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Original question:

TPS923655: delay created by the IC from PWM or Dimming input

TPS923655DMTREVM: Response Time in Flexible Dimming Mode Under Normal Operation

Paul Brar
Prodigy 100 points
Part Number: TPS923655DMTREVM
Other Parts Discussed in Thread: TPS922055DMTREVM

Tool/software:

Hi,

I have the following setup with TPS922055DMTREVM operating in flexible dimming mode:

ADIM/HD and PWM/EN frequencies are set @ 20 KHz and 240 Hz, respectively. I captured the following waveforms during normal operation (after startup and dimming mode has been selected):

1. ADIM/HD duty cycle @ 20% 

2. ADIM/HD duty cycle @ 50% 

3. ADIM/HD duty cycle @ 70%  

It would be great if someone can answer the following questions:

1. Why is there a delay for current waveform w.r.t. PWM signal when it goes high?

2. Where is this delay coming from? As per my understanding, the IC will start/stop switching without any delay? Is there a way to minimize this delay?

3. Also, I observed from waveforms that as I increase the duty cycle of ADIM (20% --> 50% --> 70%), the delay reduces. Could you please explain why?

Any help is much appreciated.

Regards,

Paul

  • +1
    TI__Mastermind 23150 points

    Hi Paul,

    Sorry. I am currently out of office. I will reply to you next Monday when I am back to office. Thanks for your understanding.

    Best Regards,

    Steven

  • +1
    TI__Mastermind 23150 points

    Hi Paul,

    Please refer to my comments below.

    1. Most likely this is due to the fact that it needs some time for the inductor current to charge up the output capacitors before it goes into the LED.

    2. Yes. The IC will start/stop switching without any delay when you input PWM waveforms into the EN/PWM pin (after startup and dimming mode has been selected). I assume the delay is due to the output capacitors. Please try reducing the output capacitance and see if it helps.

    3. This is due to the fact that larger current needs less time to charge up the output capacitors.

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Prodigy 100 points

    Hi Steven,

    Thank you for a prompt response. 

    Doesn't the rise time of current waveform show the charging of output capacitors? Could this delay be due to the overcoming of V_forward of laser diodes? For example, no current flow through the laser diodes until the V_forward (~30.5V) is achieved. 

    Looking forward to your response!

    Regards,

    Paul

  • 0 in reply to Paul Brar
    TI__Mastermind 23150 points

    Hi Paul,

    Paul Brar said:
    Doesn't the rise time of current waveform show the charging of output capacitors? Could this delay be due to the overcoming of V_forward of laser diodes? For example, no current flow through the laser diodes until the V_forward (~30.5V) is achieved. 

    Yes, it shows. And the delay is due to overcoming of V_forward of laser diodes. I think we are talking about the same idea.

    Best Regards,

    Steven