MIC5020 Datasheet, PDF (6/7 Pages) Micrel Semiconductor – Current-Sensing Low-Side MOSFET Driver

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 filament is warm enough for its current to decrease
(resistance increases). The sense resistor is selected so the
voltage across the sense resistor drops below the sense
threshold (50mV) as the filament becomes warm. The
MOSFET is no longer pulsed to limit current and the lamp
turns completely on.
10µF
TTL Input
(0V/5V)
MIC5020
1
8
V_DD
Gate
2
7
Input Sense−
3
6
Fault Sense+
4
C_T
5
Gnd
“( )” values apply to
demo circuit. See text.
V+
(+11V to +12V)
Incandescent
Lamp (#1157)
N-Channel
Power MOSFET
(IRF540)
R_SENSE
(0.041Ω)
Micrel
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)
+11V to +50V
(+13.2V)
10µF
TTL Input
(0V/5V)
MIC5020
1
8
V_DD
Gate
2
7
Input Sense−
3
6
Fault Sense+
4
C_T
5
Gnd
“( )” values apply to
demo circuit. See text.
(3Ω, > 60W)
N-Channel
Current Sensing
Power MOSFET
(IRCZ24)
R_SENSE
(10Ω)
Figure 1. Lamp Driver with
Figure 3. Using a Current Sensing MOSFET
Current Sensing
The MOSFET current source is used to develop a voltage
A lamp may not fully turn on if the filament does not heat up ^{across a sense resistor. This voltage is monitored by the}
adequately. Changing the duty cycle, sense resistor, or both ^{MIC5020 (SENSE + and SENSE – pins) to identify an overcur-}
to match the filament characteristics can correct the problem. ^{rent condition.}
5
Soft start can be demonstrated using a #1157 dual-filament
automotive lamp. The value of R_S shown in figure 1 allows for
soft start of the higher-resistance filament (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+
+11V to +50V
Solenoid
Diode
10µF
TTL Input
MIC5020
1
8
V_DD
Gate
2
7
Input Sense−
3
6
Fault Sense+
4
C_T
5
Gnd
N-Channel
Power MOSFET
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 character-
istics.
Figure 2. Solenoid Driver,
Without Current Sensing
A diode across the load protects the MOSFET from the
voltage 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 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 configuration as
figure 2).
October 1998
5-167