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TPS92691-Q1: flicker issue when pwm duty.cycle less.than 4.25%

Jiawei Hu1
Expert 5555 points
Part Number: TPS92691-Q1
Other Parts Discussed in Thread: TPS92691

Hi team.

My customer use tps92691 for their head lighting, and they found LED flicker issie when pwm duty cycle less than 4.25%. The frequency of PWM is 250Hz. Could.you kindly tell me what is the possible reason?

The red is current and the yellow is PWM.

BR

Jiawei

  • 0
    TI__Expert 5555 points

    Hi team,

    From above waveforms, red is output current, yellow is PWM, we can see unstable current. We do some debug and share with you:

    1. We changed PWM frequency from 250Hz to 200Hz, it doesn't work, still flickering.
    2. We changed soft start cap from 0.1uF to 10nF, it doesn't work.
    3. We changed COMP network, change R(increase) and C(increase and decrease), it doesn't work.

    My support need:

    1. We don't use DRRV and only use PWM dimming, could you kindly tell me how to get stable output current at Vin=13V?
    2. Which parameters will influence PWM dimming?

    Thanks!

    BR

    Jiawei

  • 0 in reply to Jiawei Hu1
    TI__Mastermind 18890 points

    Hello Jiawei,

    You will need to have the PWM dimming FET driven from DRRV if you are PWM dimming.  Please have them incorporate the FET and retest.

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    TI__Expert 5555 points

    Hi Tuan.

    Why is high pwm duty ok even without drrv? We only can achieve pwm dimming woth drrv pon? Thanks

  • 0 in reply to Jiawei Hu1
    TI__Mastermind 18890 points

    Hello,

    As I mention, for your application with 4.25% you will need to have the PWM dimming FET.  It is used to disconnect the load from the output caps so it does not discharge them.  For lower PWM dimming duty cycle it will have more time off and it will discharge the output cap.  What you are seeing is the result of not using the PWM dimming FET for narrow PWM dimming.  That's the reason we recommend it and have a driver designed in for it the first place.

    Thanks Tuan