January 2004

M0235-121903

MIC2085/2086

Micrel

MIC2085/MIC2086

Single Channel Hot Swap Controllers

General Description

The MIC2085 and MIC2086 are single channel positive
voltage hot swap controllers designed to allow the safe
insertion of boards into live system backplanes. The MIC2085
and MIC2086 are available in 16-pin and 20-pin QSOP
packages, respectively. Using a few external components
and by controlling the gate drive of an external N-Channel
MOSFET device, the MIC2085/86 provide inrush current
limiting and output voltage slew rate control in harsh, critical
power supply environments. Additionally, a circuit breaker
function will latch the output MOSFET off if the current limit
threshold is exceeded for a programmed period of time. The
devices' array of features provide a simplified yet robust
solution for many network applications in meeting the power
supply regulation requirements and affords protection of
critical downstream devices and components.

All support documentation can be found on Micrel's web
site at www.micrel.com.

Typical Application

GND

/FAULT

8200pF

0.1

SENSE

VCC

/FAULT

CPOR

COMP--

/POR

COMPOUT

REF

GATE

COMP+

GND

CRWBR

CFILTER

0.1

Long

Backplane

Connector

PCB Edge

Connector

Long

Overvoltage (Input) = 13.3V
Undervoltage Lockout = 10.8V
Undervoltage (Output) &
Power-Good (Output) = 11.4V

Short

Medium

(or Short)

3.3

SENSE

0.007

Si7884DP

(PowerPAK

SO-8)

*R6

1.82k

16.2k

100k

47k

10k

MIC2085

0.022

0.01

0.033

LOAD

220

127k

PWRGD

/RESET

Power-On Reset

Output

Output Signal

(Power Good)

**R9

180

TCR22-4

2N4401

OUT

12V@5A

12V

LOGIC

CONTROLLER

POR/START-UP DELAY = 60ms
Circuit-Breaker Response Time = 500

*R6 is an optional component used for noise filtering
**R9 needed when using a sensitive gate SCR

R11

47k

R10

47k

LOGIC

Features

· MIC2085: Pin for pin functional equivalent to the

LTC1642

· 2.3V to 16.5V supply voltage operation
· Surge voltage protection to 33V
· Operating temperature range 40

C to 85

· Active current regulation limits inrush current

independent of load capacitance

· Programmable inrush current limiting
· Analog foldback current limiting
· Electronic circuit breaker
· Dual-level overcurrent fault sensing
· Fast response to short circuit conditions (< 1

· Programmable output undervoltage detection
· Undervoltage lockout protection
· Power-on reset (MIC2085/86) and

power-good (MIC2086) status outputs

· /FAULT status output
· Driver for SCR crowbar on overvoltage

Applications

· RAID systems
· Cellular base stations
· LAN servers
· WAN servers
· InfiniBandTM Systems
· Industrial high side switching

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

InfiniBand is a trademark of InfiniBand Trade Association
PowerPAK is a trademark of Vishay Intertechnology Inc.

MIC2085/2086

Micrel

M0235-121903

January 2004

Pin Description

Pin Number

Pin Name

Pin Function

MIC2086

MIC2085

CRWBR

Overvoltage Timer and Crowbar Circuit Trigger: A capacitor connected to
this pin sets the timer duration for which an overvoltage condition will trigger
an external crowbar circuit. This timer begins when the OV input rises above
its threshold as an internal 45

A current source charges the capacitor. Once

the voltage reaches 470mV, the current increases to 1.5mA.

CFILTER

Current Limit Response Timer: A capacitor connected to this pin defines the
period of time (t

OCSLOW

) in which an overcurrent event must last to signal a

fault condition and trip the circuit breaker. If no capacitor is connected, then
t

OCSLOW

defaults to 5

CPOR

Power-On Reset Timer: A capacitor connected between this pin and ground
sets the start-up delay (t

START

) and the power-on reset interval (t

POR

). When

VCC rises above the UVLO threshold, the capacitor connected to CPOR
begins to charge. When the voltage at CPOR crosses 1.24V, the start-up
threshold (V

START

), a start cycle is initiated if ON is asserted while capacitor

POR

is immediately discharged to ground. When the voltage at FB rises

above V

, capacitor C

POR

begins to charge again. When the voltage at

CPOR rises above the power-on reset delay threshold (V

), the timer

resets by pulling CPOR to ground, and /POR is deasserted.
If C

POR

= 0, then t

START

defaults to 20

Pin Configuration

CRWBR

CFILTER

CPOR

/POR

/FAULT

GND

16 VCC

SENSE

GATE

REF

COMP

COMP+

COMPOUT

MIC2085

16-Pin QSOP (QS)

Ordering Information

Part Number

Fast Circuit Breaker Threshold

Discharge Output

Package

MIC2085-xBQS

x = J, 95mV

16-pin QSOP

x = K, 150mV*

x = L, 200mV*

x = M, Off

MIC2086-xBQS

x = J, 95mV

Yes

20-pin QSOP

x = K, 150mV*

x = L, 200mV*

x = M, Off

*Contact factory for availability.

CRWBR

CFILTER

CPOR

/POR

PWRGD

/FAULT

GND

20 VCC

VCC

SENSE

GATE

REF

DIS

COMP

COMP+

COMPOUT

MIC2086

20-Pin QSOP (QS)

January 2004

M0235-121903

MIC2085/2086

Micrel

Pin Description (Cont.)

Pin Number

Pin Name

Pin Function

MIC2086

MIC2085

ON Input: Active high. The ON pin, an input to a Schmitt-triggered compara-
tor used to enable/disable the controller, is compared to a V

reference

with 100mV of hysteresis. Once a logic high is applied to the ON pin
(V

> 1.24V), a start-up sequence is initiated as the GATE pin starts

ramping up towards its final operating voltage. When the ON pin receives a
low logic signal (V

< 1.14V), the GATE pin is grounded and /FAULT is

high if VCC is above the UVLO threshold. ON must be low for at least 20

in order to initiate a start-up sequence. Additionally, toggling the ON pin
LOW to HIGH resets the circuit breaker.

/POR

Power-On Reset Output: Open drain N-Channel device, active low. This pin
remains asserted during start-up until a time period t

POR

after the FB pin

voltage rises above the power-good threshold (V

). The timing capacitor

POR

determines t

POR

. When an output undervoltage condition is detected

at the FB pin, /POR is asserted for a minimum of one timing cycle, t

POR

. The

/POR pin has a weak pull-up to VCC.

N/A

PWRGD

Power-Good Output: Open drain N-Channel device, active high. When the
voltage at the FB pin is lower than 1.24V, the PWRGD output is held low.
When the voltage at the FB pin is higher than 1.24V, then PWRGD is
asserted. A pull-up resistor connected to this pin and to VCC will pull the
output up to VCC. The PWRGD pin has a weak pull-up to VCC.

/FAULT

Circuit Breaker Fault Status Output: Open drain N-Channel device, active
low. The /FAULT pin is asserted when the circuit breaker trips due to an
overcurrent condition. Also, this pin indicates undervoltage lockout and
overvoltage fault conditions. The /FAULT pin has a weak pull-up to VCC.

