Introduction to Digital Power Conversion
XMC4000/1000 Family
PWM Generation
Application Guide 31 V1.0, 2015-01
4.8 Fixed Off-Time (FOFFT)
In Fixed Off-Time switch mode, each PWM On-Time pulse is variable and terminated when the
inductor current slope hits the peak-current detection level.
On each of these events the current slope will fall, with a fixed Off-time, before next pulse, controlled
by a timer. This could be the PWM timer itself for example.
FOFFT by Load Timer Function – How it works
The load-timer mode is an alternative way of creating a fixed Off-time interval.
The compare register value is copied into the timer register by a load-timer request on a peak
detection event.
From this event, until period match, the rest of the timer cycle is a fixed time (period value minus
compare value).
FOFFT mode with On-Time Limitation
The advantage of using the load-timer mode solution in Fixed Off-Time PWM generation, is that an
optional On-Time limitation is left “for free”, to be an add-on feature in the extension; i.e. a compare
event will come and terminate the On-Time even if a load-timer request does not appear.
FOFFT with Blanking
To avoid inaccurate peak current detections due to noise from the power switches, blanking time
zones should be invoked, during which all detection events by the analog comparator should be
rejected.
Blanking should be synchronized to the start event of each PWM cycle (See section 6.4).
Figure 20 PWM with Fixed-Off-Time (FOFFT)
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