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MIC5020
Lamp Driver Application
Incandescent lamps have a high inrush current (low resis-
tance) when turned on. The MIC5020 can perform a “soft
start” by pulsing the MOSFET (overcurrent condition) until
the ?lament is warm enough for its current to decrease (re-
sistance increases). The sense resistor is selected so the
voltage across the sense resistor drops below the sense
threshold (50mV) as the ?lament becomes warm. The
MOSFET is no longer pulsed to limit current and the lamp
turns completely on.
V+
(+11V to +12V)
Micrel, Inc.
Current Sensing MOSFET Application
A current sensing MOSFET allows current sensing without
adding additional resistance to the power switching circuit.
A current sensing MOSFET has two source connections: a
“power source” for power switching and a “current source”
for current sensing. The current from the current source is
approximately proportional to the current through the power
source, but much smaller. A current sensing ratio (I SOURCE /
I SENSE ) is provided by the MOSFET manufacturer.
V+
(+13.2V, > 4.4A)
10μF
2
TTL Input
(0V/5V)
MIC5020
1
TTL Input
2
(0V/5V)
3
MIC5020
1
V DD Gate
Input Sense-
3
Fault Sense+
4
C T Gnd
“( )” values apply to
demo circuit. See text.
8
7
6
5
Incandescent
Lamp (#1157)
N-Channel
Power MOSFET
(IRF540)
R S E N S E
(0.041 ?)
+11V to +50V
(+13.2V)
10μF V DD Gate
Input Sense-
Fault Sense+
4
C T Gnd
“( )” values apply to
demo circuit. See text.
8
7
6
5
(3?, > 60W)
N-Channel
Current Sensing
Power MOSFET
(IRCZ24)
R SENSE
(10?)
+11V to +50V
10μF
1 8
N-Channel
Power MOSFET
2 7
Figure1. LampDriverwith
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.
Soft start can be demonstrated using a #1157 dual-?lament
automotive lamp. The value of R S shown in ?gure 1 allows
for soft start of the higher-resistance ?lament (measures
approx. 2.1Ω cold or 21Ω hot).
Solenoid Driver Application
The MIC5020 can be directly powered by the control voltage
supply in typical 11Vdc through 50Vdc control applications.
Current sensing has been omitted as an example.
V+
Solenoid Diode
MIC5020
V DD Gate
TTL Input Input Sense-
3 6
Fault Sense+
4 5
C T Gnd
Figure 2. Solenoid Driver,
Without Current Sensing
A diode across the load protects the MOSFET from the volt-
age spike generated by the inductive load upon MOSFET
turn off. The peak forward current rating of the diode should
be greater than the load current.
Figure 3. Using a Current Sensing MOSFET
The MOSFET current source is used to develop a voltage
across a sense resistor. This voltage is monitored by the
MIC5020 ( SENSE + and SENSE – pins) to identify an over-
current condition.
The value of the sense resistor can be estimated with:
R SENSE = (r V TRIP R DS(ON) ) / (I LOAD R DS(ON) – V TRIP )
where:
R SENSE = external “sense” resistor
V TRIP = 50mV (0.050V) for the MIC5020
r = manufacturer ’s current sense ratio: (I SOURCE /I SENSE )
R DS(ON) = manufacturer ’s power source on resistance
I LOAD = load current (power source current)
The drain to source voltage under different fault conditions
affects the behavior of the MOSFET current source; that is, the
current source will respond differently to a slight over-current
condition (V DS(ON) very small) than to a short circuit (where
V DS(ON) is approximately equal to the supply voltage).
Adjustment of the sense resistor value by experiment starting
from the above formula will provide the quickest selection
of R SENSE .
Refer to manufacture’s data sheets and application notes
for detailed information on current sensing MOSFET char-
acteristics.
Figure 3 includes values which can be used to demonstrate
circuit operation. The IRCZ24 MOSFET has a typical sense
ratio of 780 and a R DS(ON) of 0.10Ω. A large 3Ω wirewound
load resistor will cause inductive spikes which should be
suppressed using a diode (using the same con?guration as
?gure 2).
MIC5020
6
July 2005
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