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TPS92691: What is the minimum PWM frequency and pulse width, and how fast can it start using the SS line instead?

Michael Shonle
Prodigy 220 points
Part Number: TPS92691

Hi, we are wanting to use the TPS92691 to control some LEDs in 'strobe' mode, e.g. pulse it for e.g. 1-10 ms at a rate between approx. 1 per second and 60 times per second. The Part datasheet does not mention any minimum PWM frequency, but the Eval Board datasheet states a minimum frequency of about 100 Hz. Is there a minimum frequency and pulse width, and if so, what are they?  Also, how fast can it start if using the SS line instead?  We would like short pulses (~1 millisecond or less).

Thanks,

-Michael

  • 0
    TI__Mastermind 18890 points
    Hi Michael,

    The TPS92691 is not designed to be used in strobe mode the way you are planning to use. The issue will using SS for strobing is the sof-start time itself can be greater than 1mS since it has to charge the output caps prior to PWM'ng it.

    If you use PWM'ng for pulsing then the the ouput voltage will droop when PWM is low for 1 second because we tri-state the part and do not switch to maintain the output voltage. So when the PWM is high we can experience an over-current condition for the switch current on IS.

    Thanks Tuan
  • 0 in reply to Tuan Tran74
    Prodigy 220 points
    Hi, thanks so much for the response. It looks like we will be able to run it at about 30 Hz, is that going to work with the PWM signal? Also, what's the minimum PWM "On" time? Is the 4% min actually a function of the PWM frequency or is it in microseconds, e.g. 166 microseconds, regardless of frequency?

    Thanks a lot,
    -Michael
  • 0 in reply to Michael Shonle
    TI__Mastermind 18890 points
    Hi Michael,

    30Hz would be fine but the key is to have enough capacitance on the output such that it does not droop too much during the PWM OFF so you do not trigger IS over current. The 4% of 30Hz is just a conservative case. If you want to push the limit then the MINIMUM PWM ON time depends on the the switching frequency and loop response of the control loop. Note that switching frequency is determined by Rt.

    For example: If your switching freqency is 400KHz you will need a least 20 switching pulse on the the 400KHz and the loop has to be stable and LED current has reach the controlled level. You will need to monitor the LED current with a current probe when PWM first goes high and see when the LED current is ramped up and stable for minimum of 20 PWM cycle from the 200KHz swithcing frequency. This is the minimm PWM time that you can have in your system. This is because when you are PWM'ng the ouput current it's a sampling system and the LED current has to reach the controlled level with some sampling time.

    For the example above if you have done correct design you can get minimum PWM ON time of 20 pulses of 400KHZ which translate to 50uS.

    Thanks Tuan
  • 0 in reply to Tuan Tran74
    Prodigy 220 points

    Thanks for the detailed reply!