U3525

LINEAR INTEGRATED CIRCUIT

UNISONIC TECHNOLOGIES CO., LTD

8 of 10

www.unisonic.com.tw

QW-R103-032.B

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APPLI CAT I ON I N FORM AT I ON AN D CI RCU I T

SHUTDOWN OPTIONS (see Block Diagram)

Since both the compensation and soft-start terminals (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 alternate approach is the use of the shutdown circuitry of Pin 10 which has been improved to enhance the

available shutdown options. Activating this circuit by applying a positive signal on Pin 10 performs two functions: the

PWM latch is immediately set providing the fastest turn-off signal to the outputs; and a 150

μA current sink begins to

discharge the external soft-start capacitor. If the shutdown command is short, the PWM signal is terminated 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, recycling slow turn-on upon release.

Pin 10 should not be left floating as noise pickup could conceivably interrupt normal operation.

OSCILLATOR SCHEMATIC