LT1158CS View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1158CS' PDF : 20 Pages View PDF
LT1158
APPLICATIONS INFORMATION
from 2 to 5 times the final value will be present for a few
µs, followed by an interval in which IDS = 0. The current
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.
ISC
value of RSENSE for the 5-lead MOSFET increases by the
current sensing ratio (typically 1000 – 3000), thus elimi-
nating the need for a low valued shunt. ∆V is in the range
of 1V to 3V in most applications.
Assuming a dead short, the MOSFET dissipation will rise
to VSUPPLY × ISC. For example, with a 24V supply and ISC
= 10A, the dissipation would be 240W. To determine how
long the MOSFET can remain at this dissipation level
before 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.
5µs/DIV
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 ISC regulated by the
LT1158 as shown in Figure 8:
ISC
=
150mV
RSENSE
Standard 3-Lead
RSENSE
=
150mV
ISC
MOSFET
( ) r 150mV
ISC = RSENSE
1−
150mV
∆V
−2
5-Lead
( ) r
RSENSE =
150mV
ISC
1−
150mV
∆V
−2
MOSFET
r = current sense ratio, ∆V = VGS = VGS − VT
The time for the current to recover to ISC following the
initial current spike is approximately QGS/0.5mA, where
QGS is the MOSFET gate-to-source charge. ISC need not be
set higher than the required start-up current for motors
(see Starting High In-Rush Current Loads). Note that the
If Short-Circuit Protection is Not Required
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.
Self-Protection with Automatic Restart
When using the current sense circuits of Figures 6 and 7,
local shutdown can be achieved by connecting the fault pin
through resistor RF to the enable pin as shown in Figure 9.
An optional thermostat mounted to the load or MOSFET
heatsink can also be used to pull enable low.
An internal 25µA current source normally keeps the en-
able
to V
+c,apfoarciVto+r<CE7N.5cVh)a.rWgehdetno
the 7.5V clamp
a fault occurs,
voltage (or
CEN is dis-
charged to 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:
tSHUTDOWN = (100 + 0.8RF) CEN DC input
13
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