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[UCC28810] the Relationship between PWM freq. range and VSENSE control scheme

Other Parts Discussed in Thread: UCC28811, UCC28810

Hello~

Please let me ask one question for the sluu355a (UCC28810EVM-002 User's Guide).

There are dimming control with external or on-board PWM in buck Side (UCC28811).

PWM signal for dimming is connected to VSENSE pin of UCC28811.

The User's Guide of UCC28810EVM-002 (page 3) say freq. Range is 200Hz to 1000Hz, but I couldn't understand exactly the relationship between PWM freq. range and VSENSE control scheme.

Would you please provide some help what I can understand control scheme of PWM dimming?

It will be very helpful to us to move forward our design.

Thanks...

  • Hello,

    The Vsense pin is being used to disable the UCC28811 when PWM dim is pulled high.  Notice the parallel connection 'shutdown buck' does the same thing.  The UCC28811 Vsense pin shuts off the converter when transistor Q4 pulls it to zero volts.

    By PWMing the Vsense pin the UCC28811 is shutting off and on at the rate of 200 to 1000Hz depending on what the PWM frequency is (users choice).  The output current will be controlled by the duty cycle, not the frequency.  Going beyond 1000 Hz PWM it may start to go non-linear.

    The bandwidth of the buck stage is much higher than a PFC control loop.  The resistor divider to Vsense sets it at the higher operating error amplifier current.  This is using the Isense pin and the current regulator.

    Note:  This will only work if the bandwidth is high (as it is in this buck stage), it won't be possible to do PWM dimming this was if the UCC28811 or UCC28810 is being used as a power factor controller.

    Thanks,