LT1158
APPLICATIONS INFORMATION
the value of R
for the 5-lead MOSFET increases by
of the LT1158 current limit loop, an initial current spike of
SENSE
the current sensing ratio (typically 1000 – 3000), thus
eliminating the need for a low valued shunt. ΔV is in the
range of 1V to 3V in most applications.
from 2 to 5 times the final value will be present for a few
μs, followed by an interval in which I = 0. The current
DS
spike is normally well within the safe operating area (SOA)
of the MOSFET, but can be further reduced with a small
(0.5μH) inductor in series with the output.
Assuming a dead short, the MOSFET dissipation will rise
to V
• I . For example, with a 24V supply and I
SUPPLY SC
SC
= 10A, the dissipation would be 240W. To determine how
longtheMOSFETcanremainatthisdissipationlevelbefore
it must be shut down, refer to the SOA curves given in
the MOSFET data sheet. For example, an IRFZ34 would
be safe if shut down within 10ms.
A Tektronix A6303 current probe is highly recommended
for viewing output fault currents.
I
SC
If Short-Circuit Protection is Not Required
5μs/DIV
In applications which do not require the current sense
capability of the LT1158, the sense pins 11 and 12 should
both be connected to pin 13, and the FAULT pin 5 left
open. The enable pin 4 may still be used to shut down
the device. Note, however, that when unprotected the top
MOSFET can be easily (and often dramatically) destroyed
by even a momentary short.
LT1158 F08
Figure 8. Top MOSFET Short-Circuit Turn-On current
If neither the enable nor input pins are pulled low in
response to the fault indication, the top MOSFET current
will recover to a steady-state value I regulated by the
LT1158 as shown in Figure 8:
SC
Self-Protection with Automatic Restart
150mV
ISC =
When using the current sense circuits of Figures 6 and 7,
RSENSE
local shutdown can be achieved by connecting the FAULT
150mV
ISC
RSENSE
=
pin through resistor R to the enable pin as shown in
F
Figure 9. An optional thermostat mounted to the load or
−2
MOSFET heatsink can also be used to pull enable low.
r 150mV
RSENSE
(
)
150mV
⎛
⎞
ISC =
1−
⎜
⎝
⎟
⎠
Aninternal25μAcurrentsourcenormallykeepstheenable
ΔV
+
capacitorCENchargedtothe7.5Vclampvoltage(ortoV ,
−2
+
r 150mV
(
)
150mV
for V < 7.5V). When a fault occurs, CEN is discharged to
⎛
⎞
RSENSE
=
1−
⎜
⎝
⎟
⎠
below the enable low threshold (1.15V typ) which shuts
down both MOSFETs. When the FAULT pin or thermostat
releases, CEN recharges to the upper enable threshold
where restart is attempted. In a sustained short circuit,
FAULT will again pull low and the cycle will repeat until the
short is removed. The time to shut down for a DC input
or thermal fault is given by:
ISC
ΔV
r = current senseratio, ΔV = VGS = VGS − VT
The time for the current to recover to I following the
initial current spike is approximately Q /0.5mA, where
SC
GS
Q
is the MOSFET gate-to-source charge. I need not
GS
SC
be set higher than the required start-up current for mo-
t
= (100 + 0.8R ) C
EN
DC input
SHUTDOWN
F
tors (see Starting High In-Rush Current Loads). Note that
1158fb
14
LINEAR_DIMENSIONS [ Linear Dimensions ]
