SNIU028D February 2016 – September 2020 UCD3138 , UCD3138064 , UCD3138064A , UCD3138128 , UCD3138A , UCD3138A64
There are several registers in the Loop Mux related to Burst mode/Light Load Mode. This mode works based on the output of a selected filter. In its simplest form, it disables DPWM output pulses when the filter output goes below a threshold – LoopMuxRegs.LLDISTHRESH. It reenables them when the filter goes above another threshold – LoopMuxRegs.LLENTHRESH.bit.TURN_ON_THRESH. There are bits in the LLCTRL register to enable Light Load Mode, and to select which filter output is used to drive the Light Load control.
To enable the single Light Load Module to control a DPWM module, the BURST_EN bit in DPWMCTRL1 must be set.
In normal mode, if PWM pulses get very short, they will stop altogether. The output voltage will drop by a small amount, and the filter output will rise again, and it will start up again. There are added features which permit output of fixed size pulses in burst mode. This function is very application specific. Consult the reference firmware code provided with the UCD3138 EVM for the desired topology for further information.
It may be necessary to adjust the thresholds as a function of Vin, for example.