U3525

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

7 of 9

www.unisonic.com.tw

QW-R103-032,A

APPLICATION INFORMATION AND CIRCUIT

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 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 150µA 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

Q9

Q8

400μA

25KΩ

BLANKING

TO OUTPUT

TO PWM

COMPARATOR

2kΩ

2kΩ

Q10 Q11

5pF

23KΩ

1KΩ

Q16

1KΩ

Q15

Q14

DISCHARGE

SYNC.

GND

Q12

Q13

3KΩ

250Ω

CLOCK

4

Q17

14KΩ

7.4KΩ

15

6

5

Q6

Q5

Q1

Q3

7

Q2

Q4

Q7