MIC5021_05 Datasheet, PDF (8/10 Pages) Micrel Semiconductor – High-Speed High-Side MOSFET Driver

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
Input
7
Gate
3
C_T
6
Sense-
4
5 R1
Gnd Sense+
N-Channel
Power MOSFET
V_R1 = V_R2
to avoid skewing
the 50mV trip point.
(5mV suggested)
5µA
V_{R 1}
R2
15µA
V_{R 2}
R_S
50mV nominal
(at trip)
Load
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
10µF
TTL Input
MIC5021
1
8
V _DD
V _BOOST
2
Input
7
Gate
3
C_T
4
Gnd
6
Sense-
5
Sense+
D2
11DQ03
D3
11DQ03
2.7
nF
R1
1.0k
R2
330Ω
D1
N-Channel
Power MOSFET
R_{S E N S E}
Inductive
Load
Figure 8. Protection from Inductive Kick
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)
10µF
TTL Input
MIC5021
1
8
V_DD
V _BOOST
2
Input
7
Gate
3
C_T
4
Gnd
6
Sense-
5
Sense+
0.01
µF
R_{S E N S E}
(< 0.01Ω)
N-Channel
Power MOSFET
(IRFZ44)
Wirewound
Resistor
(3Ω)
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+
10µF
TTL Input
MIC5021
1
8
V_DD
V _BOOST
2
7
Input
Gate
3
C_T
4
Gnd
6
Sense-
5
Sense+
(+12V)
N-Channel
Power MOSFET
(IRF540)
0.01
µF
R_{S E N S E}
(0.041Ω)
"( )" values apply to demo circuit.
See text.
Incandescent
Lamp (#1157)
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 ap-
prox. 2.1Ω cold or 21Ω hot).
MIC5021
8
July 2005