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TPS61185: TPS61185 Frequency Program PWM Dimming

tateo yamashiro
Guru 10890 points
Part Number: TPS61185

Hi,

We got a questions from customer about TPS61185.
Could you help us?

[Question]
They measured PWM dimming frequency in frequency program PWM dimming by EVM. The result is below.

 Vin=10.8V
 LED=8S4P
 Riset=56k

They think PWM dimming frequency is poor accuracy at Rfpwmo=100k. Why is it poor accuracy at Rfpwmo=100k?

Best Regards,
tateo

  • 0
    TI__Prodigy 320 points
    Hi Tateo,

    Sorry for the delay. I've asked the design team to get clear information.
    Please let me share if any update.

    Best regards,
    Milo
  • 0 in reply to Milo Kim
    Guru 10890 points

    Thank you for your reply.
    Please let us know if you get the feedback.

    By the way, we got additional question.

    - How much is the minimum PWM input ON time?
      My customer should be specified 1% Duty at 20kHz PWM input frequency by frequency program PWM dimming. They have consern about the PWM input minimum ON time.

    Best Regards,
    tateo

  • 0 in reply to tateo yamashiro
    TI__Expert 3340 points

    Hi,

    according to the datasheet pg15, the minimum on time in each dimming cycle is 5us, which means the minimum duty of PWM dimming can reach down to 1% when dimming freq is less than 2KHz.

  • 0 in reply to Sheng Jin
    Guru 10890 points

    Thank you for your reply.
    PWM input minimum on time and PWM dimming minimum on time are same value that is 5usec. So when they use frequency program PWM under the following condition, minimum PWM dimming Duty is 10%. Is it right?

     - Condition -
     PWM input frequency=20kHz
     Rfpwmo=100kHz(PWM dimming frequency=1.5kHz)

    And please give us your feedback about first question.

    Best Regards,
    tateo

  • 0
    TI__Prodigy 670 points
    Hi Tateo,

    For dimming frequency setting, the integrated oscillator need to be trimmed to ensure the accuracy. We could only trim the oscillator at one condition. The 210Hz is quite popular dimming frequency when we were designing TPS61185, so we trim the frequency at 210Hz. So the accuracy for dimming frequency will be good at 210Hz and worse at higher frequency. In the other hand, I would suggest customer to use 210Hz for dimming frequency instead of 1.5KHz, because 1.5kHz is in sensitive audiable frequency range and the acoustic noise on ceramic capacitor at this frequency is much easier to be heard.

    Best regards,
    Xianghao
  • 0 in reply to Xianghao Meng
    Guru 10890 points

    Thank you for your support.
    Please let me confirm your answer about PWM input minimum on time.

    PWM input minimum on time and PWM dimming minimum on time are same value that is 5usec. So when they use frequency program PWM under the following condition, minimum PWM dimming Duty is 10%. Is it right?

    - Condition -
    PWM input frequency=20kHz
    Rfpwmo=100kHz(PWM dimming frequency=1.5kHz)

    Best Regards,
    tateo

  • 0 in reply to tateo yamashiro
    TI__Prodigy 670 points

    Below is recommended application conditions in datasheet.

    The minimum 5us Ton is only for FPWMO. So i think the minimum PWM dimming duty is 1% for PWMIN=20kHz and PWMO=1.5kHz.

    BEst regards,

    Xianghao