MIC5021 Micrel Semiconductor, MIC5021 Datasheet - Page 8

• Manufacturer Part Number: MIC5021
• Description: High-Speed High-Side MOSFET Driver
• Manufacturer: Micrel Semiconductor

MIC5021
External Protection
Resistors placed in series with each
the current drawn from the internal Schottky diodes during a
negative transient. This minimizes the forward drop across
the diodes.
During normal operation, sensing current from the sense pins
is unequal (5 A and 15 A). 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.
10µF
TTL Input
Figure 8. Protection from Inductive Kick
V
to avoid skewing
the 50mV trip point.
(5mV suggested)
R1
1
2
3
4
R1
V
Input
C
Gnd
= V
Figure 7. Resistor Voltage Drop
DD
3
T
1
2
3
4
MIC5021
R2
V
Input
C
Gnd
R2
DD
T
Sense
Sense
MIC5021
V
BOOST
Gate
Sense
Sense
V
BOOST
Gate
11DQ03
.
8
7
6
5
11DQ03
D2
8
7
6
5
D3
15µA
5µA
R1
R2
V
V
R1
R2
2.7
nF
1.0k
330
R1
R2
D1
SENSE
+12V to +36V
R
N-Channel
Power MOSFET
S
50mV nominal
(at trip)
R
N-Channel
Power MOSFET
connection limit
SENSE
Inductive
Load
5-176
High-Side Sensing
Sensing the current on the high side of the MOSFET isolates
the
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.
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 cold or 21 hot).
TTL Input
SENSE
TTL Input
10µF
10µF
pins from the inductive spike.
"( )" values apply to demo circuit.
Figure 10. Lamp Driver with
Figure 9. High Side Sensing
1
2
3
4
1
2
3
4
Current Sensing
V
Input
C
Gnd
See text.
DD
T
V
Input
C
Gnd
MIC5021
DD
T
MIC5021
Sense
Sense
V
Sense
Sense
BOOST
V
Gate
BOOST
Gate
S
8
7
6
5
8
7
6
5
shown in Figure 10 allows
0.01
µF
+12V to +20V
0.01
µF
V+
(+12V)
N-Channel
Power MOSFET
(IRF540)
R
(0.041 )
(+12V)
R
(< 0.01 )
N-Channel
Power MOSFET
(IRFZ44)
SENSE
Incandescent
Lamp (#1157)
October 1998
SENSE
Wirewound
Resistor
(3 )
Micrel