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TPS92691: TPS92691 dimming ratio and High-side series FET problems

Iris Shen
Expert 3491 points
Part Number: TPS92691

Hi Tuan,

     Sorry for bothering you again. Recently I am learning TPS92691 for my review presentation, so I still have some questions:

     1. How to calculate the 1000:1 PWM dimming ratio? I know the ratio=1/minimum duty cycle=PWM minimum on time/T. But you said the PWM minimum on time depends on the system, and the PWM frequency can be 100Hz~1KHz. So how does the 1000 come from?

    2. I want to know how to select low-side series NFET or High-side PFET for PWM dimming. Why the 001EVM choose high-side? Is it for short circuit protect consideration?

I know to control the slew rate the low or high demands different circuit design, is it an important issue to consider?

    3. I found if set average Iout=300mA, peak Iout=400mA, duty cycle=75%, the PWM dimming efficiency is lower than analog Iout=300mA.

Is it normal and what cause it? 

     Thanks again!

  • 0
    TI__Intellectual 1675 points

    Hi Iris,

    I apologize for the delay on this. We will answer you within 24 hours.

    Best Regards,

    Caroline

  • +1 in reply to Caroline Fuschino
    TI__Genius 17640 points

    Iris,

    1.  The dimming ratio is defined as the PWM on time divided by the 1/frequency of the PWM'ing.  Another word, for 100Hz dimming frequency and the ON time of the dimming is 10uS then the dimming ratio = 10uS divide by .01 which equals to 0.001 or ratio of 1:1000.

    2.  The EVM has both high side and low side dimming FET in the design.  This is to let the customer have the flexibility of choosing whatever is appropriate for their design.  The FET should be chosen to carry both the current and voltage that is being PWM to the LED load.

    3.  Efficiency depends on operating conditions of the input voltage and load ect for both PWM dimming and analog dimming.  It's not always the same ad your theoretical so you cannot just compare them that way.

    Thanks Tuan