U3525 Datasheet, PDF (7/9 Pages) Unisonic Technologies – REGULATING PWM IC

U3525
LINEAR INTEGRATED CIRCUIT
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
V_REF 15
Q1
Q2
Q4
Q5
7.4KΩ
R_{T 6}
Q6
Q3
C_{T 5}
Q8
Q9
TO PWM
COMPARATOR
2kΩ
SYNC.
14KΩ
Q7
DISCHARGE
7
400μA
GND
2kΩ
Q10 Q11
1KΩ
Q17
25KΩ
BLANKING
5pF
TO OUTPUT
Q15
Q14
23KΩ
Q12
Q13
1KΩ ^Q16
250Ω
3KΩ
4 ^CLOCK
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
7 of 9
QW-R103-032,A