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Original question:

TPS92512: Problems with PWM Dimming

TPS92512: The max PWM frequency of TPS92512

Kane Zhang
Intellectual 585 points
Part Number: TPS92512

Dear TI Experts,

Our customer used TPS92512 as LED controller in their products,

and they found that when TPS92512 work in the low frequency the voltage ripple is very high, when TPS92512 work in 12Khz, the voltage ripple is ok;

From the datasheet showed the PWM frequency is 100Hz to 1Khz , i just want to ask that if TPS92512 can work in 12Khz?

  • 0
    TI__Mastermind 28655 points

    Hello Kane,

    When PWM is used it turns the current from the LEDs on and off.  Voltage ripple is not important since this is a current regulator.  Why are they concerned with voltage ripple?

    12 KHz is out of the datasheet recommended operating range of 100 Hz to 1 KHz.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Intellectual 585 points

    Hi Irwin,

    i know the PWM frequency is overpass the recommend of datasheet，

    for the voltage ripple，because they need to concern EMI certification，it is important for medical application。

  • 0 in reply to Kane Zhang
    TI__Mastermind 28655 points

    Hello Kane,

    I forwarded this to a colleague for his response.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Intellectual 585 points

    Dear experts，

    i donot receive your colleagues's respon.

  • 0
    TI__Intellectual 2590 points

    Kane,

    Irwin already responded to this.  The 12kHz PWM dimming frequency is outside the recommended range of PWM dimming frequencies.  When you speak of voltage ripple, I don't know if you are talking about input or output.  However, radiated EMI will not be majorly effected by PWM dimming frequency voltage ripple as the edge rates in the PWM dimming case are quite slow (not like the switch node of the converter itself which is much faster).  And conducted EMI only really depends on the current ripple, as Irwin mentioned.  The current slew rate is controlled by the inductor of the buck converter and is also quite slow in comparison to the switch node of the converter.

    Regardless, I cannot recommend higher frequency PWM dimming as it is not recommended in the device datasheet.  If they have conducted EMI concerns, then they can put an input filter in front of the buck converter to attenuate the effective EMI signature.  If they have radiated EMI concerns then they can slow down the effective PWM edge rate by using a larger inductor for the buck converter power stage.

    -JP