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TPS922055DRRREVM: Determination of Led driver Vout

bekir safa kilic
Prodigy 55 points
Part Number: TPS922055DRRREVM

Tool/software:

Hi,

I did some test with tps922055evm and somethings confused my head. When supply voltage is 28V and no load condition, measurement of output voltage is about 26V. In load condition, my led forward voltage is 3.4V and 7 series. So ı measure output voltage and saw 23.8V in %100duty cycle. There is no problem so far. But when ı changed the load (3V x 5 led), Vout still 23.8V and my led blow. First question, why ı did measure 26V in first place? Second, Why didn't the output voltage change automatically when I changed the load?Because the datasheet says "VOUT is the sum of the voltage across LED load". Third question is how can ı be sure my led driver is working in cc mode? 

Thank you for feedback.

  • 0
    TI__Mastermind 30761 points

    Dear Sir,

    Could you pls kindly share me the photo of your EVM board and the configuration/setup? I would like to first check that there is no problem with the hardware and setup. Besides, in theory, it should work with not only 7 LED series but also 5LED series. Not sure if you change any components of the EVM board?

    Thank you!

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    Prodigy 55 points

    Dear Lucia,

    I have made some progress on evm board. That is why ı have new questions. Firstly, ı did not change any components of EVM board. When ı connected  3.4V and 7 series led series, the output voltage in%100 duty is correct. Changing of duty cycle is effecting Vout. After that, ı changed the load 3V x 5 led series and started from %0.1 duty. When duty comes %1, Vout is 15.6V and led current is typicall value, so ı can not increase duty anymore, because Vout increasing. I don't want to more voltage than 15.6V. I mean led brightness acting like pwm %100 in %1 duty.  It shows when change two different series led in same Duty, low one will blow. How can ı fix that problem?

    Note: I checked Dimming Mode Configuration, I used PWM Dimming mode.

    Example : Vin:28V Vf: 3V Series:5 Max output voltage:16V F:4khz 

    D %0.01 =12.47V D %0.1=12.99V D %0.5=14.23V D %1 =15.6V

    /resized-image/__size/1024x1024/__key/communityserver-discussions-components-files/196/Led-driver-tps922055.jpeg

  • 0 in reply to bekir safa kilic
    TI__Mastermind 30761 points

    Dear Sir,

    Kindly let me check your question and get back to you soon. Thank you!

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    TI__Mastermind 30761 points

    Dear Sir,

    Thanks for your feedback. I checked your question but maybe I am not fully understand what is the issue now. Actually, for LED, it's a normal characteristic that the LED voltage will increase with the current go through the LED. You can refer to the LED datasheet for it's IV curve to double confirm. I am not sure why the PWM dimming duty can't be increased from your side. Are you worried about that with the increase of the LED voltage, it might exceed 16V?

    Thank you!

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    TI__Mastermind 30761 points

    Dear Sir,

    As we got email feedback that issue is resolved, will close this thread first. Thank you!

    BRs

    Lucia