Power-Good Threshold Input: This input is internally compared to a 1.24V
reference with 3mV of hysteresis. An external resistive divider may be used
to set the voltage at this pin. If this input momentarily goes below 1.24V,
then /POR is activated for one timing cycle, t

POR

, indicating an output

undervoltage condition. The /POR signal de-asserts one timing cycle after
the FB pin exceeds the power-good threshold by 3mV. A 5

s filter on this pin

prevents glitches from inadvertently activating this signal.

9,10

GND

Ground Connection: Tie to analog ground.

OV Input: When the voltage on OV exceeds its trip threshold, the GATE pin
is pulled low and the CRWBR timer starts. If OV remains above its threshold
long enough for CRWBR to reach its trip threshold, the circuit breaker is
tripped. Otherwise, the GATE pin begins to ramp up one POR timing cycle
after OV drops below its trip threshold.

COMPOUT

Uncommitted Comparator's Open Drain Output.

COMP+

Comparator's Non-Inverting Input.

COMP-

Comparator's Inverting Input.

DIS

Discharge Output: When the MIC2086 is turned off, a 550

internal resistor

at this output allows the discharging of any load capacitance to ground.

REF

Reference Output: 1.24V nominal. Tie a 0.1

F capacitor to ground to ensure

stability.

GATE

Gate Drive Output: Connects to the gate of an external N-Channel
MOSFET. An internal clamp ensures that no more than 13V is applied
between the GATE pin and the source of the external MOSFET. The GATE
pin is immediately brought low when either the circuit breaker trips or an
undervoltage lockout condition occurs.

MIC2085/2086

Micrel

M0235-121903

January 2004

Pin Description (Cont.)

Pin Number

Pin Name

Pin Function

MIC2086

MIC2085

SENSE

Circuit Breaker Sense Input: A resistor between this pin and VCC sets the
current limit threshold. Whenever the voltage across the sense resistor
exceeds the slow trip current limit threshold (V

TRIPSLOW

), the GATE voltage

is adjusted to ensure a constant load current. If V

TRIPSLOW

(48mV) is

exceeded for longer than time period t

OCSLOW

, then the circuit breaker is

tripped and the GATE pin is immediately pulled low. If the voltage across the
sense resistor exceeds the fast trip circuit breaker threshold, V

TRIPFAST

, at

any point due to fast, high amplitude power supply faults, then the GATE pin
is immediately brought low without delay. To disable the circuit breaker, the
SENSE and VCC pins can be tied together.

The default V

TRIPFAST

for either device is 95mV. Other fast trip thresholds

are available: 150mV, 200mV, or OFF(V

TRIPFAST

disabled). Please contact

factory for availability of other options.

19,20

VCC

Positive Supply Input: 2.3V to 16.5V. The GATE pin is held low by an
internal undervoltage lockout circuit until VCC exceeds a threshold of 2.18V.
If VCC exceeds 16.5V, an internal shunt regulator protects the chip from
VCC and SENSE pin voltages up to 33V.

January 2004

M0235-121903

MIC2085/2086

Micrel

Absolute Maximum Ratings

(1)

(All voltages are referred to GND)

Supply Voltage (V

) ..................................... 0.3V to 33V

SENSE Pin .......................................... 0.3V to V

+ 0.3V

GATE Pin ....................................................... 0.3V to 22V

ON, DIS, /POR, PWRGD, /FAULT,

COMP+, COMP, COMPOUT ....................... 0.3V to 20V

CRWBR, FB, OV, REF ..................................... 0.3V to 6V

Maximum Currents

Digital Output Pins ..................................................... 10mA

(/POR, /FAULT, PWRGD, COMPOUT)

DIS Pin ....................................................................... 30mA

ESD Rating:

Human Body Model ................................................... 2kV
Machine Model ........................................................ 200V

Operating Ratings

(2)

Supply Voltage (V

) .................................... 2.3V to 16.5V

Operating Temperature Range .................. 40

C to +85

Junction Temperature (T

) ........................................ 125

Package Thermal Resistance R

(J-A)

16-pin QSOP ..................................................... 112

C/W

20-pin QSOP ....................................................... 91

C/W

Electrical Characteristics

(3)

= 5.0V, T

= 25

C unless otherwise noted. Bold indicates specifications over the full operating temperature range of 40

C to +85

Symbol

Parameter

Condition

Min

Typ

Max

Units

Supply Voltage

2.3

16.5

Supply Current

1.6

2.5

Undervoltage Lockout Threshold

rising

2.05

2.18

2.28

falling

1.85

2.0

2.10

UVHYST

UV Lockout Hysteresis

180

FB (Power-Good) Threshold Voltage

FB rising

1.19

1.24

1.29

FBHYST

FB Hysteresis

OV Pin Threshold Voltage

OV pin rising

1.19

1.24

1.29

OV Pin Threshold Voltage

2.3V < V

< 16.5V

Line Regulation

OVHYST

OV Pin Hysteresis

OV Pin Current

0.2

POR Delay and Overcurrent (CFILTER) V

CPOR

, V

CFILTER

rising

1.19

1.24

1.29

Timer Threshold

CPOR

Power-On Reset Timer Current

Timer on

2.5

2.0

1.5

Timer off

TIMER

Current Limit /Overcurrent

Timer on

30

15

Timer Current (CFILTER)

Timer off

2.5

CRWBR Pin Threshold Voltage

2.3V < V

< 16.5V

445

470

495

CRWBR Pin Threshold Voltage

2.3V < V

< 16.5V

Line Regulation

CRWBR Pin Current

CRWBR On, V

CRWBR

= 0V

60

30

CRWBR On, V

CRWBR

= 2.1V

1.5

1.0

CRWBR Off, V

CRWBR

= 1.5V

3.3

TRIP

Circuit Breaker Trip Voltage

TRIP

= V

SENSE

TRIPSLOW

(Current Limit Threshold)

2.3V

16.5V

TRIPFAST

x = J

110

x = K

150

x = L

200

External Gate Drive

GATE

< 3V

5V < V

< 9V

9V < V

< 15.0V

4.5

21V

MIC2085/2086

Micrel

M0235-121903

January 2004

Electrical Characteristics (Cont.)

