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PWM termination using the analogue comparator

Question asked by Robert Vause on May 23, 2016
Latest reply on May 23, 2016 by xiangjun.rong


Please can you list all of the Kinetis and LPC family members that have a PWM capable timer and analogue comparator that can perform the following function in hardware (no software intervention):


1. You set up the timer to perform a fixed frequency PWM output with a duty cycle of 50%

2. You set up the analogue comparator to trigger when the pin is connected to a fixed voltage reference and the - pin is connected to an analogue input voltage.

3. The analogue comparator output automatically terminates the "ON" time of the PWM output if the input exceeds the reference threshold before the high level PWM output reaches 50% duty cycle. The only delay in terminating the PWM output in the system is the propagation delay of the analogue comparator (no additional clock cycles).

4. Once the PWM period has finished and a new period commences, the PWM duty cycle returns to 50% unless the comparator output is still high or the comparator trigger terminates the PWM "ON" time before it reaches 50%.


Please note that the comparator trigger output must reset itself once the input falls below the reference threshold (no software intervention).


This process is repeated continuously.


The effect of this set up is to control the "ON" time of the PWM period by the analogue comparator output, and not through any software adjustment of the duty cycle on the timer registers.


The software will control the OFF time (period) by making corrective changes to the period, but not on a cycle by cycle basis (example, the period will be adjusted every 5 or 10 PWM outputs)


The purpose of this function is to carry out Digital Power conversion using the MCU whereby the PWM output drives a MOSFET that is switching an inductor on/off to step up or step down a voltage.


I am also interested to know which Kinetis & LPC family members can be set up for:


1. Dual MOSFET control from one comparator for Push-Pull, half-bridge, LLC converters or dual phase converters

2. Quad MOSFET control from one comparator for full bridge control or Multi-phase converters

3. Independent channels using two or more internal comparators to terminate the PWM timers for examples such as

          a. PFC + flyback (1 x PWM timer + 1 x PWM timer + 2 x comparators)

          b. PFC + Forward converter + synchronous rectification (1 x PWM timer + 1 x PWM timer + 2 x PWM timer + 3 x comparators)


In each case the PWM timer must have its own timer that is independent of the other timers, so they don't have to share the same period (frequency).