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TPS92512: Problems with PWM Dimming

KIM Andy
Expert 2530 points
Part Number: TPS92512
Other Parts Discussed in Thread: TPS92515, TPS92641, LM3409

Hi
Currently, our customers are developing the following LED Module using TPS92512.

Vin: 41Vdc, Load: 12 lines Serial (3x12 = 36V)

PWM dimming is changing LED brightness and color temperature.

However, if the PWM frequency is set to 6kHz or less, the LED brightness and color temperature change normally, but audible noise is generated.

If the PWM frequency is set to 20kHz or higher, audible noise will disappear.

However, the LED blinks at certain intervals.

The actual PWM Dimming frequency is 20kH.

From the datasheet, most of them seem to be designed to be less than 1kH at 250Hz.

Could the corresponding PWM frequency be a problem?

Is there anything I need to review? Or can you recommend another IC?

  • 0
    TI__Mastermind 28655 points
    Hello,

    Yes, the high PWM frequency can be an issue. The PWM dimming is enabling and disabling the converter. This requires the current to ramp up and ramp down in the inductor. The ramp up and ramp down rate depend on Vin, Vout, the inductor value and LED current. During the disable the compensation is held constant. It is trying to hold the regulation to the set-point of the design. If the PWM frequency is high the current waveform will not look like a square wave so the converter is trying to regulate a DC current level when it is not true DC when on and not ture zero when off. If you look at the inductor current when you are PWM dimming you can see this. If you need high frequency PWM you may want to look at a part that can be SHUNT FET dimmed, such as the TPS92641, TPS92515 or the LM3409. The TPS92515 and LM3409 will also work better with high frequency PWM dimming since they are hysterestic control.

    Best Regards,