Symbol

Parameter

Condition

Min

Typ

Max

Units

GATE

GATE Pin Pull-up Current

Start cycle, V

GATE

= 0V

=16.5V

22

= 2.3V

20

GATEOFF

GATE Pin Sink Current

/FAULT = 0, V

GATE

>1V

= 16.5V

= 2.3V

ON Pin Threshold Voltage

ON rising

1.19

1.24

1.29

ON falling

1.09

1.14

1.19

ONHYST

ON Pin Hysteresis

100

ON Pin Input Current

= V

0.5

START

Undervoltage Start-up

CPOR

rising

1.19

1.24

1.29

Timer Threshold

/FAULT, /POR, PWRGD Output

OUT

= 1.6mA

0.4

Voltage

(PWRGD for MIC2086 only)

PULLUP

Output Signal Pull-up Current

/FAULT, /POR, PWRGD = GND

/FAULT, /POR, PWRGD, COMPOUT

(PWRGD for MIC2086 only)

REF

Reference Output Voltage

LOAD

= 0mA; C

REF

= 0.1

1.21

1.24

1.27

LNR

Reference Line Regulation

2.3V < V

< 16.5V

LDR

Reference Load Regulation

OUT

= 1mA

2.5

7.5

RSC

Reference Short-Circuit Current

REF

= 0V

3.5

COS

Comparator Offset Voltage

= V

REF

CHYST

Comparator Hysteresis

= V

REF

DIS

Discharge Pin Resistance

ON pin toggles from HI to LOW

100

550

1000

AC Electrical Characteristics

(4)

Symbol

Parameter

Condition

Min

Typ

Max

Units

OCFAST

Fast Overcurrent Sense to GATE

= 5V

Low Trip Time

SENSE

= 100mV

GATE

= 10nF, See Figure 1

OCSLOW

Slow Overcurrent Sense to Gate

= 5V

Low Trip Time

SENSE

= 50mV

FILTER

= 0, See Figure 1

ONDLY

ON Delay Filter

FBDLY

FB Delay Filter

Notes:

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

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

3. Specification for packaged product only.

4. Specification for packaged product only.

MIC2085/2086

Micrel

M0235-121903

January 2004

Typical Characteristics

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

-40 -20

80 100

SUPPLY CURRENT (mA)

TEMPERATURE (

Supply Current

vs. Temperature

= 2.3V

= 5V

= 16.5V

1.4

1.6

1.8

2.0

2.2

2.4

2.6

-40 -20

80 100

CPOR

(µA)

TEMPERATURE (

Power-On Reset Timer Current

vs. Temperature

= 16.5V

= 5V

= 2.3V

-40 -20

80 100

CPOR

(mA)

TEMPERATURE (

Power-On Reset Timer (Off) Current

vs. Temperature

= 16.5V

= 5V

= 2.3V

-40 -20

80 100

TIMER

(µA)

TEMPERATURE (

Overcurrent Timer Current

vs. Temperature

= 16.5V

= 5V

= 2.3V

-40 -20

80 100

TIMER

(mA)

TEMPERATURE (

Overcurrent Timer (Off) Current

vs. Temperature

= 16.5V

= 5V

= 2.3V

-40 -20

80 100

GATE

(µA)

TEMPERATURE (

Gate Pull-Up Current

vs. Temperature

= 16.5V

= 5V

= 2.3V

10 12 14 16 18

)

(V)

Gate Pull-Up Current

vs. V

-40 -20

80 100

(V)

TEMPERATURE (

External Gate Drive

vs. Temperature

= 2.3V

= 16.5V

= 5V

10 12 14 16 18

(V)

External Gate Drive

vs. V

100

-40 -20

80 100

GATEOFF

(mA)

TEMPERATURE (

Gate Sink Current

vs. Temperature

= 2.3V

= 16.5V

= 5V

100

200

300

400

500

600

GATEOFF

(mA)

GATE

(V)

Gate Sink Current

vs. Gate Voltage

12V

1.20

1.21

1.22

1.23

1.24

1.25

-40 -20

80 100

(mV)

TEMPERATURE (

POR Delay/Overcurrent

Timer Threshold

vs. Temperature

= 2.3V

= 16.5V

= 5V

January 2004

M0235-121903

MIC2085/2086

Micrel

Typical Characteristics

100

105

110

115

120

-40 -20

80 100

TRIPFAST

(mV)

TEMPERATURE (

Current Limit Threshold

(Fast Trip)

vs. Temperature

= 2.3V

= 16.5V

= 5V

-40 -20

80 100

TRIPSLOW

(mV)

TEMPERATURE (

Current Limit Threshold

(Slow Trip)

vs. Temperature

= 5V

= 16.5V

= 2.3V

1.7

1.8

1.9

2.0

2.1

2.2

2.3

2.4

2.5

-40 -20

80 100

UVLO THRESHOLD (V)

TEMPERATURE (

UVLO Threshold

vs. Temperature

UVLO

UVLO+

1.15

1.20

1.25

1.30

-40 -20

80 100

ON THRESHOLD (V)

TEMPERATURE (

ON Pin Threshold (Rising)

vs. Temperature

= 16.5V

= 5V

= 2.3V

1.05

1.10

1.15

1.20

-40 -20

80 100

ON THRESHOLD (V)

TEMPERATURE (

ON Pin Threshold (Falling)

vs. Temperature

= 16.5V V

= 5V

= 2.3V

-40 -20

80 100

ON PIN INPUT CURRENT (nA)

TEMPERATURE (

ON Pin Input Current

vs. Temperature

= 16.5V

= 5V

= 2.3V

1.15

1.20

1.25

1.30

-40 -20

80 100

FB THRESHOLD (V)

TEMPERATURE (

FB (Power-Good) Threshold

vs. Temperature

= 16.5V

= 5V

= 2.3V

1.15

1.20

1.25

1.30

-40 -20

80 100

OVERVOLTAGE PIN THRESHOLD (V)

TEMPERATURE (

Overvoltage Pin Threshold

vs. Temperature

= 16.5V

= 2.3V

-40 -20

80 100

PULLUP

(µA)

TEMPERATURE (

Output Signal Pull-Up Current

vs. Temperature

= 16.5V

= 2.3V

= 5V

0.0

0.1

0.2

0.3

0.4

0.5

-40 -20

80 100

COMPARATOR OFFSET VOLTAGE (V)

TEMPERATURE (

Comparator Offset Voltage

vs. Temperature

= 16.5V

= 2.3V

= 5V

200

300

400

500

600

700

800

900

1000

-40 -20

80 100

TEMPERATURE (

Discharge Pin Resistance

vs. Temperature

16.5V

2.3V

DIS

(

)

MIC2085/2086

Micrel

M0235-121903

January 2004

Functional Description

Hot Swap Insertion

When circuit boards are inserted into live system backplanes
and supply voltages, high inrush currents can result due to
the charging of bulk capacitance that resides across the
supply pins of the circuit board. This inrush current, although
transient in nature, may be high enough to cause permanent
damage to on-board components or may cause the system's
supply voltages to go out of regulation during the transient
period which may result in system failures. The MIC2085/86
acts as a controller for external N-Channel MOSFET devices
in which the gate drive is controlled to provide inrush current
limiting and output voltage slew rate control during hot plug
insertions.

Power Supply

VCC is the supply input to the MIC2085/86 controller with a
voltage range of 2.3V to 16.5V. The VCC input can withstand
transient spikes up to 33V. In order to help suppress tran-
sients and ensure stability of the supply voltage, a capacitor
of 1.0

F to 10

F from VCC to ground is recommended.

Alternatively, a low pass filter, shown in the typical application
circuit, can be used to eliminate high frequency oscillations as
well as help suppress transient spikes.

