MIC2583R-NBQS - Micrel, Inc.

April 2001 1 MIC2582/MIC2583

MIC2582/MIC2583 Micrel

MIC2582/MIC2583

Single Channel Hot Swap Controller

Preliminary Information

General Description

The MIC2582 and MIC2583 are single channel positive

voltage hot-swap controllers designed to allow the safe

insertion of boards into live system backplanes. Using few

external components, the parts act in conjunction with an

external N-Channel MOSFET device, for which the gate drive

is controlled to provide inrush current-limiting and output

voltage slew rate control. The MIC2582/3 are rated for

operation from supply voltages from 2.3V to 13.2V, but can

withstand transient voltages as high as 20V without damage.

Overcurrent fault protection is provided along with a program-

mable over-current threshold. Active current regulation dur-

ing start-up ensures that inrush current never exceeds the

programmed threshold. The MIC2582/3 provide a circuit

breaker function that latches the output MOSFET off if the

current limit threshold is exceeded for a programmed period

of time. Dual-level fault detection allows very fast response

to short-circuit faults while preventing lower level overcurrent

transients from causing “nuisance tripping” of the circuit

breaker. A /FAULT signal is provided indicating overcurrent

or undervoltage fault conditions.

Typical Application

SENSE

0.1µF

3.3V

Open-Drain

Output

Logic Level

Input

12.4k

OUT

0.1µF

100µF

VCC SENSE GATE

/POR

CPOR

MIC2582

0.1µF

GND

Logic V

MIC2582 Typical Application Circuit

Features

•Provides safe insertion and removal from live

backplanes

•2.3V to 13V supply voltage operation

•Surge voltage protection up to 20V

•Undervoltage Lockout protection

•Programmable inrush current-limiting

•Current regulation limits inrush current regardless of

load capacitance

•Dual-level overcurrent fault sensing eliminates false

tripping

•Fast response to short circuit conditions (< 1 µs)

•Power-On Reset and Power-Good status outputs

(MIC2583)

•Programmable output undervoltage detection

•Electronic circuit breaker

•/FAULT status output

•Auto-restart function (MIC2583R)

Applications

•RAID systems

•Base stations

•PC Board Hot-Swap insertion and removal

•Hot-Swap CompactPCI Cards

•Network Switches

Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com

RSENSE

0.1µF

VIN 12V

Logic

VCC

Logic Level

Input

12.4k

VOUT

0.1µFCL

100µF

VCC SENSE GATE

/FAULT

CFILTER

MIC2583

CFILTER

GND

CPOR

0.1µF

Open-Drain

Output

/PWRGD

/POR

DIS

Logic VCC

Power

Good

Power

Reset

10k R1

10k

MIC2583 Typical Application Circuit

MIC2582/MIC2583 Micrel

MIC2582/MIC2583 2 April 2001

Ordering Information

Part Number Circuit Breaker Threshold Circuit Breaker Package

MIC2582-xBM x = J, 50mV Latched off 8 pin SOIC

x = K, 100mV

x = L, 150mV

x = M, 200mV

x = N, Off

MIC2582/83-xBQS x = J, 50mV Latched off 16 pin QSOP

x = K, 100mV

x = L, 150mV

x = M, 200mV

x = N, Off

MIC2583R-xBQS x = J, 50mV Auto-retry 16 pin QSOP

x = K, 100mV

x = L, 150mV

x = M, 200mV

x = N, Off

Pin Configuration

1/POR

CPOR

GND

8 VCC

SENSE

GATE

8-Pin SOIC

1/POR

/PWRGD

CPOR

CFILTER

GND

16 VCC

SENSE

GATE

DIS

/FAULT

16-Pin QSOP

April 2001 3 MIC2582/MIC2583

MIC2582/MIC2583 Micrel

Pin Description

Pin Name SOIC QSOP Pin Function

/FAULT NA 11 Circuit Breaker Fault Output. Open-Drain, Active-Low. Asserted when the

circuit breaker is tripped.

ON 2 3 ON input. With this pin high, the device is enabled, a start-up sequence is

initiated, and the GATE pin starts ramping up towards its final operating

voltage. The ON input is compared to a VTH reference which has 25mV of

hysteresis.

Toggling ON is also used to reset the circuit breaker. ON must be low for

20µs in order to initiate a start-up sequence. This pin should not be taken

higher than VCC + 0.3V, or 7.5V, whichever is less.

