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TPS92518-Q1: Shunt FET Dimming

Meng Liu
Intellectual 1755 points
Part Number: TPS92518-Q1
Other Parts Discussed in Thread: TPS92518, LM3409

Hi Expert

I have two question about TPS92518-Q1

1. what is shunt dimming? what difference between shunt dimming and PWM dimming, i just fund 'Shunt FET PWM dimming avoids this issue as the average current is affected during switching ON and OFF
times. Shunt FET PWM dimming can out-perform PWM dimming, but is more complicated to implement' in datasheet but I don't understood it?

2. I want to test TPS92518 EVM but I don't have LED, could I use load that work on constant current source mode for test?

  • +1
    TI__Intellectual 2620 points

    Hello,

    1. Shunt dimming is a form of PWM dimming where you bypass the LED string with a parallel FET, using the PWM signal. This allows the current in the buck LED driver to maintain the same current through the inductor, and the FET determines the duration for which current flows in the LED string. This gives more precise dimming than simple PWM dimming. If you look at LM3409 datasheet in section 3.8.3 the principle of external FET/shunt FET dimming shown as well. 

    2. Shunt FET dimming requires fast dynamics. An electronic load (which is what I think you mean by constant current source mode) will not be able to handle this type of dimming. It should be done with LEDs.

    Thanks

    Sumeet