Start-Up Cycle

When the voltage on the ON pin rises above its threshold of
1.24V, the MIC2085/86 first checks that its supply (V

) is

above the UVLO threshold. If so, the device is enabled and
an internal 2

A current source begins charging capacitor

POR

to 1.24V to initiate a start-up sequence (i.e., start-up

delay times out). Once the start-up delay (t

START

) elapses,

CPOR is pulled immediately to ground and a 15

A current

source begins charging the GATE output to drive the external
MOSFET that switches V

to V

OUT

. The programmed start-

up delay is calculated using the following equation:

0.62 C

( F)

START

POR

CPOR

POR

(1)

where V

, the POR delay threshold, is 1.24V, and I

CPOR

the POR timer current, is 2

A. As the GATE voltage contin-

ues ramping toward its final value (V

+ V

) at a defined

slew rate (See

"Load Capacitance"/"Gate Capacitance Domi-

nated Start-Up"

sections), a second CPOR timing cycle

begins if: 1)/FAULT is high and 2)CFILTER is low (i.e., not
an overvoltage, undervoltage lockout, or overcurrent state).
This second timing cycle, t

POR

, starts when the voltage at the

FB pin exceeds its threshold (V

) indicating that the output

voltage is valid. The time period t

POR

is equivalent to t

START

and sets the interval for the /POR to go Low-to-High after
"power is good" (See Figure 2 of

"Timing Diagrams"

). Active

current regulation is employed to limit the inrush current
transient response during start-up by regulating the load
current at the programmed current limit value (See

"Current

Limiting and Dual-Level Circuit Breaker"

section). The fol-

lowing equation is used to determine the nominal current
limit value:

48mV

LIM

TRIPSLOW

SENSE

(2)

where V

TRIPSLOW

is the current limit slow trip threshold found

in the electrical table and R

SENSE

is the selected value that

will set the desired current limit. There are two basic start-up
modes for the MIC2085/86: 1)Start-up dominated by load
capacitance and 2)start-up dominated by total gate capaci-
tance. The magnitude of the inrush current delivered to the
load will determine the dominant mode. If the inrush current
is greater than the programmed current limit (I

LIM

), then load

capacitance is dominant. Otherwise, gate capacitance is
dominant. The expected inrush current may be calculated
using the following equation:

INRUSH

15 A

GATE

LOAD

GATE

LOAD

GATE

µ ×

(3)

where I

GATE

is the GATE pin pull-up current, C

LOAD

is the

load capacitance, and C

GATE

is the total GATE capacitance

ISS

of the external MOSFET and any external capacitor

connected from the MIC2085/86 GATE pin to ground).

Load Capacitance Dominated Start-Up

In this case, the load capacitance, C

LOAD

, 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, `M' option). During start-up under this
condition, the load current is regulated at the programmed
current limit value (I

LIM

) and held constant until the output

voltage rises to its final value. The output slew rate and
equivalent GATE voltage slew rate is computed by the
following equation:

Output Voltage Slew Rate, dV

/dt

OUT

LIM

LOAD

(4)

where I

LIM

is the programmed current limit value. Conse-

quently, the value of C

FILTER

must be selected to ensure that

the overcurrent response time, t

OCSLOW

, exceeds the time

needed for the output to reach its final value. For example,
given a MOSFET with an input capacitance C

ISS

= C

GATE

4700pF, C

LOAD

is 2200

F, and I

LIMIT

is set to 6A with a 12V

input, then the load capacitance dominates as determined by
the calculated INRUSH > I

LIM

. Therefore, the output voltage

slew rate determined from Equation 4 is:

Output Voltage Slew Rate, dV

/dt

2200 F

2.73

OUT

and the resulting t

OCSLOW

needed to achieve a 12V output is

approximately 4.5ms. (See

"Power-On Reset, Start-Up, and

Overcurrent Timer Delays"

section to calculate t

OCSLOW

GATE Capacitance Dominated Start-Up

In this case, the value of the load capacitance relative to the
GATE capacitance is small enough such that the load current
during start-up never exceeds the current limit threshold as
determined by Equation 3. The minimum value of C

GATE

that

will ensure that the current limit is never exceeded is given by
the equation below:

(min)

GATE

LIM

LOAD

(5)

January 2004

M0235-121903

MIC2085/2086

Micrel

where C

GATE

is the summation of the MOSFET input

capacitance (C

ISS

) and the value of the external capacitor

connected to the GATE pin of the MOSFET. Once C

GATE

determined, use the following equation to determine
the output slew rate

for gate capacitance dominated start-up.

/dt (output)

OUT

GATE

(6)

Table 1 depicts the output slew rate for various values of C

GATE

GATE

= 15

GATE

OUT

/dt

0.001

15V/ms

0.01

1.5V/ms

0.1

0.150V/ms

0.015V/ms

Table 1. Output Slew Rate Selection for GATE

Capacitance Dominated Start-Up

Current Limiting and Dual-Level Circuit Breaker

Many applications will require that the inrush and steady state
supply current be limited at a specific value in order to protect
critical components within the system. Connecting a sense
resistor between the VCC and SENSE pins sets the nominal
current limit value of the MIC2085/86 and the current limit is
calculated using Equation 2. However, the MIC2085/86 ex-
hibits foldback current limit response. The foldback feature
allows the nominal current limit threshold to vary from 24mV
up to 48mV as the FB pin voltage increases or decreases.
When FB is at 0V, the current limit threshold is 24mV and for
FB

0.6V, the current limit threshold is the nominal 48mV.

(See Figure 4 for Foldback Current Limit Response charac-
teristic).

The MIC2085/86 also features a dual-level circuit breaker
triggered via 48mV and 95mV current limit thresholds sensed
across the VCC and SENSE pins. The first level of the circuit
breaker functions as follows. Once the voltage sensed across
these two pins exceeds 48mV, the overcurrent timer, its
duration set by capacitor C

FILTER

, starts to ramp the voltage

at CFILTER using a 2

A constant current source. If the

voltage at CFILTER reaches the overcurrent timer threshold
(V

) of 1.24V, then CFILTER immediately returns to ground

as the circuit breaker trips and the GATE output is immedi-
ately shut down. For the second level, if the voltage sensed
across VCC and SENSE exceeds 95mV at any time, the
circuit breaker trips and the GATE shuts down immediately,
bypassing the overcurrent timer period. To disable current
limit and circuit breaker operation, tie the SENSE and VCC
pins together and the CFILTER pin to ground.

Output Undervoltage Detection

The MIC2085/86 employ output undervoltage detection by
monitoring the output voltage through a resistive divider
connected at the FB pin. During turn on, while the voltage at
the FB pin is below the threshold (V

), the /POR pin is

asserted low. Once the FB pin voltage crosses V

, a 2

current source charges capacitor C

POR

. Once the CPOR pin

voltage reaches 1.24V, the time period t

POR

elapses as the

CPOR pin is pulled to ground and the /POR pin goes HIGH.
If the voltage at FB drops below V

for more than 10

s, the

/POR pin resets for at least one timing cycle defined by t

POR

(see Applications Information for an example).