/POR 1 1 Power-On Reset Output. Open-Drain. Asserted during start-up until a time

period tPOR after the Power-Good threshold is reached. Also asserted

during undervoltage lockout fault conditions. (CPOR determines time period

tPOR)

CPOR 3 4 Power-On-Reset Timer. A capacitor connected between this pin and ground

sets the Power-On-Reset interval, tPOR, and start-up delay tDELAY. When

VCC rises above the UVLO, the capacitor connected to CPOR begins to

charge. When the voltage at CPOR crosses 0.3V, a start cycle is initiated if

ON is asserted. CPOR is reset to zero volts.

When the voltage at FB rises above VFBTH, CPOR begins to charge again.

When the voltage at CPOR rises above VPORDLY, the timer resets, pulls

CPOR to ground, and /POR is de-asserted. If CPOR = 0, then tDELAY is set

to 20µs.

SENSE 7 15 Circuit Breaker Sense Input. A resistor between this pin and VCC sets the

current-limit threshold. When the current-limit threshold of 50mV is ex-

ceeded for tOCSLOW (internally set at 5µs for the MIC2582), the circuit

breaker is tripped and the GATE pin is immediately pulled low. If the voltage

across the sense resistor exceeds VTHSLOW due to fast high amplitude faults

such as short-circuits, then the GATE pin is immediately brought low (no

delay).

VTHSLOW is available for values of 50mV, 100mV, 200mV, or Off. Whenever

the voltage across the sense resistor exceeds the current-limit threshold, the

GATE voltage is adjusted to ensure a constant load current. To disable the

circuit breaker, the SENSE and VCC pins can be tied together.

GATE 6 14 Gate Drive Output. Connects to the gate of an N-Channel MOSFET. An

internal clamp ensures that no more than 8V is applied between the GATE

and Source. When the circuit breaker trips or when undervoltage lockout

occurs the GATE pin is immediately brought low.

FB 5 12 Power-Good Threshold Input. This input is internally compared to a 1.24V

reference which has 30mV of hysteresis. Whenever this input goes below

1.24V, /POR is asserted. A 5µs filter on this pin prevents negative-going

glitches from causing nuisance activation of this pin. If this input momentarily

goes below 1.24V, then /POR is activated for one timing cycle, tPOR.

VCC 8 16 Positive Supply Input. 2.3V - 13.2V. The GATE pin is held low by an internal

undervoltage lockout circuit until VCC exceeds a threshold of 2.1V. If VCC

exceeds 13.2V, an internal shunt regulator protects the chip from transient

voltages up to 20V at the VCC and SENSE pins.

CFILTER NA 5 Current-limit Response Timer. A capacitor connected to this pin defines the

period of time tOCSLOW in which an overcurrent event must last to signal a

fault condition and trip the circuit breaker. If no capacitor is connected to this

pin, then tOCSLOW is set to 5µs.

During auto-retry, the MIC2583R exhibits a preset duty cycle of 10%. The

value of CFILTER will set the interval between auto-retry attempts.

See Applications section.

MIC2582/MIC2583 Micrel

MIC2582/MIC2583 4 April 2001

Pin Name SOIC QSOP Pin Function

/PWRGD NA 2 Power-Good Output. Open Drain. When the voltage at the FB pin is lower

than 1.24V, /PWRGD output is held low. When the voltage at the FB pin is

higher than 1.24V, then /PWRGD is de-asserted. A pull-up resistor con-

nected to this pin and to VCC will pull the output up to VCC.

DIS NA 13 Discharge output. When the MIC2582/MIC2583 is turned off, a 500Ω resistor

connected to this output allows the discharging of external load capacitance

to ground.

GND 4 7,8 Ground connection

NC NA 6,9,10 No Connection

April 2001 5 MIC2582/MIC2583

MIC2582/MIC2583 Micrel

Absolute Maximum Ratings (Note 1)

All voltages are referred to GND

Supply Voltage (VCC) ..................................... –0.3V to 20V

/POR, /FAULT , /PWRGD pins....................... –0.3V to 15V

SENSE pin ............................................–0.3V to VCC+0.3V

ON pin .........-0.3V to Vcc+0.3V OR 8V, whichever is lower

GATE pin........................................................ –0.3V to 20V

FB input pins .................................................... –0.3V to 6V

Junction Temperature ............................................... 125°C

ESD Rating, Note 3

Operating Ratings (Note 2)

Supply Voltage (VCC) .................................... 2.3V to 13.2V

Thermal Resistance Rθ(J-A) (8-pin SOIC)..............163°C/W

Thermal Resistance Rθ(J-A) (16-pin SOIC).......... 112°C/W

Operating Temperature Range ..................–40°C to +85°C

Electrical Characteristics

VCC = 5.0V, TA = 25°C unless otherwise noted. Bold values indicate –40°C ≤ TA ≤ +85°C.

