July 2005

9

MIC5013

MIC5013

Micrel, Inc.

(using TAB and SOURCE for forcing, and SENSE and

KELVIN for sensing) is the best method of evaluating “R.”

Alternatively, “R” can be estimated for large MOSFETs

smaller MOSFETs.

High-Side Driver with Current Sensing MOSFET (Figure

5). The design starts by determining the value of “S” and

“R” for the MOSFET (use the guidelines described for the

point is somewhat reduced when the output is at ground as

the voltage drop across R1 is zero. No clamping is required

for inductive loads, but may be added to reduce power dis-

sipation in the MOSFET.

Typical Applications

Start-up into a Dead Short. If the MIC5013 attempts to turn

on a MOSFET when the load is shorted, a very high current

flows. The over-current shutdown will protect the MOSFET,

but only after a time delay of 5 to 10µs. The MOSFET must

be capable of handling the overload; consult the device’s

SOA curve. If a short circuit causes the MOSFET to exceed

its 10µs SOA, a small inductance in series with the source

can help limit di/dt to control the peak current during the 5

to 10µs delay.

When testing short-circuit behavior, use a current probe

rated for both the peak current and the high di/dt.

The over-current shutdown delay varies with comparator

overdrive, owing to noise filtering in the comparator.Adelay

ofupto100µscanbeobservedatthethresholdofshutdown.

A 20% overdrive reduces the delay to near minimum.

Incandescent Lamps. The cold filament of an incandes-

cent lamp exhibits less than one-tenth as much resistance

as when the filament is hot. The initial turn-on current of

a #6014 lamp is about 70A, tapering to 4.4A after a few

hundred milliseconds. It is unwise to set the over-current

trip point to 70A to accommodate such a load. A “resistive”

short that draws less than 70A could destroy the MOSFET

by allowing sustained, excessive dissipation. If the over-

current trip point is set to less than 70A, the MIC5013 will

not start a cold filament. The solution is to start the lamp

with a high trip point, but reduce this to a reasonable value

after the lamp is hot.

The MIC5013 over-current shutdown circuit is designed to

handle this situation by varying the trip point with time (see

viding a current limit of approximately twice that required by

to approximately 10 times the steady-state lamp current.

The high initial trip point decays away according to a 20ms

results in a very high over-current threshold. As a rule of

thumb design the over-current circuitry in the conventional

tor that provides the desired time constant working against

When the MIC5013 is turned off, the threshold pin (2) ap-

constant, and it simulates the thermal response of the fila-

ment. If the lamp is pulse-width modulated, the current limit

ModifyingSwitchingTimes.Donotaddexternalcapacitors

to the gate to slow down the switching time. Add a resistor

(1kΩ to 51kΩ) in series with the gate of the MOSFET to

achieve this result.

Bootstrapped High-Side Driver (Figure 6). The speed

of a high-side driver can be increased to better than 10µs

by bootstrapping the supply off of the MOSFET source.

This topology can be used where the load is pulse-width

modulated (100Hz to 20kHz), or where it is energized

Fault

V+

Gate

1

2

3

4

8

MIC5013

Gnd

7

6

5

Thresh

Sense

Source

Input

12V

IRCZ44

10µF

43Ω

R1

3.9kΩ

Control Input

+

#6014

22kΩ

R

TH1

22µF

1kΩ

R

TH2

Fault

V+

Gate

1

2

3

4

8

MIC5013

Gnd

7

6

5

Thresh

Sense

Source

Input

IRF540

10µF

20kΩ

Control Input

+

100Ω

R

18mΩ

LOAD

R2

100nF

1N4001 (2)

1N5817

7 to 15V

+

V

1mA

R1=

S

Figure 5. Time-Variable

Trip Threshold

Figure 6. Bootstrapped

High-Side Driver