Datasheet

LT1158
15
1158fb
APPLICATIONS INFORMATION
Figure 9. Self-Protection with Auto Restart
t
SHUTDOWN
becomes more diffi cult to analyze when the
output is shorted with a PWM input. This is because the
FAULT pin only conducts when fault currents are actually
present in the MOSFET. FAULT does not conduct while the
input is low in Figures 6 and 7 or during the interval I
DS
=
0 in Figure 8. Thus t
SHUTDOWN
will safely increase when
the duty cycle of the current in the top MOSFET is low,
maintaining the average MOSFET current at a relatively
constant level.
The length of time following shutdown before restart is
attempted is given by:
t
V
A
CC
RESTART EN EN
=
⎛
⎝
⎜
⎞
⎠
⎟
=×
()
15
25
610
4
.
μ
In Figure 9, the top MOSFET would shut down after being
in DC current limit for 0.9ms and try to restart at 60ms
intervals, thus producing a duty cyle of 1.5% in short
circuit. The resulting average top MOSFET dissipation
during a short is easily measured by taking the product of
the supply voltage and the average supply current.
Starting High In-Rush Current Loads
The LT1158 has a V
DS
sensing function which allows more
than I
SC
to fl ow in the top MOSFET providing that the
Note that for the fi rst event only, t
SHUTDOWN
is approximately
twice the above value since C
EN
is being discharged all
the way from its quiescent voltage. Allowable values for
R
F
are from zero to 10k.
SENSE
–
pin is within 1.2V of supply. Under these condi-
tions the current is limited only by the R
DS(ON)
in series
with R
SENSE
. For a 5-lead MOSFET the current is limited
by R
DS(ON)
alone, since R
SENSE
is not in the output path
(see Figure 7). Again adjusting R
DS(ON)
for temperature,
the worst-case start currents are:
I
V
RR
START
DS ON
SENSE
=
+
()
+
()
12
1
.
∂
3-Lead MOSFET
II
V
R
START
DS ON
=
+
()
()
12
1
.
∂
5-Lead MOSFET
Properly sizing the MOSFET for I
START
allows inductive
loads with long time constants, such as motors with high
mechanical inertia, to be started.
Returning to the example used in Power MOSFET Selec-
tion, an IRFZ34 (R
DS(ON)
= 0.05Ω max) was selected for
operation at 5A. If the short-circuit current is also set at 5A,
what start current can be supported? From the equation
for R
SENSE
, a 0.03Ω shunt would be required, allowing
the worst-case start current to be calculated:
I
V
A
START
=
()
+
=
12
17 005 003
10
.
.. .ΩΩ
This calculation gives the minimum current which could
be delivered with the IRFZ34 at T
J
= 125°C without activat-
ing the FAULT pin on the LT1158. If more start current is
required, using an IRFZ44 (R
DS(ON)
= 0.028Ω max) would
increase I
START
to over 15A at T
J
= 110°C, even though
the short-circuit current remains at 5A.
In order for the V
DS
sensing function to work properly, the
supply pins for the LT1158 must be connected at the drain
of the top MOSFET, which must be properly decoupled
(see Ugly Transient Issues).
Driving Lamps
Incandescent lamps represent a challenging load because
they have much in common with a short circuit when cold.
The top gate driver in the LT1158 can be confi gured to turn
on large lamps while still protecting the power MOSFET
FAULT
LT1158
C
EN
1μF
1158 F09
R
F
1k
7.5V
1.15V
ENABLE
OPTIONAL THERMOSTAT
CLOSE ON RISE
AIRPAX #67FXXX
25μA
7.5V
+