Symbol Parameter Condition Min Typ Max Units

VCC Supply voltage Temp = –20°C to +85°C 2.3 13.2 V

Temp = –40°C to +85°C2.7 13.2 V

ICC Supply current VON = 2V 1.6 2.5 mA

VTRIP Circuit breaker trip voltage VTRIP = VCC – VSENSE VTRIPSLOW 37 43 50 mV

VTRIPFAST 85 91 105 mV

VGS External Gate Drive |VCC – VGATE|V

CC > 3V 5.5 6.3 8 V

VCC = 2.3V 2.7 3.8 5 V

IGATEON GATE pin pull-up current Start cycle, VGATE = 0V

VCC =13.2V 10 16 24 µA

VCC = 2.3V 9 14 19 µA

IGATEOFF GATE pin sink current /FAULT = 0, VGATE>1V VCC = 13.2V 100 mA

VCC = 2.3V 50 mA

Turn off 110 µA

ITIMER Current-limit timer current VCC – VSENSE > VTRIPSLOW 4.5 6.5 8.5 µA

VCC – VSENSE < VTRIPSLOW –8.5 –6.5 –4.5 µA

IPOR Power-On-Reset timer current Timer On 12 14 16 µA

Timer Off 0.5 1.45 mA

VTH POR Delay and CFILTER timer VCPOR rising

threshold VCFILTER rising 1.19 1.24 1.29 V

VUV Undervoltage Lockout threshold VCC rising 2.1 2.2 2.3 V

VCC falling 1.9 2.05 2.2 V

VUVHYS Undervoltage Lockout hysteresis 175 mV

VONTH ON pin threshold voltage ON rising 1.19 1.23 1.29 V

ON falling 1.16 1.21 1.26 V

ION ON pin input current VON = VCC –0.5 µA

VONHYS ON pin hysteresis 25 mV

VDELAYTH Under-voltage start-up timer VCPOR rising .28 .33 .38 V

threshold

VAUTO Auto-restart threshold voltage Upper Threshold 1.24 V

Lower Threshold 0.33 V

MIC2582/MIC2583 Micrel

MIC2582/MIC2583 6 April 2001

Symbol Parameter Condition Min Typ Max Unit

IAUTO Auto-restart current charge current 12 µA

discharge current 1.2 µA

VFBTH Power-Good threshold voltage FB rising 1.19 1.24 1.29 V

FB falling 1.16 1.21 1.26 V

VFBHYS FB hysteresis 30 mV

VOL /FAULT , /POR , /PWRGD output IOUT = 1mA 0.4 V

voltage

RDIS Output discharge resistance 500 1000 Ω

AC Parameters

tOCFAST Fast overcurrent SENSE to GATE VCC = 5V 1 µs

low trip time VCC - VSENSE = 100mV

CGATE = 10nF

Figure 1

tOCSLOW Slow overcurrent SENSE to GATE VCC = 5V 5 µs

low trip time CTIM = 0, VIN - VSENSE = 50mV

Figure 1

tONDLY ON delay filter 20 µs

tFBDLY FB delay filter 20 µs

Note 1. Exceeding the absolute maximum rating may damage the device.

Note 2. The device is not guaranteed to function outside its operating rating.

Note 3. Devices are ESD sensitive. Handling precautions recommended.

Timing Diagrams

GATE

(VCC – VSENSE)

VTRIPFAST

50mV

tOCSLOW tOCFAST

Figure 1. Current -Limit Rsponse

/POR

/PWRGD

1.5V

1.2V

POR

Figure 2. Power-on Reset Response

GATE

UVLO

DELAY

Figure 3. Power-on Start-up Delay Timing

April 2001 7 MIC2582/MIC2583

MIC2582/MIC2583 Micrel

Functional Description

Functional Overview

When ON is asserted high, the device is enabled and a start-

up sequence is initiated. The GATE begins to ramp up at a

rate determined by CGATE. When the FB pin exceeds the

internal 1.24V threshold reference, /PWRGD is de-asserted,

signaling an available output. Internal current regulation

ensures that the inrush current will not exceed the pro-

grammed threshold. A circuit breaker function will latch the

output MOSFET off if the current limit threshold is exceeded

for a period, tOCSLOW, determined by CFILTER (tOCSLOW = 5µs

with no CFILTER).

