Assembly Drawing and List of Materials
30
SLUUBX3B – June 2019 – Revised November 2019
Copyright © 2019, Texas Instruments Incorporated
UCC25640EVM-020 Evaluation Module
The burst mode threshold is controlled by the standby circuit using transistor Q5. When in standby mode,
the base of Q5 is pulled low, connecting the top of the LL/SS resistor divider to RVCC. When out of
standby mode, Q5 is turned off, changing the burst mode threshold to the minimum burst threshold (0.2V).
For applications where the output voltage set point is decreased when in standby mode, it is possible to
get in and out of burst mode by changing the output voltage regulation set point. This is because the
change in necessary peak to peak voltage on the VCR waveform to regulate at the higher output voltage
is greater than the programmed burst threshold. For these applications, the burst threshold adjustment
circuit on the primary may not be necessary.
Figure 32. Burst Mode Threshold Adjustment
Please note that this external standby circuit was added to evaluation module only to mimic system level
behavior. This circuit is not required in order to design with UCC25640x.