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LT1158

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.

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 25

µA current source normally keeps the en-

able capacitor CEN charged to the 7.5V clamp voltage (or

to V

+, for V+ < 7.5V). When a fault occurs, C

EN 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

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:

Standard 3-Lead

MOSFET

5-Lead

MOSFET

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

(0.5

µH) inductor in series with the output.

µs/DIV

LT1158 F08

ISC

Figure 8. Top MOSFET Short-Circuit Turn-On current

ISC =

= () −









= () −









−

150mV

r 150mV

150mV

r 150mV

150mV

sense ratio, V = V

SENSE

∆

current

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

APPLICATIONS INFORMATION

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