Oscillator Schematic

UC1525B-SP

PRINCIPLES OF OPERATION AND TYPICAL CHARACTERISTICS (continued)

Since both the compensation and soft-start terminals (Pins 9 and 8) have current source pullups, either can

readily accept a pulldown 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 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, ultimately discharges this external capacitor, recycling slow turn-on

upon release.

Copyright © 2009, Texas Instruments Incorporated

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