SHUTDOWN OPTIONS (see Block Diagram)

Since both the compensation and soft-start termi-

nals (Pins 9 and 8) have current source pull-ups,

either can readily accept a pull-down signal which

only has to sink a maximum of 100

µA to turn off the

outputs.This is subject to the added requirement of

discharging whatever external capacitance may be

attached to these pins.

An alternateapproachis the useof the shutdowncir-

cuitry of Pin 10 which has been improved to en-

hance the available shutdown options. Activating

this circuit by applying a positive signal on Pin 10

performs two functions : the PWM latch is immedi-

ately set providing the fastest turn-off signal to the

outputs ; and a 150

µA current sink begins to dis-

charge the external soft-start capacitor. If the shut-

down command is short, the PWM signal is termi-

nated without significant discharge of the soft-start

capacitor, thus, allowing, for example, a convenient

implementation of pulse-by-pulse current limiting.

Holding Pin 10 high for a longer duration, however,

will ultimately discharge this external capacitor, re-

cycling slow turn-on upon release.

Pin 10 should not be left floating as noise pickup

could conceivably interrupt normal operation.

Figure 5 : Error Amplifier.

PRINCIPLES OF OPERATION

SG2525A-SG3525A

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