ON Pin (Maximum Voltage)

The ON pin for the MIC2582/3 has an absolute maximum

voltage rating of (VCC + 0.3V) or 8V, whichever is less. At

operating voltages of 2.3 to 7.5V, the ON pin may be con-

nected directly to the VCC pin through a pull-up resistor

(100kΩ suggested). When operating at voltages between 7.5

and 13.2V with ON tied to VCC, it is necessary to include a

voltage divider from the ON pin to ground to keep the ON pin

from exceeding the absolute maximum rating of 8V. A 100kΩ

resistor is also recommended for the divider.

Start-Up Cycle

There are two basic modes of operation that may occur

during start-up with the MIC2582/3.

Load Capacitance Dominated Start-up

In this case the load capacitance, CL, is large enough to

cause the inrush current to exceed the programmed current-

limit but is less than the fast-trip threshold (or the fast-trip

threshold is disabled, ‘N’ option). When this occurs the output

current is regulated and held constant until the output voltage

rises to its final value. The output rise-time is computed by the

following equation:

OutputRiseTime IC

LIMIT

dv/dt

In this case the value of CFILTER must be selected to ensure

the overcurrent response time, tOCSLOW exceeds this value

to prevent the circuit-breaker from tripping.

GATE Capacitance Dominated Start-up

In this case the value of the load capacitance relative to the

GATE capacitance is small enough such that during start-up

the output current never exceeds the current-limit threshold.

The minimum value of CG that will ensure that the current-

limit is never exceeded is given by the equation below:

C (Min.) I

GGATE

LIMIT L

=×

Where CG is the summation of the MOSFET input capaci-

tance (CISS) specification and the value of the capacitor

connected to the GATE pin of the MOSFET, CGATE. CL is the

load capacitance connected to the output and ILIMIT and

IGATE are respectively the current limit and gate charge

current specifications found in the Electrical Characteristics

Table. Once CG is determined use the equation below to

determine the output slew rate

dv/dt

(load) IC

GATE

Table 1 depicts the output slew rate for various values of CG.

IGATE = 16µA

CGdv/dt (load)

0.001µF 16000V/s

0.01µF 1600V/s

0.1µF 1600V/s

1µF 16V/s

Table 1. Output Slew-Rate Selection for GATE

Capacitance Dominated Start-Up

Supply Bypass

For local supply bypass, a 0.1µF ceramic capacitor is recom-

mended

Voltage Divider at FB Pin

There are two important points to note here. First, consider-

ation should be taken to determine the desired output value

for MIC2582/3 turn on. For example, if the application is

operating at 12V, the desired turn on may be 11V. The two FB

resistors are selected to divide 11V accordingly, with 1.24V

set at the FB pin. Second, the ratio of these two resistors

should be chosen to maintain relatively low power consump-

tion while maintaining good accuracy in the face of input bias

currents. A current of approximately 100µA is recommended.

The following example determines the resistor values for the

output voltage divider at the FB pin.

RVRR

OUT turn on FB

() .

−=+









=+











2124 12

12VOUT ±5% (supply tolerance) gives 11.4V to 12.6V output

range.

Select 11V output turn-on.

Choose

RVAV

2100 124

100 12 4 1=µ=µ=Ω

..,%

Find R1

RR V

OUT

12124









=.

VOUT R2 = 1.24V(R1 + R2). Simplifying,

Rkk

111 12 4 1 24 12 4

124

=×

()

−×

()

...

R1 = 97.6kΩ, also a standard 1% resistor value.

Power-On-Reset and Overcurrent Filter Delays

The Power-On-Reset delay, tPOR, is the period of time for

/POR to de-assert after the Power-Good threshold is reached.

The value of capacitor CPOR will determine tPOR. The follow-

ing equation is used to calculate tPOR:

tCV

POR POR TH

POR

=×

MIC2582/MIC2583 Micrel

MIC2582/MIC2583 8 April 2001

where VTH is the typical power-on reset delay threshold and

IPOR is the typical power-on reset timer current.

Capacitor CFILTER is utilized as part of the MIC2582/83 dual-

level overcurrent fault detection. Overcurrent conditions which

last longer than time period tOCSLOW will trip the circuit

breaker. The following equation is used to calculate tOCSLOW:

tCV

OCSLOW FILTER TH

TIMER

=×

where VTH is the CFILTER timer threshold and ITIMER is the

typical current-limit timer current.

