SNIU028D February 2016 – September 2020 UCD3138 , UCD3138064 , UCD3138064A , UCD3138128 , UCD3138A , UCD3138A64
The ramp module is also used for Analog Peak Current Mode (APCM). Normally the PCM_START_SEL bit is set so that a filter output is used to drive the starting point of the ramp. The ramp end point is set to a low value so that the ramp will always go downward.
In this mode, the ramp trigger simply starts the ramp. The step comes from the Front End at a 32 nanosecond rate. There is a comparator in the Front End. This comparator compares the ramp value (in the DAC) to the EADC input value. When the EADC input exceeds the ramp value, the APCM fault output of the Front End goes active. This bit can be sent to the DPWM pins via the Loop Mux and Fault Mux.
The Front End drawing omits it for simplicity, but the DAC (and the input to the EADC amplifier) are actually differential. The PCM comparator, however, is single ended. So there is a differential to single ended converter between the DAC and the PCM comparator. This comparator must be enabled for PCM to work. The D2S_COMP_EN bit in the EADCCTRL register performs this function.
For additional information consult the reference firmware provided with UCD3138PSFBEVM-029 EVM.