Input Overvoltage Protection

The MIC2085/86 monitors and detects overvoltage condi-
tions in the event of excessive supply transients at the input.
Whenever the overvoltage threshold (V

) is exceeded at

the OV pin, the GATE is pulled low and the output is shut off.
The GATE will begin ramping one POR timing cycle after the
OV pin voltage drops below its threshold. An external CRWBR
circuit, as shown in the typical application diagram, provides
a time period that an overvoltage condition must exceed in
order to trip the circuit breaker. When the OV pin exceeds the
overvoltage threshold (V

), the CRWBR timer begins charg-

ing the CRWBR capacitor initially with a 45

A current source.

Once the voltage at CRWBR exceeds its threshold (V

) of

0.47V, the CRWBR current immediately increases to 1.5mA
and the circuit breaker is tripped, necessitating a device reset
by toggling the ON pin LOW to HIGH.

Power-On Reset, Start-Up, and Overcurrent Timer
Delays

The Power-On Reset delay, t

POR

, is the time period for the

/POR pin to go HIGH once the voltage at the FB pin exceeds
the power-good threshold (V

). A capacitor connected to

CPOR sets the interval, t

POR

, and t

POR

is equivalent to the

start-up delay, t

START

(see Equation 1).

A capacitor connected to CFILTER is used to set the timer
which activates the circuit breaker during overcurrent condi-
tions. When the voltage across the sense resistor exceeds
the slow trip current limit threshold of 48mV, the overcurrent
timer begins to charge for a period, t

OCSLOW

, determined by

FILTER

. If no capacitor is used at CFILTER, then t

OCSLOW

defaults to 5

s. If t

OCSLOW

elapses, then the circuit breaker

is activated and the GATE output is immediately pulled to
ground. The following equation is used to determine the
overcurrent timer period, t

OCSLOW

0.062 C

( F)

OCSLOW

FILTER

TIMER

FILTER

(7)

where V

, the CFILTER timer threshold, is 1.24V and

TIMER

, the overcurrent timer current, is 20

A. Tables 2 and

3 provide a quick reference for several timer calculations
using select standard value capacitors.

MIC2085/2086

Micrel

M0235-121903

January 2004

POR

= t

START

0.01

6ms

0.02

12ms

0.033

18.5ms

0.05

30ms

0.1

60ms

0.33

200ms

Table 2. Selected Power-On Reset and

Start-Up Delays

FILTER

OCSLOW

1800pF

100

4700pF

290

8200pF

500

0.010

620

0.020

1.2ms

0.033

2.0ms

0.050

3.0ms

0.1

6.2ms

0.33

20.75ms

Table 3. Selected Overcurrent Timer Delays

Applications Information

Output Undervoltage Detection

For output undervoltage detection, the first consideration is to
establish the output voltage level that indicates "power is
good." For this example, the output value for which a 12V
supply will signal "good" is 11V. Next, consider the tolerances
of the input supply and FB threshold (V

). For this example,

the 12V supply varies

5%, thus the resulting output voltage

may be as low as 11.4V and as high as 12.6V. Additionally,
the FB threshold has

50mV tolerance and may be as low as

1.19V and as high as 1.29V. Thus, to determine the values of
the resistive divider network (R5 and R6) at the FB pin, shown
in Figure 5, use the following iterative design procedure.

1) Choose R6 so as to limit the current through the

divider to approximately 100

A or less.

100 A

1.29V

100 A

12.9k

FB(MAX)

R6 is chosen as 13.3k

1%.

2) Next, determine R5 using the output "good"

voltage of 11V and the following equation:

R5 R6

OUT(Good)

(

)

(8)

Using some basic algebra and simplifying Equation 8 to
isolate R5, yields:

OUT(Good)

FB(MAX)

(8.1)

where V

FB(MAX)

= 1.29V, V

OUT(Good)

= 11V, and R6 is

13.3k

. Substituting these values into Equation 8.1 now

yields R5 = 100.11k

. A standard 100k

1% is selected.

Now, consider the 11.4V minimum output voltage, the lower
tolerance for R6 and higher tolerance for R5, 13.17k

and

101k

, respectively. With only 11.4V available, the voltage

sensed at the FB pin exceeds V

FB(MAX)

, thus the /POR and

PWRGD (MIC2086) signals will transition from LOW to
HIGH, indicating "power is good" given the worse case
tolerances of this example.

Input Overvoltage Protection

The external CRWBR circuit shown in Figure 5 consists of
capacitor C4, resistor R7, NPN transistor Q2, and SCR Q3.
The capacitor establishes a time duration for an overvoltage
condition to last before the circuit breaker trips. The CRWBR
timer duration is approximated by the following equation:

.01 C4( F)

OVCR

(

)

(9)

where V

, the CRWBR pin threshold, is 0.47V and I

, the

CRWBR pin current, is 45

A during the timer period (see the

CRWBR timer pin description for further description). A
similar design approach as the previous undervoltage detec-
tion example is recommended for the overvoltage protection
circuitry, resistors R2 and R3 in Figure 5. For input overvolt-
age protection, the first consideration is to establish the input
voltage level that indicates an overvoltage triggering a sys-
tem (output voltage) shut down. For this example, the input
value for which a 12V supply will signal an "output shut down"
is 13.2V (+10%). Similarly, from the previous example:

1) Choose R3 to satisfy 100

A condition.

100 A

1.19V

100 A

11.9k

OV(MIN)

R3 is chosen as 13.7k

1%.

2) Thus, following the previous example and

substituting R2 and R3 for R5 and R6, respec-
tively, and 13.2V overvoltage for 11V output
"good", the same formula yields R2 of 138.3k

The next highest standard 1% value is 140k

Now, consider the 12.6V maximum input voltage (V

+5%),

the higher tolerance for R3 and lower tolerance for R2, 13.84k
and 138.60k

, respectively. With a 12.6V input, the voltage

sensed at the OV pin is below V

OV(MIN)

, and the MIC2085/86

will not indicate an overvoltage condition until V

exceeds

at least 13.2V.

MIC2085/2086

Micrel

M0235-121903

January 2004

PCB Connection Sense

There are several configuration options for the MIC2085/86's
ON pin to detect if the PCB has been fully seated in the
backplane before initiating a start-up cycle. In the typical
applications circuit, the MIC2085/86 is mounted on the PCB
with a resistive divider network connected to the ON pin. R2
is connected to a short pin on the PCB edge connector. Until
the connectors mate, the ON pin is held low which keeps the
GATE output charge pump off. Once the connectors mate,
the resistor network is pulled up to the input supply, 12V in this
example, and the ON pin voltage exceeds its threshold (V

)

GND

/ON_OFF

0.05

SENSE

VCC

CPOR

/POR

GATE

GND

/FAULT

Long

Backplane

Connector

PCB Edge

Connector

Long

Undervoltage (Output) = 11.4V
POR/START-UP DELAY = 30ms
*Q2 is TN0201T (SOT-23)
Additional pins omitted for clarity.

