MIC5013_05 Datasheet, PDF (9/15 Pages) Micrel Semiconductor – Protected High- or Low-Side MOSFET Driver

MIC5013
Applications Information (Continued)
12V
1N5817
Micrel, Inc.
7 to 15V
Control Input
R_TH2
1kΩ
C_TH
22µF
R_TH1
22kΩ
MIC5013
10µF +
1 _{Input Fault} 8
2 _{Thresh V+} 7
3 _{Sense Gate} 6
4 _{Source Gnd} 5
IRCZ44
Control Input
R_TH
20kΩ
MIC5013
1 _{Input Fault} 8
2 _{Thresh V+} 7
3 _{Sense Gate} 6
4 _{Source Gnd} 5
100nF
1N4001 (2)
+
10µF
IRF540
43Ω
R1
3.9kΩ
#6014
R1= ^V+
1mA
100Ω
R2
R18_SmΩ
LOAD
Figure 5. Time-Variable
Trip Threshold
(using TAB and SOURCE for forcing, and SENSE and
KELVIN for sensing) is the best method of evaluating “R.”
Alternatively, “R” can be estimated for large MOSFETs
(R_DS(ON) ≤ 100mΩ) by simply halving the stated R_DS(ON),
or by subtracting 20 to 50mΩ from the stated R_DS(ON) for
smaller MOSFETs.
High-Side Driver with Current Sensing MOSFET (Figure
5). The design starts by determining the value of “S” and
“R” for the MOSFET (use the guidelines described for the
low-side version). Let V_TRIP = 100mV, and calculate R_S for
a desired trip current. Next calculate R_TH and R1. The trip
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, but may be added to reduce power dis-
sipation in the MOSFET.
Typical Applications
Start-up into a Dead Short. If the MIC5013 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 shutdown.
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
Figure 6. Bootstrapped
High-Side Driver
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-
current trip point is set to less than 70A, the MIC5013 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 MIC5013 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, pro-
viding 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 capaci-
tor that provides the desired time constant working against
R_TH2 and the internal 1kΩ resistor.
When the MIC5013 is turned off, the threshold pin (2) ap-
pears as an open circuit, and C_TH is discharged through
R_TH1 and R_TH2. This is much slower than the turn-on time
constant, and it simulates the thermal response of the fila-
ment. If the lamp is pulse-width modulated, the current limit
will be reduced by the residual charge left in C_TH.
Modifying Switching Times. Do not add external capacitors
to the gate to slow down the switching time. Add a resistor
(1kΩ to 51kΩ) in series with the gate of the MOSFET to
achieve this result.
Bootstrapped High-Side Driver (Figure 6). The speed
of a high-side driver can be increased to better than 10µs
by bootstrapping the supply off of the MOSFET source.
This topology can be used where the load is pulse-width
modulated (100Hz to 20kHz), or where it is energized
July 2005
9
MIC5013