LT1158CS datasheet(14/20 Pages) LINER

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LT1158

tSHUTDOWN becomes more difficult 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 IDS =

0 in Figure 8. Thus tSHUTDOWN 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:

RESTART









=×









610

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 VDS sensing function which allows more

than ISC to flow in the top MOSFET providing that the

Figure 9. Self-Protection with Auto Restart

This calculation gives the minimum current which could

be delivered with the IRFZ34 at TJ = 125°C without activat-

ing the fault pin on the LT1158. If more start current is

required, using an IRFZ44 (RDS(ON)= 0.028Ω max.) would

increase ISTART to over 15A at TJ = 110°C, even though the

short-circuit current remains at 5A.

In order for the VDS 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 configured to turn

on large lamps while still protecting the power MOSFET

Note that for the first event only, tSHUTDOWN is approxi-

mately twice the above value since CEN is being discharged

all the way from its quiescent voltage. Allowable values for

RF are from zero to 10k.

FAULT

LT1158

CEN

µF

1158 F09

7.5V

1.15V

ENABLE

OPTIONAL THERMOSTAT

CLOSE ON RISE

AIRPAX #67FXXX

µA

7.5V

sense– pin is within 1.2V of supply. Under these condi-

tions the current is limited only by the RDS(ON) in series

with RSENSE. For a 5-lead MOSFET the current is limited by

RDS(ON) alone, since RSENSE is not in the output path (see

Figure 7). Again adjusting RDS(ON) for temperature, the

worst-case start currents are:

START

DS ON

SENSE

START

DS ON

()

∂

Properly sizing the MOSFET for ISTART 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 (RDS(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 RSENSE, a 0.03Ω shunt would be required, allowing the

worst-case start current to be calculated:

START =

()

17 0 05

0 03

ΩΩ

5-Lead MOSFET

3-Lead MOSFET

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