Datasheet

UC1525B UC1527B

UC2525B UC2527B

UC3525B UC3527B

PRINCIPLES OF OPERATION AND TYPICAL

CHARACTERISTICS

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 100µA to turn off the outputs. This is subject

to the added requirement of discharging whatever exter-

nal capacitance may be attached to these pins.

An alternate approach is the use of the shutdown cir-

cuitry of Pin 10 which has been improved to enhance the

available shutdown options. Activating this circuit by ap

plying 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 termi-

nated without significant discharge of the soft-start ca-

pacitor, 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.

UDG-95060 UDG-95061

The low source impedance of the output drivers provides

rapid charging of power FET input capacitance while mini

mizing external components.

Low power transformers can be driven directly by the

UC1525B. Automatic reset occurs during dead time, when

both ends of the primary winding are switched to ground.

UDG-95062

UC1525B Oscillator Schematic