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TPS92515: What is the dimming ratio at 20kHz Shunt FET Dimming

Art Mecina
Genius 16505 points
Part Number: TPS92515

Hello Guys,

Good day.

Our customer wanted some clarifications on TPS92515 dimming ratio: There is a dimming range sited on the first page of the TPS92515 datasheet for each of the three methods: 10,000:1 for Shunt PWM, 1,000:1 PWM, and 200:1 Analog. In Section 8.3.11 an example is given that for (Series) PWM the dimming range reduces from 1,000:1 to 500:1 at 10kHz, because of the 200ns minimum gate delay + slew time. That, the customer understand. What he do not understand, and would like to understand, is how to similarly estimate the Shunt PWM dimming range (through theory not simulation).

He understand the external FET could be a limiting factor, but assuming that is well chosen, what about the TPS92515 limits the dimming range? For example 10,000:1 has been chosen, how was that calculated?

Thanks and regards,

Art

  • 0
    TI__Mastermind 28655 points

    Hello Art,

    10,000:1 is when shunt FET dimming at 500 Hz, see Figure 27.  This is the performance of the design created, not calculated.  200 ns gate delay doesn't have anything to do when shunt FET dimming, that describes PWM dimming using the PWM pin.  There is more to this as well since the inductor current cannot slew that fast.  The TPS92515 can regulate current with a shorted output if the design is setup correct.  The delays will be the Shunt FET and Shunt FET drive.  The second Roff will be needed for this to keep current regulating when the output shunted.  So you could have a 50 ns on time.

    What you will run into is narrow on-times will interact with the current ripple.  Generally to avoid perceivable flicker the minimum LED on-time should be 10X the switching frequency.  If current ripple is +/- 30% and you try to dim to a narrow on pulse of 200 ns you could see the light vary by 30% since the PWM in asynchronous to the switching frequency.  Higher switching frequency helps dimming range.  Lower current ripple also reduces this effect.

    Best Regards,