April 2001 9 MIC2582/MIC2583

MIC2582/MIC2583 Micrel

Applications Information

Sense Resistor Selection

The MIC2582, MIC2583 and MIC2583R use a low-value

sense resistor to measure the current flowing through the

MOSFET switch (and therefore the load). This sense resistor

is nominally valued at 43mV/ILOAD(CONT). To accommodate

worst-case tolerances for both the sense resistor (allow ±3%

over time and temperature for a resistor with ±1% initial

tolerance) and still supply the maximum required steady-

state load current, a slightly more detailed calculation must be

used.

The current limit threshold voltage (the “trip point”) for the

MIC2582/3 may be as low as 37mV, which would equate to

a sense resistor value of 37mV/ILOAD(CONT). Carrying the

numbers through for the case where the value of the sense

resistor is 3% high, this yields RSENSE(MAX) =

37mV/(1.03)(ILOAD(CONT)) = 35.9mV/ILOAD(CONT).

Once the value of RSENSE has been chosen in this manner,

it is good practice to check the maximum ILOAD(CONT) which

the circuit may let through in the case of tolerance build-up in

the opposite direction. Here, the worst-case maximum is

found using a 53mV trip voltage and a sense resistor that is

3% low in value. The resulting current is ILOAD(CONT,

MAX) = 53mV/(0.97)(RSENSE(NOM)) = 51.5mV/(RSENSE(NOM)).

As an example, if an output must carry a continuous 1.4A

without nuisance trips occurring, RSENSE for that output

should be 35.9mV/1.4A = 25.64mΩ. The nearest standard

value is 25.0mΩ, so a 25.0mΩ ±1% resistor would be a good

choice. At the other set of tolerance extremes, ILOAD(CONT,

MAX) for the output in question is then simply 51.5mV/25.0mΩ

= 2.06A. Knowing this final datum, we can determine the

necessary wattage of the sense resistor, using P = I2R, where

I will be ILOAD(CONT, MAX), and R will be (0.97)(RSENSE(NOM)).

These numbers yield the following: PMAX = (2.06A2)(24.25mΩ)

= 0.103W. In this example, a 0.25W sense resistor would

work well.

MOSFET Selection

Selecting the proper external MOSFET for use with the

MIC2582/3 involves three straightforward tasks:

•Choice of a MOSFET which meets minimum

voltage requirements

•Selection of a device to handle the maximum

continuous current (steady-state thermal issues)

•Verify the selected part’s ability to withstand any

peak currents (transient thermal issues)

MOSFET Voltage Requirements

The first voltage requirement for the MOSFET is easily

stated: the drain-source breakdown voltage of the MOSFET

must be greater than VIN(MAX). For instance, a 12V input may

reasonably be expected to see high-frequency transients as

high as 15V. Therefore, the drain-source breakdown voltage

of the MOSFET must be at least 16V. For ample safety

margin and standard availability, the closest value will be

20V.

The second breakdown voltage criterion that must be met is

a bit subtler than simple drain-source breakdown voltage, but

is not hard to meet. In MIC2582/3 applications, the gate of the

external MOSFET is driven up to approximately 19.5V by the

internal output MOSFET (again, assuming 12V operation).

At the same time, if the output of the external MOSFET (its

source) is suddenly subjected to a short, the gate-source

voltage will go to (19.5V – 0V) = 19.5V. This means that the

external MOSFET must be chosen to have a gate-source

breakdown voltage of 20V or more, which is an available

standard maximum value. However, if operation is at or

above 13V, the 20V gate-source maximum will likely be

exceeded. As a result, an external Zener diode clamp should

be used to prevent breakdown of the external MOSFET when

operating at voltages above 8V. A Zener diode with 10V

rating is recommended as shown in Figure 4. At the present

VCC SENSE

SENSE

GATE

0.1µF

/POR

2.3V to 13.2V

CPOR

MIC2582

0.1µF

GND

Open-Drain

Output

Logic

Level Input

12.4k

OUT

1N5240B

10V

47Ω

Figure 4.

MIC2582/MIC2583 Micrel

MIC2582/MIC2583 10 April 2001

time, most power MOSFETs with a 20V gate-source voltage

rating have a 30V drain-source breakdown rating or higher.

As a general tip, look to surface-mount devices with a drain-

source rating of 30V as a starting point.

Finally, the external gate drive of the MIC2582/3 requires a

low-voltage logic level MOSFET when operating at voltages

lower than 3V. There are 2.5V logic level MOSFETs available

(See

MOSFET and Sense Resistor Vendors

for suggested

manufacturers).

