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TPS92691: Buck topology is not providing constant LED current

keshava hv
Prodigy 50 points
Part Number: TPS92691

Hello,

we are using the TPS92691 Led driver we are trying to drive the led's at 9V @ 2.8A the input voltage to the driver varies from 9.4V to 12.6V but we are not getting the constant led current at the output, as input varies the LED current is also varying could you please suggest us a solution.  we are using the refrence design as per the datasheet whose image is attached below.

  • 0
    TI__Mastermind 18890 points

    Hi Keshava,

    There is a typo on the feedback to CSP and CSP.  The signals should be swapped on the feedback with Rcs.  We have this and will correct the data sheet on the next revision.

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Prodigy 50 points

    Hi Tuan,

    We have swapped CSP and CSN but still there is no improvement in the output , we are still getting linear current output but we need constant current output.

    Regards,

    Keshava.

  • 0 in reply to keshava hv
    TI__Mastermind 18890 points

    Hi Keshava,

    For input of 9.4V you will  not get any current since the floating buck has the diode in series and that is not enough to forward bias the LED but it should work with 10V input and above. 

    The information provide here is hard for us to trouble shoot what is going on.  Please share with us your actual schematic along with part list and layout.  We would like to see scope capture of the following signals on the same capture with input voltage at 11V.

    CH1: Soft start (pin 2)       CH2:  GATE (pin 15)         CH3:  Inductor current      CH4:  LED Current

    Note that inductor current can be measured with inductor lifted and wire a small loop for current probe.

    Thanks Tuan

  • 0 in reply to Tuan Tran74
    Prodigy 50 points

    Hi Tuan,

    i have attached the schematic for your refrence, will share the scope capture of the signals you have requested shortly.

    TI_LED_DRIVER.pdf

  • 0 in reply to keshava hv
    TI__Mastermind 18890 points

    Hi Keshava,

    As I mentioned before, with input of 9.4V to 12.6V and output voltage of 9V I would not recommend the buck topology.  I would recommend the buck boost topology.  You should look into getting an evaluation board and modifying it for your design. 

    As for your schematic C40 should be 1000pF instead of 0.47uF.  That high of a capacitance value will filter out the 390Khz current pulse that we are trying to measure.  The FET you use is an old FET and might not be good for the switching frequency you are trying to run at with the TPS92691 gate drive.  IMON has a 0.47uF on it...It should have 10pF.  This is first look.  Again, I highly recommend you get an evaluation board and use the buck boost topoloty.

    Thanks Tuan