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TPS92515: Current still went through LED string with PWM 100% + IADJ 0V

CC Lai
Prodigy 110 points
Part Number: TPS92515

Hello Everyone,

This is our design schematics, basically it is based on datasheet instruction.

We have two input signals to in our design.

 

When we try to input PWM 100% (3.3V) + IADJ 0V, we can see LED string light up.

As I know,  there should be no current go through LED string when IADJ input is 0V.

Can someone help to explain this situation and give the suggestion how to solve this problem ?

  • 0
    TI__Intellectual 2590 points
    CC,

    Your understanding of the IADJ pin operation is incorrect. First off, when IADJ = 0, there could still be a positive nonzero current reference voltage due to the inherent input offset voltage of the buffer and comparator. More importantly though, there is leading edge blanking of the turn on time which yields a minimum on-time (see datasheet parameter). During this minimum on-time, the MosFET will turn on and slew the current up according to the slope (Vin-Vout)/L. Then when the minimum on-time expires, the FET will immediately turn off as it will already be past the current reference. To minimize this minimum output current, you can use a larger inductor (L) and a lower frequency target (off-timer). If you want true zero current, simply change your PWM input to 0% for true shutdown.

    -JP
  • 0 in reply to James Patterson
    Prodigy 110 points

    Hello James,

    Thanks for your reply.

    Is PWM signal a must item for TPS92515 ? can we just use analog signal to dim ?

    (Just image if the architecture has no PWM output, there is only one analog signal from Potentiometer)

    Thanks,

    Ryan

  • 0 in reply to CC Lai
    TI__Intellectual 2590 points
    I am not saying that you can't use IADJ to dim. It will indeed reduce the LED current reference linearly from the maximum to a minimum to create analog dimming function. All I was commenting about is your question about dimming to true zero current. For that functionality, as I explained, you would need to toggle the PWM pin low.

    -JP
  • 0 in reply to James Patterson
    Prodigy 110 points

    Thanks for your answer, I will do it.