MIC5013 Micrel Semiconductor, MIC5013 Datasheet - Page 9

• Manufacturer Part Number: MIC5013
• Description: Protected High- or Low-Side MOSFET Driver
• Manufacturer: Micrel Semiconductor

MIC5013
Applications Information
includes bond resistances. A Kelvin-connected ohmmeter
(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
by subtracting 20 to 50m
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
desired trip current. Next calculate R
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
dissipation 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 10 s. 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 10 s SOA, a small inductance in series with the source
can help limit di/dt to control the peak current during the 5
to 10 s 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 100 s 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
July 2000
R
C
22µF
1k
TH2
TH
DS(ON)
Control Input
100m ) by simply halving the stated R
R
22k
TH1
Figure 5. Time-Variable
TRIP
Trip Threshold
1
2
3
4
= 100mV, and calculate R
Thresh
Source
Input
Sense
MIC5013
from the stated R
Fault
Gate
Gnd
V+
(Continued)
TH
8
7
6
5
3.9k
10µF
and R1. The trip
R1
43
+
DS(ON)
DS(ON)
12V
S
IRCZ44
#6014
for a
, or
for
9
Control Input
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 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
providing a current limit of approximately twice that required
by the lamp. R
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
with C
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
start-up. Let R
tor that provides the desired time constant working against
R
When the MIC5013 is turned off, the threshold pin (2)
appears as an open circuit, and C
R
constant, and it simulates the thermal response of the
filament. If the lamp is pulse-width modulated, the current
limit will be reduced by the residual charge left in C
Modifying Switching Times. Do not add external capaci-
tors to the gate to slow down the switching time. Add a
resistor (1k to 51k ) in series with the gate of the MOS-
FET to achieve this result.
Bootstrapped High-Side Driver (Figure 6). The speed of
a high-side driver can be increased to better than 10 s by
bootstrapping the supply off of the MOSFET source. This
topology can be used where the load is pulse-width modu-
TH2
TH1
R
20k
and the internal 1k resistor.
and R
TH
TH
working against the internal 1k resistor, but this
TH2
1
2
3
4
TH1
TH2
Input
Thresh
Sense
Source
TH2
. This is much slower than the turn-on time
Figure 6. Bootstrapped
MIC5013
functions in the conventional manner,
= (R
acts to increase the current limit at turn-
High-Side Driver
Fault
Gate
Gnd
TH1
V+
TH2
8
7
6
5
10)–1k , and choose a capaci-
/C
TH
R1=
TH
. R
network to allow for lamp
TH
1N5817
100nF
1mA
V
TH2
+
is discharged through
could be eliminated
100
+
10µF
R2
7 to 15V
LOAD
IRF540
MIC5013
1N4001 (2)
R
18m
S
TH
Micrel
.