15.4.3.3.3 Push-Pull Output Mode

The Push-Pull Output mode is similar to Independent Edge mode, however, the PWM cycle, as defined by the MODSEL[2:0] bits, is repeated twice each time a SOC trigger is received. The EOC trigger event and updates from Data registers are held off until the end of the second PWM cycle. Figure 15-16 shows the 2nd cycle that is invoked when using Push-Pull Output mode.

Operating the PWM in Push-Pull mode will double the period for a complete cycle, as there are two timer matches per cycle. If PGxTRIGy timers are used for event timing, the STEER signal can be used to gate application timing.

Push-Pull PWM mode is typically used in transformer coupled circuits to ensure that no net DC currents flow through the transformer. Push-Pull mode ensures that the same duty cycle PWM pulse is applied to the transformer windings in alternate directions. The phase of the push-pull count period can be determined by reading the STEER status bit (PGxSTAT[2]). If STEER = 0, the PWM Generator is generating the first PWM pulse. If STEER = 1, the PWM Generator is generating the second PWM pulse.

Since dead time is not available in Push-Pull mode, delays can be emulated in the Push-Pull Output mode by introducing a small phase offset with the PGxPHASE register. Similarly, the maximum duty cycle may be limited in software to avoid a pulse that ends too close to the start of the next PWM cycle.

Push-Pull Operation with Center-Aligned Modes

When the PWM Generator is operated in one of the two Center-Aligned modes, and the Push-Pull mode is selected, a complete PWM cycle will comprise four time base cycles.

Figure 15-17 shows the operation of the module with Push-Pull Output mode and Center-Aligned PWM mode. This combination of modes limits PWM buffer updates and interrupt events to every fourth time base cycle. Therefore, the same pulse is produced on the PWMxH and PWMxL pins before any changes to the duty cycle are allowed. Similar interrupt behavior also occurs when Dual Edge Center-Aligned mode (one update per cycle) is selected (MODSEL[2:0] = 110).

Figure 15-18 shows the operation of the module with Push-Pull Output mode and Dual Edge Center-Aligned PWM mode (two updates per cycle, MODSEL[2:0] = 111) or Double Update Center-Aligned mode. This combination of modes allows a buffer update and interrupt event on every time base cycle. This operating configuration does not attempt to maintain symmetrical pulses on the PWMxH and PWMxL outputs, which is a requirement for many push-pull applications. User software can change the edge times of the center-aligned pulses after every edge event, which minimizes control loop latency.