MIC5021

Micrel

5-176

October 1998

External Protection

the current drawn from the internal Schottky diodes during a

negative transient. This minimizes the forward drop across

the diodes.

Input

Gnd

Gate

Sense

−

Sense

+

N-Channel

Power MOSFET

MIC5021

1

2

3

4

8

7

6

5

5µA

15µA

R1

R2

to avoid skewing

(5mV suggested)

R1

≅ 3 × R2

50mV nominal

(at trip)

Figure 7. Resistor Voltage Drop

During normal operation, sensing current from the sense pins

is unequal (5

µA and 15µA). The internal Schottky diodes are

reverse biased and have no effect. To avoid skewing the trip

voltage, the current limiting resistors must drop equal volt-

ages at the trip point currents. See Figure 7. To minimize

resistor tolerance error, use a voltage drop lower than the trip

voltage of 50mV. 5mV is suggested.

External Schottky diodes are also recommended. See D2

and D3 in Figure 8. The external diodes clamp negative

transients better than the internal diodes because their larger

size minimizes the forward voltage drop at higher currents.

Input

Gnd

Gate

Sense

Sense

TTL Input

Inductive

Load

N-Channel

Power MOSFET

+12V to +36V

MIC5021

1

2

3

4

8

7

6

5

10µF

2.7

nF

1.0k

330

Ω

R2

D2

11DQ03

D3

11DQ03

D1

R1

Figure 8. Protection from Inductive Kick

High-Side Sensing

Sensing the current on the high side of the MOSFET isolates

Input

Gnd

Gate

Sense

Sense

TTL Input

Wirewound

Resistor

(3

Ω)

N-Channel

Power MOSFET

(IRFZ44)

+12V to +20V

(< 0.01

Ω)

(+12V)

MIC5021

1

2

3

4

8

7

6

5

10µF

0.01

µF

Figure 9. High Side Sensing

Lamp Driver Application

Incandescent lamps have a high inrush current (low resis-

tance) when turned on. The MIC5021 can perform a “soft

start” by pulsing the MOSFET (overcurrent condition) until

the filament is warm and its current decreases (resistance

increases). The sense resistor value is selected so the

voltage drop across the sense resistor decreases below the

sense threshold (50mV) as the filament becomes warm. The

FET is no longer pulsed and the lamp turns completely on.

Input

Gnd

Gate

Sense

−

Sense

+

TTL Input

(0.041

Ω)

N-Channel

Power MOSFET

(IRF540)

V+

MIC5021

1

2

3

4

8

7

6

5

10µF

0.01

µF

Incandescent

Lamp (#1157)

(+12V)

"( )" values apply to demo circuit.

See text.

Figure 10. Lamp Driver with

Current Sensing

A lamp may not fully turn on if the filament does not heat up

adequately. Changing the duty cycle, sense resistor, or both

to match the filament characteristics can correct the problem.

Soft start can be demonstrated using a #1157 dual filament

automotive lamp. The value of R

for soft start of the higher-resistance filament (measures

approx. 2.1

Ω cold or 21Ω hot).