SLUSC66E March 2015 – February 2017 UCD3138A
The DPWM is a complex logic system which is highly configurable to support several different power supply topologies. This section discusses waveforms, timing, and register settings, rather than ON-logic design.
The DPWM functions using a period counter, which increments from 0 to PRD, then resets and begins to increment again.
The DPWM logic causes transitions in many digital signals when the period counter reaches the target value for that signal.
In Normal mode, the Filter output determines the pulse width on DPWM A. DPWM B fits into the rest of the switching period, with a dead time separating it from the DPWM A on-time. It is useful for buck topologies. The drawing of the Normal Mode waveforms is shown in Figure 7-17.
Cycle adjust A can be used to adjust pulse widths on individual phases of a multi-phase system. This can be used for functions like current balancing. The Adaptive Sample Triggers can be used to sample in the middle of the on-time (for an average output), or at the end of the on-time (to minimize phase delay) The Adaptive Sample Register provides an offset from the center of the on-time. This can compensate for external delays, such as MOSFET and gate driver turn on times.
Blanking A-Begin and Blanking A-End can be used to blank out noise from the MOSFET turn on at the beginning of the period (DPWMA rising edge). Blanking B could be used at the turn off time of DPWMB. The other edges are dynamic, so blanking is more difficult.
Cycle Adjust B has no effect in Normal Mode.
The device uses multi-mode functionality in systems where each phase has only one driver signal. It enables each DPWM peripheral to drive two phases with the same pulse width, but with a time offset between the phases, and with different cycle adjusts for each phase.
The diagram for Multi-Mode is shown in Figure 7-18.
Event 2 and Event 4 are not relevant in Multi mode.
DPWMB can cross over the period boundary safely, and still have the proper pulse width, so full 100% pulse width operation is possible. DPWMA cannot cross over the period boundary.
Since the rising edge on DPWM B is also fixed, Blanking B-Begin and Blanking B-End can be used for blanking this rising edge.
Cycle Adjust B is usable on DPWM B.
This mode provides a symmetrical waveform where DPWMA and DPWMB have the same pulse width. As the switching frequency changes, the dead times between the pulses remain the same.
The filter has two outputs, filter duty and filter period. In this case, the Filter is configured so that the filter period is twice the filter duty. So when there were no dead times, each DPWM pin in ON for half of the period. For dead-time handling, the device substracts the average of the two dead times from the filter duty for both DPWM pins. Therefore, both pins have the same ON-time, and the dead times are fixed regardless of the period. The only edge which is fixed relative to the start of the period is the rising edge of DPWM A. This is the only edge for which the blanking signals can be used easily.
Triangular mode provides a stable phase shift in interleaved PFC and similar topologies. In this case, the PWM pulse is centered in the middle of the period, rather than starting at one end or the other. In Triangular Mode, only DPWM-B is available. The diagram for Triangular Mode is shown in Figure 7-20.
All edges are dynamic in triangular mode, so fixed blanking is not that useful. The adaptive sample trigger is not needed. It is very easy to put a fixed sample trigger exactly in the center of the FET on-time, because the center of the on-time does not move in this mode.
Leading edge mode is very similar to Normal mode, reversed in time. The DPWM A falling edge is fixed, and the rising edge moves to the left, or backwards in time, as the filter output increases. The DPWM B falling edge stays ahead of the DPWMA rising edge by a fixed dead time. The diagram of the Leading Edge Mode is shown in Figure 7-21.
As in the Normal mode, the two edges in the middle of the period are dynamic, so the fixed blanking intervals are mainly useful for the edges at the beginning and end of the period.