Short

PCB Connection Sense

Si7860DP

(PowerPAK

SO-8)

16.2k

10k

20k

100

*Q2

MIC2085

0.01

LOAD

220

20k

127k

OUT

12V@5A

12V

SENSE

0.008

Downstream
Signals

Figure 6. PCB Connection Sense with ON/OFF Control

of 1.24V and the MIC2085/86 initiates a start-up cycle. In
Figure 6, the connection sense consisting of a logic-level
discrete MOSFET and a few resistors allows for interrupt
control from the processor or other signal controller to shut off
the output of the MIC2085/86. R4 keeps the GATE of Q2 at
V

until the connectors are fully mated. A logic LOW at the

/ON_OFF signal turns Q2 off and allows the ON pin to pull up
above its threshold and initiate a start-up cycle. Applying a
logic HIGH at the /ON_OFF signal will turn Q2 on and short
the ON pin of the MIC2085/86 to ground which turns off the
GATE output charge pump.

January 2004

M0235-121903

MIC2085/2086

Micrel

Higher UVLO Setting

Once a PCB is inserted into a backplane (power supply), the
internal UVLO circuit of the MIC2085/86 holds the GATE
output charge pump off until V

exceeds 2.18V. If VCC falls

below 2V, the UVLO circuit pulls the GATE output to ground
and clears the overvoltage and/or current limit faults. For a
higher UVLO threshold, the circuit in Figure 7 can be used to
delay the output MOSFET from switching on until the desired
input voltage is achieved. The circuit allows the charge pump

0.1

SENSE

VCC

CPOR

GATE

GND

/POR

Undervoltage Lockout (Rising) = 11.0V
Undervoltage Lockout (Falling) = 10.1V
Undervoltage (Output) = 11.4V
POR/START-UP Delay = 60ms
Additional pins omitted for clarity.

IRF7822

(SO-8)

16.2k

392k

49.9k

MIC2085

0.01

LOAD

220

R4
127k

OUT

12V@4A

12V

SENSE

0.010

Downstream
Signal

Figure 7. Higher UVLO Setting

to remain off until V

exceeds 1

1.24V

. The GATE

drive output will be shut down when V

falls below

1.14V

. In the example circuit (Figure 7), the rising

UVLO threshold is set at approximately 11V and the falling
UVLO threshold is established as 10.1V. The circuit consists
of an external resistor divider at the ON pin that keeps the
GATE output charge pump off until the voltage at the ON pin
exceeds its threshold (V

) and after the start-up timer

elapses.

MIC2085/2086

Micrel

M0235-121903

January 2004

Fast Output Discharge for Capacitive Loads

In many applications where a switch controller is turned off by
either removing the PCB from the backplane or the ON pin is
reset, capacitive loading will cause the output to retain
voltage unless a `bleed' (low impedance) path is in place in
order to discharge the capacitance. The MIC2086 is equipped
with an internal MOSFET that allows the discharging of any
load capacitance to ground through a 550

path. The dis-

charge feature is configured by wiring the DIS pin to the
output (source) of the external MOSFET and becomes active

(DIS pin output is low) once the ON pin is deasserted. Figure
8(a) illustrates the use of the discharge feature with an
optional resistor (R5) that can be used to provide added
resistance in the output discharge path. For an even faster
discharge response of capacitive loads, the configuration of
Figure 8(b) can be utilized to apply a crowbar to ground
through an external SCR (Q3) that is triggered when the DIS
pin goes low which turns on the PNP transistor (Q2). See the
different

"Functional Characteristic"

curves for a comparison

of the discharge response configurations.

47kW

SENSE

VCC

SENSE

0.007W

19,20

0.022mF

Si7892DP

(PowerPAK

SO-8)

0.022mF

OUT

12V@5A

LOAD

1500mF

TCR22-4

ZTX788A

GND

CPOR

CFILTER

9,10

GATE

DIS

/POR

Undervoltage (Output) = 11V

POR/START-UP Delay = 6ms

Circuit Breaker Response Time = 620ms

*R5 of Figure 8(a) is optional to combine in series

with internal 550W.

Additional pins omitted for clarity.

0.01mF

1mF

14.7kW

Downstream

Signal

1.5kW

220W

4.4kW

110kW

10W

MIC2086

12V

ON Signal

47kW

SENSE

VCC

SENSE

0.007W

19,20

0.022mF

Si7892DP

(PowerPAK

SO-8)

OUT

12V@5A

LOAD

1500mF

GND

CPOR

CFILTER

9,10

GATE

/POR

DIS

PWRGD

0.01mF

(a)

(b)

0.01mF

1mF

14.7kW

Downstream

Signals

*R5

110kW

10W

MIC2086

12V

ON Signal

Figure 8. MIC2086 Fast Discharge of Capacitive Load

January 2004

M0235-121903

MIC2085/2086

Micrel

Auto-Retry Upon Overcurrent Faults

The MIC2085/86 can be configured for automatic restart after
a fault condition. Placing a diode between the ON and
/FAULT pins, as shown in Figure 9, will enable the auto-
restart capability of the controller. When an application is
configured for auto-retry, the overcurrent timer should be set
to minimize the duty cycle of the overcurrent response to
prevent thermal runaway of the power MOSFET. See

"MOSFET Transient Thermal Issues"

section for further

detail. A limited duty cycle is achieved when the overcurrent
timer duration (t

OCSLOW

) is much less than the start-up delay

timer duration (t

START

) and is calculated using the following

equation:

Auto Retry Duty Cycle

100%

OCSLOW

START

(10)

An InfiniBandTM Application Circuit

The circuit in Figure 10 depicts a single 50W InfiniBandTM
module using the MIC2085 controller. An InfiniBandTM
backplane distributes bulk power to multiple plug-in modules
that employ DC/DC converters for local supply requirements.

The circuit in Figure 10 distributes 12V from the backplane to
the MIC2182 DC/DC converter that steps down +12V to
+3.3V for local bias. The pass transistor, Q1, isolates the
MIC2182's input capacitance during module plug-in and
allows the backplane to accommodate additional plug-in
modules without affecting the other modules on the backplane.
The two control input signals are VBxEn_L (active LOW) and
a Local Power Enable (active HIGH). The MIC2085 in the
circuit of Figure 10 performs a number of functions. The gate
output of Q1 is enabled by the two bit input signal VBxEn_L,
Local Power Enable = [0,1]. Also, the MIC2085 limits the drain
current of Q1 to 7A, monitors VB_In for an overvoltage
condition greater than 16V, and enables the MIC2182 DC/DC
converter downstream to supply a local voltage rail. The
uncommitted comparator is used to monitor VB_In for an
undervoltage condition of less than 10V, indicated by a logic
LOW at the comparator output (COMPOUT). COMPOUT
may be used to control a downstream device such as another
DC/DC converter. Additionally, the MIC2085 is configured for
auto-retry upon an overcurrent fault condition by placing a
diode (D1) between the /FAULT and ON pins of the controller.

0.02

SENSE

VCC

CPOR

GATE

GND

/POR

Undervoltage (Output) = 4.27V
POR/START-UP Delay = 12ms
Circuit Breaker Response Time = 290

Auto-Retry Duty Cycle = 2.5%
Additional pins omitted for clarity.

