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N-Channel
Power MOSFET
R S 50mV nominal
the 50mV trip point. V R 2
Load
R S E N S E
2 7
3
6
(IRFZ44)
1 8
10μF
N-Channel
TTL Input Input Gate
Power MOSFET
R S E N S E
1 8
10μF
N-Channel
TTL Input
Power MOSFET
(IRF540)
0.01
See text.
MIC5021
External Protection
Resistors placed in series with each SENSE connection limit
the current drawn from the internal Schottky diodes during a
negative transient. This minimizes the forward drop across
the diodes.
MIC5021
1 8
V DD V BOOST
2 7
Input Gate
3 6
C T Sense-
4 5 R1
Gnd Sense+
5μA
V R 1 (at trip)
R2
V R 1 = V R 2
to avoid skewing 15μA
(5mV suggested)
R1 ? 3 × R2
Figure 7. Resistor Voltage Drop
During normal operation, sensing current from the sense pins
is unequal (5μA 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.
+12V to +36V
MIC5021
V DD V BOOST
2 7
3 6 2.7
C T Sense- nF
4 5 R1
Gnd Sense+
1.0k
D2
11DQ03
R2
Micrel, Inc.
High-Side Sensing
Sensing the current on the high side of the MOSFET isolates
the SENSE pins from the inductive spike.
+12V to +20V
(+12V)
MIC5021
10μF 1 8
V DD V BOOST (< 0.01 ?)
TTL Input Input Gate N-Channel
Power MOSFET
C T Sense-
4 5
Gnd Sense+
0.01
μF
Wirewound
Resistor
(3?)
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 ?lament 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 ?lament becomes warm. The FET
is no longer pulsed and the lamp turns completely on.
V+
(+12V)
MIC5021
V DD V BOOST
2 7
Input Gate
3 6
C T Sense-
4 5 μF
Gnd Sense+
R S E N S E
(0.041 ?)
"( )" values apply to demo circuit. Incandescent
Lamp (#1157)
D3
11DQ03
330?
D1
Inductive
Load
Figure 10. Lamp Driver with
Current Sensing
A lamp may not fully turn on if the ?lament does not heat up
adequately. Changing the duty cycle, sense resistor, or both to
match the ?lament characteristics can correct the problem.
Figure 8. Protection from Inductive Kick
Soft start can be demonstrated using a #1157 dual ?lament
automotive lamp. The value of R S shown in Figure 10 allows
for soft start of the higher-resistance ?lament (measures ap-
prox. 2.1Ω cold or 21Ω hot).
MIC5021
8
July 2005
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