6

Rev. 1.0

01/29/04

IRU3138

www.irf.com

Short-Circuit Protection

The outputs are protected against the short-circuit. The

IRU3138 protects the circuit for shorted output by sens-

ing the output voltage (through the external resistor di-

vider). The IRU3138 turns off both drivers, when the out-

put voltage drops below 0.4V.

The IRU3138 also protects the output from over-voltaging

when the control FET is shorted. This is done by turning

on the sync FET with the maximum duty cycle.

Under-Voltage Lockout

The under-voltage lockout circuit assures that the

MOSFET driver outputs remain in the off state whenever

the supply voltage drops below set parameters. Lockout

occurs if Vc and Vcc fall below 3.5V and 4.25V respec-

tively. Normal operation resumes once Vc and Vcc rise

above the set values.

Shutdown

The converter can be shutdown by pulling the soft-start

pin below 2.8V. This can be easily done by using an

external small signal transistor. During shutdown both

MOSFET drivers turn off.

the output start-up time is the time period when soft-

start capacitor voltage increases from 1V to 2V. The start-

up time will be dependent on the size of the external

soft-start capacitor. The start-up time can be estimated

by:

For a given start up time, the soft-start capacitor can be

estimated as:

MOSFET Drivers

The driver capabilities of both high and low side drivers

are optimized to maintain fast switching transitions. They

are sized to drive a MOSFET that can deliver up to 20A

output current.

An internal dead time control is implemented to prevent

cross-conduction and allows the use of several kinds of

MOSFETs.

20

Switching Frequency VS. RT

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Figure 4A - Switching frequency vs. external resistor