MOSFET Steady-State Thermal Issues

The selection of a MOSFET to meet the maximum continuous

current is a fairly straightforward exercise. First, arm yourself

with the following data:

•The value of ILOAD(CONT, MAX.) for the output in

question (see

Sense Resistor Selection

)

•The manufacturer’s data sheet for the candidate

MOSFET

•The maximum ambient temperature in which the

device will be required to operate

•Any knowledge you can get about the heat

sinking available to the device (e.g., can heat be

dissipated into the ground plane or power plane,

if using a surface-mount part? Is any airflow

available?)

Now it gets easy. The data sheet will almost always give a

value of on resistance given for the MOSFET at a gate-source

voltage of 4.5V, and another value at a gate-source voltage

of 10V. As a first approximation, add the two values together

and divide by two to get the on resistance of the part with 7V

of enhancement. This is conservative, but it works. Call this

value RON. Since a heavily enhanced MOSFET acts as an

ohmic (resistive) device, almost all that’s required to deter-

mine steady-state power dissipation is to calculate I2R. The

one addendum to this is that MOSFETs have a slight increase

in RON with increasing die temperature. A good approxima-

tion for this value is 0.5% increase in RON per °C rise in

junction temperature above the point at which RON was

initially specified by the manufacturer. For instance, if the

selected MOSFET has a calculated RON of 10mΩ at a 25°C

TJ, and the actual junction temperature ends up at 110°C, a

good first cut at the operating value for RON would be:

10mΩ[1+ (110 - 25)(0.005)(10mΩ)] =

10mΩ[1 + (85)(0.005)(10mΩ)] ≅14.3mΩ

When performing this calculation, be sure to use the highest

anticipated ambient temperature (TA(MAX)) in which the

MOSFET will be operating as the starting temperature, and

find the operating junction temperature increase (∆TJ) from

that point. Then, as shown above, the final junction tempera-

ture is found by adding TA(MAX) and ∆TJ. Since this is not a

closed-form equation, getting a close approximation may

take one or two iterations, But it’s not a hard calculation to

perform, and tends to converge quickly.

The final step is to make sure that the heat sinking available

to the MOSFET is capable of dissipating at least as much

power (rated in °C/W) as that with which the MOSFET’s

performance was specified by the manufacturer. As a prac-

tical issue, surface-mount MOSFETs are often less than

ideally specified in this regard – it’s become common for

manufacturers to simply state that the thermal data for the

part is specified with the MOSFET “Surface mounted on FR-

4 board, t ≤10seconds,” or something equally uninformative.

So here are a few practical tips:

1. The heat from a surface-mount device such as

an SO-8 MOSFET flows almost entirely out of

the drain leads. If the drain leads can be sol-

dered down to one square inch or more of

copper the copper will act as the heat sink for

the part. This copper must be on the same layer

of the board as the MOSFET drain.

2. Airflow, if available, works wonders. This is not

the place for a dissertation on how to perform

airflow calculations, but even a few LFM (linear

feet per minute) of air will cool a MOSFET down

dramatically. If you can position the MOSFET(s)

in question near the inlet of a power supply’s

fan, or the outlet of a processor’s cooling fan,

that’s always a good free ride.

3. Although it seems a rather unsatisfactory

statement, the best test of a surface-mount

MOSFET for an application (assuming the

above tips show it to be a likely fit) is an empiri-

cal one. Check the MOSFET's temperature in

the actual layout of the expected final circuit, at

full operating current. The use of a thermocouple

on the drain leads, or infrared pyrometer on the

package, will then give a reasonable idea of the

device’s junction temperature.

4. Finally, you may end up noting the following:

modern surface-mount MOSFETs are readily

and inexpensively available with such low values

of RON that the finer points mentioned above are

often almost moot.

MOSFET Transient Thermal Issues

Having chosen a MOSFET that will withstand the imposed

voltage stresses, and the worst-case continuous I2R power

dissipation which it will see, it remains only to verify the

MOSFET’s ability to handle short-term overload power dissi-

pation without overheating. Here, nature and physics work in

our favor: a MOSFET can handle a much higher pulsed

power without damage than its continuous dissipation ratings

would imply. The reason for this is that, like everything else,

thermal devices (silicon die, lead frames, etc.) have thermal

inertia. This is very easily understood by all of us who have

stood waiting for a pot of water to boil.

