July 2000

9

MIC5013

MIC5013

Micrel

includes bond resistances. A Kelvin-connected ohmmeter

(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

(R

by subtracting 20 to 50m

Ω from the stated R

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 low-

side version). Let V

desired trip current. Next calculate R

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

dissipation 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 10

µs. 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 10

µs SOA, a small inductance in series with the source

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

to 10

µs 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. A delay

of up to 100

µs can be observed at the threshold of shut-

down. 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-cur-

rent 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

Figure 5). R

providing a current limit of approximately twice that required

by the lamp. R

on to approximately 10 times the steady-state lamp current.

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

time constant contributed by C

with C

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

thumb design the over-current circuitry in the conventional

manner, then add the R

start-up. Let R

tor that provides the desired time constant working against

R

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

appears as an open circuit, and C

R

constant, and it simulates the thermal response of the

filament. If the lamp is pulse-width modulated, the current

limit will be reduced by the residual charge left in C

Modifying Switching Times. Do not add external capaci-

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

resistor (1k

Ω to 51kΩ) in series with the gate of the MOS-

FET to achieve this result.

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

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

µs by

bootstrapping the supply off of the MOSFET source. This

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

Fault

V+

Gate

1

2

3

4

8

MIC5013

Gnd

7

6

5

Thresh

Sense

Source

Input

12V

IRCZ44

10µF

43

Ω

R1

3.9k

Ω

Figure 5. Time-Variable

Trip Threshold

Control Input

+

#6014

22k

Ω

R

TH1

C

TH

22µF

1k

Ω

R

TH2

Fault

V+

Gate

1

2

3

4

8

MIC5013

Gnd

7

6

5

Thresh

Sense

Source

Input

IRF540

10µF

20k

Ω

Figure 6. Bootstrapped

High-Side Driver

Control Input

+

100

Ω

R

18m

Ω

LOAD

R2

100nF

1N4001 (2)

1N5817

7 to 15V

+

V

1mA

R1=

S

R

TH