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TPS92602-Q1: PWM dimming frequency can up to 20KHz or not

Janson Lin
Intellectual 340 points
Part Number: TPS92602-Q1
Other Parts Discussed in Thread: TPS92515

Hi Sir,

I want to use 20KHz PWM dimming frequency on TPS92602 but the datasheet show that typ PWM dimming frequency is 2KHanz and doesn't list max frequency. When use TPS92515, according to TI designs's document. The max PWM dimming frequency can up to 30KHz when use Shunt FET PWM dimming. The PWM dimming type of TPS92602 is also Shunt FET PWM dimming, could it increase PWM dimming frequency to 20KHz.

Janson 

  • 0
    TI__Expert 5475 points
    Hi Janson,

    The TPS92515 can support 30kHz PWM dimming and high PWM dimming ratio due to the shunt FET PWM dimming feature. Shunt PWM dimming means using another FET in parallel with the LED to shunt the output current. It's not the same as what we do in TPS92602.
    Frankly, it's hard for TPS92602 to work at that high PWM frequency. The dimming ratio won't be very high.

    Best Regards,
    Shirley He
  • 0 in reply to Shirley He
    Intellectual 340 points

    Hi Shirley

    What's problem may cause when PWM dimming frequency too high? Could I know the PWM dimming frequency limit by do some experiments?

    Best regards

    Janson

  • 0 in reply to Janson Lin
    TI__Expert 5475 points
    Hi Janson,

    During PWM dimming, it takes some settling time for the output current to reach the set-value in each PWM dimming cycle, usually depends on the loop response time. So if the PWM dimming frequency is too high, the settling time will have effect on the dimming accuracy. You can test the setting time to see its effect on the PWM dimming.

    Best Regards,
    Shirley He
  • 0 in reply to Shirley He
    Intellectual 340 points
    Hi Shirley,

    Does this setting time mean the duty cycle from high to normal ? Does different switch frequency affect this setting time?

    Best regards
    Janson
  • 0 in reply to Janson Lin
    TI__Expert 5475 points
    Hi Janson,

    It means the time for the output current to reach the full range value after PWM is turned high. The switching frequency won't affect the settling time. But the settling time will take up large percent of a switching period if the period is short.
    By the way, may I ask why such high PWM dimming frequency is needed? What is the application that you use this device in? Thanks.

    Best regards,
    Shirley He
  • 0 in reply to Shirley He
    Intellectual 340 points
    Hi Shirley,

    I have done the experiment on TPS92602 EVB using 20KHz PWM frequency. I saw that the output current up when PWM on and off PWM down from 1% PWM ration to 100%. I also monitor waveform of OVFB, ISNS and DIAG pins, it looks normal. Is that OK? Is Something else need I to care ?
    I use 20KHz PWM frequency to avoid acoustic noise of driver circuit. I can hear acoustic noise when use 2KHz PWM frequency. I am developing home lighting product.

    Best regards
    Janson
  • 0 in reply to Janson Lin
    TI__Expert 5475 points

    Hi Janson,

    Yes, you can use it at 20kHz if your test result is good. What I said previously is just to remind that it's not easy to have high accuracy at low dutycycle if PWM frequency is too high. But it's ok if there's no requirement of high PWM dimming ratio in your application.

    Best Regards,

    Shirley He