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TPS92691: PWM dimming frequency range

Aaron_Rangel
Intellectual 410 points
Part Number: TPS92691
Other Parts Discussed in Thread: LM3421

Hello,


After reading the document datasheet I do not have enough clear the dimming frequency range for this device.

I am not sure about the upper limmit and I would like to know it. I mean, does it accept 30KHz or it doesn't?.


I have seen some of the application notes for this device (Boost and SEPIC), but all of them use a PWM signal that is 1KHz at the most as an upper limit.


In my case, I want to design a step-down converter whith TPS92691 and the pwm signal comes from an external device (microcontroller).

Wating for your response.


Regards,

  • 0
    TI__Mastermind 18890 points

    Hi Aaron,

    We specify the PWM gate drive impedance on page 6 being 12.8 ohm for high side and 11.4 ohm for low side worse case.  This and the PWM FET gate charge is the limiting factor in how fast you can PWM and the narrowest pulse you can PWM.  

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Intellectual 410 points

    Hello Tuan,


    I am not sure if I have understood well what you said.
    I just expected that the TPS92691device, would be as similar as it is LM3421 which accepts even 30KHz or more for a step-down configuration.

    The fact is that as per your response, the pwm dimming frequency range depends strongly of the PWM external FET gate used for dimming.
    So, I can use a PWM FET with a specific gate charge to set 20KHz and then the PWM input pin will read and accept that PWM signal, will not it?

    I assume that the needed time to charge the gate is almost 4RC if I am not wrong.

    Could you tell me how to get and calculate this with numbers? I mean, is there any document where this is described or explained in detail.

    It would be helpful to me you provided it to me.

    Regards

  • 0 in reply to Aaron_Rangel
    TI__Mastermind 18890 points

    Hi Aaron,

    The PWM circuit will accept the 20KHz pwm frequency but the main limitation is the gate drive of our driver and whether it can drive the FET that you have externally.  The simple way of thinking is what you mentioned above with the 5RC time constant but fir a FET it’s related more to gate charge to be exact.

    Thanks Tuan