SG2525A-SG3525A
Figure 5
: Error Amplifier.
PRINCIPLES OF OPERATION
SHUTDOWN OPTIONS (see Block Diagram)
Since both the compensation and soft-start termi-
nals (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 alternateapproachis the useof the shutdowncir-
cuitry of Pin 10 which has been improved to en-
hance the available shutdown options. Activating
this circuit by applying a positive signal on Pin 10
performs two functions : the PWM latch is immedi-
ately set providing the fastest turn-off signal to the
outputs ; and a 150
µA
current sink begins to dis-
charge the external soft-start capacitor. If the shut-
down command is short, the PWM signal is termi-
nated 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, re-
cycling slow turn-on upon release.
Pin 10 should not be left floating as noise pickup
could conceivably interrupt normal operation.
7/12
STMICROELECTRONICS [ STMICROELECTRONICS ]
