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
2
3
4
V
DD
Input
C
T
Gnd
V
BOOST
Gate
Sense−
Sense+
8
7
6
5
R1
N-Channel
Power MOSFET
10µF
TTL Input
1
2
3
4
R
S
50mV nominal
(at trip)
Micrel
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
V
DD
Input
C
T
Gnd
V
BOOST
Gate
Sense
Sense
8
7
6
5
0.01
µF
Wirewound
Resistor
(3Ω)
R
SENSE
(< 0.01Ω)
N-Channel
Power MOSFET
(IRFZ44)
5µA
V
R1
R2
Load
V
R1
= V
R2
to avoid skewing
the 50mV trip point.
.
(5mV suggested)
R1
≅
3
×
R2
15µA
V
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
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.
V+
(+12V)
10µF
TTL Input
1
2
3
MIC5021
V
DD
Input
C
T
Gnd
V
BOOST
Gate
Sense−
Sense+
8
7
6
5
0.01
µF
R
SENSE
(0.041Ω)
"( )" values apply to demo circuit.
See text.
Incandescent
Lamp (#1157)
N-Channel
Power MOSFET
(IRF540)
MIC5021
10µF
TTL Input
1
2
3
4
V
DD
Input
C
T
Gnd
V
BOOST
Gate
Sense
Sense
8
7
6
5
2.7
nF
R1
1.0k
R
SENSE
R2
D3
11DQ03
330Ω
Inductive
Load
N-Channel
Power MOSFET
4
D2
11DQ03
D1
Figure 8. Protection from Inductive Kick
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
S
shown in Figure 10 allows
for soft start of the higher-resistance filament (measures
approx. 2.1Ω cold or 21Ω hot).
5-176
October 1998
MICREL [ MICREL SEMICONDUCTOR ]