In terms related directly to the specification and use of power

MOSFETs, this is known as “transient thermal impedance,”

or Zθ(J-A). Almost all power MOSFET data sheets give a

Transient Thermal Impedance Curve, which is a handy tool

for making sure that you can safely get by with a less

expensive MOSFET than you thought you might need. For

example, take the following case: VIN = 12V, TFLT has been

set to 100msec, ILOAD(CONT. MAX) is 1.4A, the slow-trip

threshold is 43mV nominal, and the fast-trip threshold is

100mV. If the output is accidentally connected to an 6Ω load,

the output current from the MOSFET will be regulated to 1.4A

April 2001 11 MIC2582/MIC2583

MIC2582/MIC2583 Micrel

for 100ms (TFLT) before the part trips. During that time, the

dissipation in the MOSFET is given by:

P = E x I EMOSFET = [12V-(1.4A)(4Ω)] = 6.4V

PMOSFET = (6.4V x 1.4A) = 9W for 100msec.

Wow! Looks like we need a really hefty MOSFET to withstand

this sort of fault condition. Or do we? This is where the

transient thermal impedance curves become very useful.

Figure 5 shows the curve for the Vishay (Siliconix) Si4410DY,

a commonly used SO-8 power MOSFET:

Reading this graph is not nearly as daunting as it may at first

appear. Taking the simplest case first, we’ll assume that once

a fault event such as the one in question occurs, it will be a

long time – 10 minutes or more - before the fault is isolated

and the channel is reset. In such a case, we can approximate

this as a “single pulse” event, that is to say, there’s no

significant duty cycle (a vanishingly small repetition rate).

Then, reading up from the X-axis at the point where “Square

Wave Pulse Duration” is equal to 0.1sec (=100msec), we see

that the Zθ(J-A) of this MOSFET to a highly infrequent event of

this duration is only 8% of its continuous Rθ(J-A).

This particular part is specified as having an Rθ(J-A) of 50°C/

W for intervals of 10 seconds or less. So, some further math,

just to get things ready for the finale:

Normalized Thermal Transient Impedance, Junction-to-Ambient

Square Wave Pulse Duration (sec)

0.1

0.01 10

—4

—3

—2

—1

110

Normalized Effective T ransient

Thermal Impedance

0.2

0.1

0.05

0.02

Single Pulse

Duty Cycle = 0.5

1. Duty Cycle, D =

2. Per Unit Base = R

thJA

= 50 C/W

3. T

— T

= P

thJA(t)

Notes:

4. Surface Mounted

Figure 5. Transient Thermal Impedance

Assume TA = 55°C maximum, 1 square inch of copper at the

drain leads, no airflow.

The part has an RON of (0.0335(/2) = 17mΩ at 25°C.

Assume it has been carrying just about 1.4A for some time.

Then the starting (steady-state)TJ is:

TJ ≅TA + [17mΩ + (55°C-25°C)(0.005)(17mΩ)] x

(1.4A2) x (50°C/W)

TJ ≅ ( 55°C + (0.0383W)(50°C/W) ≅56.9°C

Iterate the calculation once to see if this value is within a few

percent of the expected final value. For this iteration we will

start with TJ equal to the already calculated value of 57°C:

TJ ≅TA + [17mΩ + (57°C-25°C)(0.005)(17mΩ)] x

(1.4A2) x (50°C/W)

TJ ≅ ( 55°C + (0.0383W)(50(C°W) ≅ 56.9°C

So our original approximation of 56.9°C was very close to the

correct value. We will use TJ = 57°C, which is close enough

for all practical purposes.

Finally, add (9W)(50°C/W)(0.08) = 36°C to the steady-state

TJ to get TJ(TRANSIENT MAX.) =93°C. This is a completely

acceptable maximum junction temperature for this part.

MIC2582/MIC2583 Micrel

MIC2582/MIC2583 12 April 2001

A final illustration of the use of the transient thermal imped-

ance curves: assume that we are using an MIC2583R, which

will auto-retry at a 10% dusty cycle into the same fault, with

a one second interval between retry attempts. This frequency

of restarts will significantly increase the dissipation in the

Si4410DY MOSFET. Will the MOSFET be able to handle the

increased dissipation? We get the following:

The same part is operating into a persistent fault, so it is

cycling in a square-wave fashion (no steady-state load) with

a duty cycle of 10% = 0.1.

From the Transient Thermal Impedance Curves, reading

up from the X-axis to the line showing a duty cycle of 0.10,

we get

Rθ(J-A) = (0.16 x 50°C/W) = 8°C/W.

Calculating the peak junction temperature:

TJ(MAX) = [(9W)(8°C/W) + 55°C] = 127°C.

