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TPS92602EVM: PWM Dimming at Low Currents

Richard Leath
Prodigy 240 points
Part Number: TPS92602EVM

We are evaluating PWM-Only dimming for a new LED driver design using the EVM module.  We configure the EVM as a buck-to-battery converter.  Power in = 19V.  Current range:  0 -700mA.  Vout at 700mA = 10V.  Sw Freq = 600kHz

For any output current above about 100mA, the duty cycle at the switching node is about 50% when PWM is ON.  This is expected. .However, as the PWM Duty Cycle is further reduced, the duty cycle at the switching node quickly increases to almost 100% and the output current rises to about 140mA.  As the PWM Duty Cycle is further reduced, the current decreases from 140mA to zero.

Exactly the same behavior occurs if the PWM Duty Cycle is increased from 0.  What is causing this "peaking" of output current at 140mA?

We are using 20kHz for the PWM Frequency.   We tried 2kHz and found that the peak is still there but it occurs at about 14mA (factor of 10 lower current).  We would like the current to decrease monotonically (not necessarily linearly) to 1mA.  Is there something we can do to achieve that?

I can send scope photos if necessary.

Thanks,

Richard

  • 0
    TI__Expert 5475 points
    Hi Richard,

    We've never seen that on our EVM. Would you mind clarifying a little bit more about the issue? At what PWM duty cycle did the switch node duty cycle and output current get wrong? It would be better if you can share some more details and scope photos.
    By the way, may I ask why you are using such high frequency (20kHz) for PWM dimming? What is the application that you want to use this device in? Thanks.

    Best Regards,
    Shirley He
  • 0 in reply to Shirley He
    Prodigy 240 points

    Hi Shirley,

    I am inserting a spreadsheet where we captured PWM Duty Cycle vs Output Current.   The PWM "index" is 0 for Duty Cycle = 0, up to 2400 for Duty Cycle = 100%.

    We are using 20kHz because we notice acoustic noise when using 2kHz.  We have 6 channels of LED drivers on our product.  We want to get the PWM frequency about audible to eliminate any audible noise.

    At the bottom of the spreadsheet, there are some scope photos showing the Switching Node and the PWM signal at several different "index" values.

    I can send you a video that shows the current being ramped up.  How can I send it to you?  It is a .MOV file that is 22MB.

    Regards,

    Richard

    TPS92602 EVB Current Curve with PWM dimming_20180410 for TI.xlsx

  • 0 in reply to Richard Leath
    TI__Expert 5475 points

    Hi Richard,

    According to our previous test, the EVM can always achieve current decreasing monotonically with PWM dutycycle decreasing, as the figure in the datasheet shows.

    The EVM can be configured as boost or boost-to-battery topology. May I ask how did you configure it as buck-to-battery? I can also help to test on the EVM and figure out what happened.

    Best Regards,

    Shirley He

  • 0 in reply to Richard Leath
    TI__Genius 14295 points
    Hi, Richard,

    I'll close this thread due to inactivity and if you have more questions please reply to it directly and it will open again.

    Please help to click "Resolve" if the problem is solved.

    Thanks.
    Regards,
    Kenneth
  • 0 in reply to Kenneth Du
    Prodigy 240 points
    Please don't close the issue yet. I am gathering more information. I'll get back to you soon.

    Richard
  • 0 in reply to Richard Leath
    Prodigy 240 points
    We are going to use PWM dimming from 0 - 35mA, and Analog Dimming from 35mA to 700 mA. We prototyped it using the EVM and it works fine.
    Issue Resolved.
    Thanks for your help.
    Richard