IRF7822

(SO-8)

14.7k

47k

33k

1N914

MIC2085

0.022

LOAD

220

R4
34k

OUT

5V@2.5A

ON SIGNAL

/FAULT

OUTPUT

4700pF

CFILTER

/FAULT

SENSE

0.012

Downstream
Signal

Figure 9. Auto-Retry Configuration

MIC2085/2086

Micrel

M0235-121903

January 2004

Sense Resistor Selection

The MIC2085 and MIC2086 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 48mV/I

LOAD(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
MIC2085/86 may be as low as 40mV, which would equate to
a sense resistor value of 40mV/I

LOAD(CONT)

. Carrying the

numbers through for the case where the value of the sense
resistor is 3% high yields:

40mV

1.03 I

38.8mV

SENSE(MAX)

LOAD(CONT)

(

)

(

)

(11)

Once the value of R

SENSE

has been chosen in this manner,

it is good practice to check the maximum I

LOAD(CONT)

which

the circuit may let through in the case of tolerance build-up in
the opposite direction. Here, the worst-case maximum cur-
rent is found using a 55mV trip voltage and a sense resistor
that is 3% low in value. The resulting equation is:

55mV

0.97 R

56.7mV

LOAD(CONT,MAX)

SENSE(NOM)

(

)

(

)

(12)

As an example, if an output must carry a continuous 6A
without nuisance trips occurring, Equation 11 yields:

38.8mV

6.5m

SENSE(MAX)

The next lowest standard value is 6.0mW. At the other set
of tolerance extremes for the output in question:

56.7mV

6.0m

9.45A

LOAD(CONT,MAX)

almost 10A. Knowing this final datum, we can determine
the necessary wattage of the sense resistor, using P = I

where I will be I

LOAD(CONT, MAX)

, and R will be

(0.97)(R

SENSE(NOM)

). These numbers yield the following:

MAX

= (10A)

(5.82m

) = 0.582W.

In this example, a 1W sense resistor is sufficient.

MOSFET Selection

Selecting the proper external MOSFET for use with the
MIC2085/86 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 that the drain-
source breakdown voltage of the MOSFET must be greater
than V

IN(MAX)

. For instance, a 16V input may reasonably be

expected to see high-frequency transients as high as 24V.
Therefore, the drain-source breakdown voltage of the MOSFET
must be at least 25V. For ample safety margin and standard
availability, the closest minimum value should be 30V.

InfiniBandTM Application

VB_Ret

Local Power

Enable

VBxEn_L

SENSE

VCC

COMP+

CPOR

COMPÐ

/FAULT

/POR

REF

GATE

COMPOUT

GND

CRWBR

0.1mF

Long

InfiniBandª

Backplane

InfiniBandª

MODULE

MIC2182

DC/DC Converter

Long

Overvoltage (Input) = 16.0V

Undervoltage (Input) = 10.0V

Undervoltage (Output) &

Power-Good (Output) = 10.0V

Circuit Breaker Response Time = 1.2ms

POR/START-UP Delay = 18.5ms

Auto-Retry Duty Cycle = 6.5%

Short

SENSE

0.007W

IRF7822

(SO-8)

10W

11kW

25.5kW

1N914

13.3kW

10kW

165kW

78.7kW

MIC2085

0.033mF

0.01mF

0.022mF

/UV

3.3V @ 4A

174kW

RUN/SS

Power-On Reset

Output

VB_In

(12V)

GND

0.022mF

CFILTER

Figure 10. A 50W InfiniBandTM Application

January 2004

M0235-121903

MIC2085/2086

Micrel

The second breakdown voltage criterion that must be met is
a bit subtler than simple drain-source breakdown voltage. In
MIC2085/86 applications, the gate of the external MOSFET
is driven up to a maximum of 21V by the internal output
MOSFET. 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 (21V 0V) = 21V. Since most
power MOSFETs generally have a maximum gate-source
breakdown of 20V or less, the use of a Zener clamp is
recommended in applications with V

8V. A Zener diode

with 10V to 12V rating is recommended as shown in Figure
11. At the present time, most power MOSFETs with a 20V
gate-source voltage rating have a 30V drain-source break-
down rating or higher. As a general tip, choose surface-mount
devices with a drain-source rating of 30V or more as a starting
point.

Finally, the external gate drive of the MIC2085/86 requires a
low-voltage logic level MOSFET when operating at voltages
lower than 3V. There are 2.5V logic level MOSFETs avail-
able. Please see Table 4,

"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 I

LOAD(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?).

The data sheet will almost always give a value of on resis-
tance 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 8V of enhancement.
Call this value R

. Since a heavily enhanced MOSFET acts

as an ohmic (resistive) device, almost all that's required to
determine steady-state power dissipation is to calculate I

The one addendum to this is that MOSFETs have a slight
increase in R

with increasing die temperature. A good

approximation for this value is 0.5% increase in R

per

rise in junction temperature above the point at which R

was

initially specified by the manufacturer. For instance, if the
selected MOSFET has a calculated R

of 10m

at a

= 25

C, and the actual junction temperature ends up

at 110

C, a good first cut at the operating value for R

would be:

10m

[1 + (110 - 25)(0.005)]

14.3m

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. 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, 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.

0.1

SENSE

VCC

CPOR

GATE

GND

/POR

Undervoltage (Output) = 11.0V
POR/START-UP Delay = 60ms
*Recommended for MOSFETs with gate-source
breakdown of 20V or less (IRF7822 VGS(MAX) = 12V)
for catastrophic output short circuit protection.
Additional pins omitted for clarity.

IRF7822

(SO-8)

*D1

1N5240B

10V

13.3k

47k

33k

MIC2085

0.01

LOAD

220

R4
100k

OUT

12V@5A

12V

/FAULT

SENSE

0.007

Downstream
Signals

Figure 11. Zener Clamped MOSFET GATE

MIC2085/2086

Micrel

M0235-121903

January 2004

2. Airflow works. Even a few LFM (linear feet per

minute) of air will cool a MOSFET down sub-
stantially. If you can, position the MOSFET(s)
near the inlet of a power supply's fan, or the
outlet of a processor's cooling fan.

3. The best test of a surface-mount MOSFET for

an application (assuming the above tips show it
to be a likely fit) is an empirical 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.

MOSFET Transient Thermal Issues

Having chosen a MOSFET that will withstand the imposed
voltage stresses, and the worse case continuous I

R 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. 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.

In terms related directly to the specification and use of power
MOSFETs, this is known as "transient thermal impedance,"
or Z

(J-A)

. Almost all power MOSFET data sheets give a

Transient Thermal Impedance Curve. For example, take the
following case: V

= 12V, t

OCSLOW

has been set to 100msec,

LOAD(CONT. MAX)

is 2.5A, the slow-trip threshold is 48mV

nominal, and the fast-trip threshold is 95mV. If the output is
accidentally connected to a 3

load, the output current from

the MOSFET will be regulated to 2.5A for 100ms (t

OCSLOW

)

before the part trips. During that time, the dissipation in the
MOSFET is given by:

P = E x I E

MOSFET

= [12V-(2.5A)(3

)] = 4.5V

MOSFET

= (4.5V x 2.5A) = 11.25W for 100msec.

