SLUSD49A September 2017 – January 2019 UCC256303
PRODUCTION DATA.
Refer to the PDF data sheet for device specific package drawings
The soft-start programming and burst mode threshold programming are multiplexed on one pin – LL/SS. In addition, when ZCS region operation happens, this pin is pulled down to ground through a resistor to increase the switching frequency.
An internal constant current source charges the soft start capacitor to generate the soft-start command. Soft start period starts right after charge boot stage is done, and ends when FBreplica becomes lower than SS pin voltage.
After soft start is done, the SS voltage is replaced by AVDD to send to the FB chain. The LL/SS pin is then used to generate the burst mode threshold. In UCC256303 we try to maintain the same burst mode power level over the input voltage range. This is done by adaptively changing the burst mode threshold with sensed BLK voltage.
The programming resistors output provide two degrees of freedom, to set the burst mode threshold, as well as how the threshold changes with BLK voltage. When programmed correctly, the power stage will always enter burst mode at a certain output current level, making the system much easier to optimize.