MOSFET Vendors Key MOSFET Type(s) Contact Information

Vishay (Siliconix) Si4420DY (SO-8 package) www.siliconix.com

Si4420DY (SO-8 package) (203) 452-5664

Si3442DV (for VCC < 3V) (SO-8 package)

International Rectifier IRF7413A (SO-8 package) www.irf.com

Si4420DY (second source to Vishay) (310) 322-3331

IRF7601 (for VCC < 3V) (SO-8 package)

Fairchild Semiconductor FDS6880A (SO-8 package) www.fairchildsemi.com

(207) 775-8100

Resistor Vendors Sense Resistors Contact Information

Vishay (Dale) “WSL” Series www.vishay.com/docswsl_30100.pdf

(203) 452-5664

IRC “OARS” Series www.irctt.com/pdf_files/OARS.pdf

“LR” Series www.irctt.com/pdf_files/LRC.pdf

(second source to “WSL”) (828) 264-8861

This is a bit marginal, but will be acceptable under fault

conditions of limited duration.

And finally, checking the RMS power dissipation just to be

complete:

PRMS =

[17mΩ+(127°C-25°C)(0.005)(17mΩ)](1.4A2)

01.

= 0.016W,

which will result in a negligible temperature rise. The

Si4410DY is suitable for this application.

MOSFET and Sense Resistor Vendors

The simplest way to address the issues of MOSFET and

Sense Resistor selection is to give the names and web

addresses of several major vendors of suitable parts. At the

same time, it’s quite possible to mention by type number

some of the more popular MOSFET and resistor types used

in the industry, which will constitute a good starting point for

most designs.

April 2001 13 MIC2582/MIC2583

MIC2582/MIC2583 Micrel

PCB Layout Considerations

Because of the low values of the sense resistors, special care

must be used to accurately measure the voltage drop across

them. Specifically, the voltage across RSENSE must be mea-

sured using Kelvin sensing, which is simply a means of

making sure that any voltage drops in the power traces

connecting to the resistors are not picked up by the traces

measuring the voltages across the sense resistors them-

selves. If accuracy must be paid for, it’s worth keeping.

Figure 6 (below) illustrates how Kelvin sensing is performed,

with the Kelvin connections between the VCC and SENSE

pins as shown. These Kelvin connection traces do not need

significant width as only signal currents will be flowing through

them. As can be seen, all the high current in the circuit flows

directly through the power PCB traces and RSENSE. The

voltage drop resulting across RSENSE is sampled in such a

way that the high currents through the power traces connect-

ing to the resistor will not introduce any extraneous IR drops.

Tempting though it may become in some layouts,

do not

tap

the sense voltages off of the power traces until all of the

current being measured will absolutely have made its way

into the sense resistor.

Finally, to minimize IR drops between the input and the load,

the PCB power traces should be as short as possible, and

should be wide enough to carry the maximum required

current with 10°C ~ 20°C maximum temperature rise.

SENSE

POWER TRACE

FROM VCC POWER TRACE

TO MOSFET DRAIN

Signal trace

to MIC2583 SENSE

Signal Trace

to MIC2583 VCC

Figure 6. Kelvin Sensing Connetions for RSENSE

MIC2582/MIC2583 Micrel

MIC2582/MIC2583 14 April 2001

Package Information

45°

0°–8°

0.244 (6.20)

0.228 (5.79)

0.197 (5.0)

0.189 (4.8) SEATING

PLANE

0.026 (0.65)

MAX)

0.010 (0.25)

0.007 (0.18)

0.064 (1.63)

0.045 (1.14)

0.0098 (0.249)

0.0040 (0.102)

0.020 (0.51)

0.013 (0.33)

0.157 (3.99)

0.150 (3.81)

0.050 (1.27)

TYP

PIN 1

DIMENSIONS:

INCHES (MM)

0.050 (1.27)

0.016 (0.40)

45°

0.2284 (5.801)

0.2240 (5.690)

SEATING

PLANE

0.009 (0.2286)

REF 0.012 (0.30)

0.008 (0.20)

0.157 (3.99)

0.150 (3.81)

0.050 (1.27)

0.016 (0.40)

0.0688 (1.748)

0.0532 (1.351)

0.196 (4.98)

0.189 (4.80)

0.025 (0.635)

BSC

PIN 1

DIMENSIONS:

INCHES (MM)

0.0098 (0.249)

0.0040 (0.102) 0.0098 (0.249)

0.0075 (0.190) 8°

0°

8-Pin SOP (M)

16-pin QSOP (QS)

April 2001 15 MIC2582/MIC2583

MIC2582/MIC2583 Micrel

MIC2582/MIC2583 16 April 2001

MICREL INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131 USA

TEL + 1 (408) 944-0800 FAX + 1 (408) 944-0970 WEB http://www.micrel.com

This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or

other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc.