Introduction to Digital Power Conversion
XMC4000/1000 Family
PWM Generation
Application Guide 27 V1.0, 2015-01
4.5 ON/OFF Control
Since all the individual ‘timer-cells’ of a CCU can be mapped to act upon virtually any external event
and function request, then theoretically any on-chip module can be considered to control a PWM. For
example, an ADC or comparators can join in the PWM control loops in this way, acting on analog
events. With such configurations, variable frequency and/or PWM pattern control is easily
accomplished.
Figure 16 PWM On/Off Control by External Events
4.6 Fixed ON-Time (FOT)
Fixed-On-Time (FOT) PWM has two essential properties:
1. The FOT Pulse Width is generated by a fixed active output state of a timer, by single-shot mode
for example.
2. The FOT Pulse Rate is controlled by external events; i.e. the timer does not decide pulse density.
Duty-cycle should be monitored for example, to enable feed-back control in the start-up phase.
Figure 17 PWM with Fixed-On-Time (FOT)
Use Case
FOT can be used in a PFC with inductor current ZCD in the control loop (See Figure 18).
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