MIC5010 Datasheet, PDF (10/16 Pages) Micrel Semiconductor – Full-Featured High- or Low-Side MOSFET Driver

MIC5010
Applications Information (Continued)
Micrel
Control Input
R
TH
20kΩ
MIC5010
V+ =15V
1 _{Inhibit Fault} 14
V
LOAD
2 _NC
3 _Input
V+ ¹³
NC ¹²
+
10µF
4 _{Thresh C1} 11
5 _{Sense Com} 10
LOAD
6 _{Source C2} 9
7 _{Gnd Gate} 8
SENSE
IRCZ44
(S=2590,
R=11mΩ)
R_S
22Ω
KELVIN
SOURCE
SR V
R_S =
TRIP
R I –V
L TRIP
2200
R=
TH _V
TRIP
–1000
For this example:
I _L =20A (trip current)
V =100mV
TRIP
Figure 4. Low-Side Driver with
Current-Sensing MOSFET
point is somewhat reduced when the output is at ground as
the voltage drop across R1 is zero. No clamping is required
for inductive loads.
Typical Applications
Start-up into a Dead Short. If the MIC5010 attempts to turn
on a MOSFET when the load is shorted, a very high current
flows. The over-current shutdown will protect the MOSFET,
but only after a time delay of 5 to 10µs. The MOSFET must
be capable of handling the overload; consult the device's
SOA curve. If a short circuit causes the MOSFET to exceed
its 10µs SOA, a small inductance in series with the source
can help limit di/dt to control the peak current during the 5
to 10µs delay.
When testing short-circuit behavior, use a current probe
rated for both the peak current and the high di/dt.
The over-current shutdown delay varies with comparator
overdrive, owing to noise filtering in the comparator. A delay
of up to 100µs can be observed at the threshold of shut-
down. A 20% overdrive reduces the delay to near minimum.
Incandescent Lamps. The cold filament of an incandes-
cent lamp exhibits less than one-tenth as much resistance
as when the filament is hot. The initial turn-on current of a
#6014 lamp is about 70A, tapering to 4.4A after a few
hundred milliseconds. It is unwise to set the over-current trip
point to 70A to accommodate such a load. A “resistive” short
that draws less than 70A could destroy the MOSFET by
allowing sustained, excessive dissipation. If the over-cur-
rent trip point is set to less than 70A, the MIC5010 will not
start a cold filament. The solution is to start the lamp with a
high trip point, but reduce this to a reasonable value after the
lamp is hot.
The MIC5010 over-current shutdown circuit is designed to
handle this situation by varying the trip point with time (see
Figure 5). R_TH1 functions in the conventional manner,
providing a current limit of approximately twice that required
by the lamp. R_TH2 acts to increase the current limit at turn-
on to approximately 10 times the steady-state lamp current.
The high initial trip point decays away according to a 20ms
time constant contributed by C_TH. R_TH2 could be eliminated
with C_TH working against the internal 1kΩ resistor, but this
results in a very high over-current threshold. As a rule of
thumb design the over-current circuitry in the conventional
manner, then add the R_TH2/C_TH network to allow for lamp
start-up. Let R_TH2 = (R_TH1 ÷ 10) – 1kΩ, and choose a
capacitor that provides the desired time constant working
against R_TH2 and the internal 1kΩ resistor.
When the MIC5010 is turned off, the threshold pin (4)
appears as an open circuit, and C_TH is discharged through
R_TH1 and R_TH2. This is much slower than the turn-on time
R_TH2
1kΩ
Control Input
R
TH1
22kΩ
MIC5010
1 _{Inhibit Fault} 14
2 _NC
V+ ¹³
3 _{Input NC} 12
4 _{Thresh C1} 11
5 _{Sense Com} 10
6 _{Source C2} 9
7 _{Gnd Gate} 8
C_TH
22µF
12V
+
10µF
IRCZ44
43Ω
3.9kΩ
#6014
Figure 5. Time-Variable
Trip Threshold
5-96
April 1998