At first glance, it would appear that a really hefty MOSFET is
required to withstand this sort of fault condition. This is where
the transient thermal impedance curves become very useful.
Figure 12 shows the curve for the Vishay (Siliconix) Si4410DY,
a commonly used SO-8 power MOSFET.

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. 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 Z

(J-A)

of this MOSFET to a highly infrequent

event of this duration is only 8% of its continuous R

(J-A)

This particular part is specified as having an R

(J-A)

C/W for intervals of 10 seconds or less. Thus:

Assume T

= 55

C maximum, 1 square inch of copper at the

drain leads, no airflow.

Recalling from our previous approximation hint, the part has

an R

of (0.0335/2) = 17m

at 25

Assume it has been carrying just about 2.5A for some time.

When performing this calculation, be sure to use the highest
anticipated ambient temperature (T

A(MAX)

) in which the

MOSFET will be operating as the starting temperature, and
find the operating junction temperature increase (

) from

that point. Then, as shown next, the final junction temperature
is found by adding T

A(MAX)

and

. 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.

Then the starting (steady-state)T

is:

A(MAX)

+ [R

+ (T

A(MAX)

)(0.005/

C)(R

)]

x I

x R

(J-A)

C + [17m

+ (55

C-25

C)(0.005)(17m

)]

x (2.5A)

x (50

C/W)

(55

C + (0.122W)(50

C/W)

61.1

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 T

equal to the already calculated value of 61.1

+ [17m

+ (61.1

C-25

C)(0.005)(17m

)]

x (2.5A)

x (50

C/W)

( 55

C + (0.125W)(50

C/W)

61.27

Normalized Thermal Transient Impedance, Junction-to-Ambient

Square Wave Pulse Duration (sec)

0.1

0.01

Nor

aliz

ed Ef

f
ectiv

r
ansient

Ther

mal 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

= P

thJA

(t)

Notes:

4. Surface Mounted

Figure 12. Transient Thermal Impedance

January 2004

M0235-121903

MIC2085/2086

Micrel

So our original approximation of 61.1

C was very close to the

correct value. We will use T

= 61

Finally, add (11.25W)(50

C/W)(0.08) = 45

C to the steady-

state T

to get T

J(TRANSIENT MAX.)

= 106

C. This is an accept-

able maximum junction temperature for this part.

PCB Layout Considerations

Because of the low values of the sense resistors used with the
MIC2085/86 controllers, special attention to the layout must
be used in order for the device's circuit breaker function to
operate properly. Specifically, the use of a 4-wire Kelvin
connection to measure the voltage across R

SENSE

is highly

recommended. Kelvin sensing is simply a means of making
sure that any voltage drops in the power traces connecting to
the resistors does not get picked up by the traces themselves.
Additionally, these Kelvin connections should be isolated
from all other signal traces to avoid introducing noise onto
these sensitive nodes. Figure 13 illustrates a recommended,

multi-layer layout for the R

SENSE

, Power MOSFET, timer(s),

overvoltage and feedback network connections. The feed-
back and overvoltage resistive networks are selected for a
12V application (from Figure 5). Many hot swap applications
will require load currents of several amperes. Therefore, the
power (V

and Return) trace widths (W) need to be wide

enough to allow the current to flow while the rise in tempera-
ture for a given copper plate (e.g., 1 oz. or 2 oz.) is kept to a
maximum of 10

C ~ 25

C. Also, these traces should be as

short as possible in order to minimize the IR drops between
the input and the load. For a starting point, there are many
trace width calculation tools available on the web such as the
following link:

http://www.aracnet.com/cgi-usr/gpatrick/trace.pl

Finally, plated-through vias are utilized to make circuit con-
nections to the power and ground planes. The trace connec-
tions with indicated vias should follow the example shown for
the GND pin connection in Figure 13.

*POWER MOSFET

(SO-8)

*SENSE RESISTOR

(2512)

Current Flow

from the Load

Via to GND Plane

Via to POWER (VCC)

Plane

140k9

13.7k9

**R4

109

100k9

13.3k9

**C

GATE

**C

POR

**C

FILTER

DRAWING IS NOT TO SCALE

*See Table 4 for part numbers and vendors

**Optional components

Trace width (W) guidelines given in "PCB Layout.

Recommendations" section of the datasheet.

MIC2085

WBR

CFIL

TER

CPOR

/POR

AUL

GND

COMPOUT

COMP+

COMP-

REF

SENSE

VCC

Current Flow

to the Load

Current Flow

to the Load

Figure 13. Recommended PCB Layout for Sense Resistor, Power MOSFET,

and Feedback/Overvoltage Network

MIC2085/2086

Micrel

M0235-121903

January 2004

MOSFET Vendors

Key MOSFET Type(s)

*Applications

Contact Information

Vishay (Siliconix)

Si4420DY (SO-8 package)

OUT

10A

www.siliconix.com

Si4442DY (SO-8 package)

OUT

= 10A-15A, V

(203) 452-5664

Si3442DV (SO-8 package)

OUT

3A, V

Si7860DP (PowerPAKTM SO-8)

OUT

12A

Si7892DP (PowerPAKTM SO-8)

OUT

15A

Si7884DP (PowerPAKTM SO-8)

OUT

15A

SUB60N06-18 (TO-263)

OUT

20A, V

SUB70N04-10 (TO-263)

OUT

20A, V

International Rectifier

IRF7413 (SO-8 package)

OUT

10A

www.irf.com

IRF7457 (SO-8 package)

OUT

10A

(310) 322-3331

IRF7822 (SO-8 package)

OUT

= 10A-15A, V

IRLBA1304 (Super220TM)

OUT

20A, V

Fairchild Semiconductor

FDS6680A (SO-8 package)

OUT

10A

www.fairchildsemi.com

FDS6690A (SO-8 package)

OUT

10A, V

(207) 775-8100

Philips

PH3230 (SOT669-LFPAK)

OUT

20A

www.philips.com

Hitachi

HAT2099H (LFPAK)

OUT

20A

www.halsp.hitachi.com
(408) 433-1990

* These devices are not limited to these conditions in many cases, but these conditions are provided as a helpful reference for customer applications.

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

Table 4. MOSFET and Sense Resistor Vendors

MOSFET and Sense Resistor Vendors

Device types and manufacturer contact information for power
MOSFETs and sense resistors is provided in Table 4. Some
of the recommended MOSFETs include a metal heat sink on
the bottom side of the package. The recommended trace for

the MOSFET Gate of Figure 13 must be redirected when
using MOSFETs packaged in this style. Contact the device
manufacturer for package information.

MIC2085/2086

Micrel

M0235-121903

January 2004

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

The information furnished by Micrel in this data sheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its use.

Micrel reserves the right to change circuitry and specifications at any time without notification to the customer.

Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product can

reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant into
the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A Purchaser's

use or sale of Micrel Products for use in life support appliances, devices or systems is at Purchaser's own risk and Purchaser agrees to fully indemnify

Micrel for any damages resulting from such use or sale.

Ð­Ð»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: MIC2086-xBQS

ÐÐ»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: MIC2086-xBQS