16 ______________________________________________________________________________________
register to the desired fan frequency [read: speed]. The
8415Hz is divided down from the MAX6661 internal
clock, and has a ±25°C tolerance.
1) Determine the fan’s maximum tachometer frequency:
f(TACH) Hz = (rpm/60s / min) ✕number of poles
Where poles = number of pulses per revolution.
Most fans are two poles; therefore, they have two
pulses per revolution.
2) Set the programmable FTD so that the frequency of
the fan tachometer divided by the prescaler value in
the FCD register falls in the 33Hz to 66Hz range.
3) Determine the value required for the fan FS register:
FS = 8415 / (fTACH ✕P)
Where P is the prescaler division ratio of the FCD
register.
Example: Fan A has a 2500rpm rating and 2 poles:
fTACH = 2500 / 60 ✕2 = 83.4Hz
MAX6661
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
______________________________________________________________________________________ 17
N
TEMP DATA
FAN GAIN (FG)
8°C, 16°C, 32°C
RANGE
FAN-SPEED
CONTROL
1 TO 63
REF FREQ
8415Hz
FAN FULL
SCALE (FS)
127 TO 255
COUNTER
FAN
TACHOMETER
PERIOD (FTP)
FAN
TACHOMETER
PERIOD LIMIT (FTPL)
COMPARATOR
FAN TACHOMETER
DIVISOR (FTD)
1, 2, 4, 8
FAN OPEN/CLOSED
LOOP
UP/DOWN
DRIVER
DAC
FAN FAIL
VFAN
TACH
FAN
TACH IN
Figure 7. MAX6661 Fan Loop Functional Diagram
REGISTER/ADDRESS FL (B1H = READ, B8H = WRITE)
COMMAND READ LIMIT/FAILURE REGISTER
Label 7
(MSB)
6
Data Bit
5
Data Bit
4
Data Bit
3
Data Bit
2
Data Bit
1
Data Bit
0
Data Bit
POR State 1 1 1 1 1 1 1 1
Table 11. Fan Tachometer Period Limit (RFTCL/WWFTCL)
Note: The fan tachometer period limit register is programmed with the maximum speed that is compared against the value in the FS
register to produce an error output to the status register.
MAX6661
The 83.4Hz value is out of the 33Hz to 66Hz decre-
ment/increment range.
Set bits in the FTD register to divide the signal down
within the 33Hz to 66Hz range. Bits 1, 0 = 10 (divide
by 2: P = 2). The result is 83.4Hz/2 = 41.7Hz.
4) Set the FS register to yield approximately 42Hz:
42Hz = 8415Hz / FS (value)
FS (value) ≈200
FS register = 11001000
5) In current-sense feedback, a pulse is generated
whenever there is a step increase in fan current. The
frequency of pulses is then not only determined by
the fan rpms and the number of poles, but also by
the update rate at which the fan driver forces an
increase in voltage across the fan.
The maximum pulse frequency is then given by:
fCHz = fTACH ✕P / (P-1)
Where f = (rpm/60) ✕poles and P is the value in
FTD.
The value required for the fan FS register is:
FS = 8415Hz / (fTACH / (P-1))
The fan speed limit in FTPL should be set to:
fL= 8415Hz / (N ✕fTACH)
A value of P = 1 cannot be used in current-sense
mode.
Fan Selection
For closed-loop operation and fan monitoring, the
MAX6661 requires fans with tachometer outputs. A
tachometer output is typically specified as an option on
many fan models from a variety of manufacturers. Verify
the nature of the tachometer output (open collector,
totem pole) and the resultant levels and configure the
connection to the MAX6661. For a fan with an open-
drain/collector output, a pullup resistor of typically 5kΩ
must be connected between TACH IN and VFAN. Note
how many pulses per revolution are generated by the
tachometer output (this varies from model to model and
among manufacturers, though two pulses per revolu-
tion are the most common). Table 12 lists the represen-
tative fan manufacturers and the model they make
available with tachometer outputs.
Low-Speed Operation
Brushless DC fans increase reliability by replacing
mechanical commutation with electronic commutation.
By lowering the voltage across the fan to reduce its
speed, the MAX6661 is also lowering the supply volt-
age for the electronic commutation and tachometer
electronics. If the voltage supplied to the fan is lowered
too far, the internal electronics may no longer function
properly. Some of the following symptoms are possible:
•The fan may stop spinning.
•The tachometer output may stop generating a signal.
•The tachometer output may generate more than two
pulses per revolution.
The problems that occur and the supply voltages at
which they occur depend on which fan is used. As a
rule of thumb, 12V fans can be expected to experience
problems somewhere around 1/4 and 1/2 their rated
speed.
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
18 ______________________________________________________________________________________
MANUFACTURER FAN MODEL OPTION
Comair Roton
All DC brushless models can be
ordered with optional tachometer
output.
EBM-Papst Tachometer output optional on
some models.
JMC Tachometer output optional.
NMB
All DC brushless models can be
ordered with optional tachometer
output.
Panasonic
Panaflo and flat unidirectional
miniature fans can be ordered
with tachometer output.
Sunon Tachometer output optional on
some models.
Table 12. Fan Manufacturers
Chip Information
TRANSISTOR COUNT: 6479
PROCESS: BiCMOS
MAX6661
Remote Temperature-Controlled Fan-Speed
Regulator with SPI-Compatible Interface
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 19
© 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
Package Information
QSOP.EPS
Pin Configuration
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
VFAN TACH IN
SDIN
SCL
GAIN
DOUT
ALERT
CS
OVERT
TOP VIEW
MAX6661
VCC
DXP
N.C.
DXN
FAN
PGND
AGND
QSOP
