JAJSEO4E July 2012 – January 2018 TPS23751 , TPS23752
PRODUCTION DATA.
Converters require a softstart on the voltage error amplifier to prevent output overshoot on startup. Figure 35 shows a common implementation of a secondary-side softstart that works with the typical TL431 error amplifier. The softstart components consist of DSS, RSS, and CSS. They serve to control the output rate-of-rise by pulling VCTL down as CSS charges through ROB, the optocoupler, and DSS. This has the added advantage that the TL431 output and CIZ are preset to the proper value as the output voltage reaches the regulated value, preventing voltage overshoot due to the error amplifier recovery. The secondary-side error amplifier does not become active until there is sufficient voltage on the secondary. The TPS23751 and TPS23752 provide a primary-side softstart which persists long enough (approximately 3ms) for secondary side voltage-loop softstart to take over. The primary-side current-loop softstart controls the switching MOSFET peak current by applying a slowly rising ramp voltage to a second PWM control input. The PWM is controlled by the lower of the softstart ramp or the CTL-derived current demand. The actual output voltage rise time is usually much shorter than the internal softstart period. Initially the internal softstart ramp limits the maximum current demand as a function of time. The current limit, secondary-side softstart, or output regulation assume control of the PWM before the internal softstart period is over.