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TPS61187: Stops working after a while

Torben Brendstrup
Prodigy 70 points
Part Number: TPS61187

Hi,

We are attempting to use the TPS61187 for driving 60 white leds at 20mA, and chips are failing after a while.

Here is the schematics, LEDs have been left out, but are exactly as specified in the datasheet and evm.

Input voltage is 12V which is supplied over a 1m to 1.5m long power cord. PWM is applied with duty cycles ranging from 0 to 1. 

Is there anything that comes to mind that would make the TPS61187 fail over time (ranging from DoA to weeks or months)?

  • 0
    TI__Intellectual 1780 points
    Hi Torben,

    Thank you for the question.

    Could you please share more about configuration of TPS61187 for driving 60 white leds at 20mA?

    What is the value of Rset in your schematic? I'm not able to clearly see that value on your schematic. Is it 19kohm in your schematic, in which case, it does not correspond to 20mA per channel.

    Also what is the output voltage (or forward voltage, Vf of the LEDs you're using)?

    Thank you,
    -Harini
  • 0 in reply to Harini Sankabattula
    Prodigy 70 points

    Hi Harini,

    Sorry for the long answer, I missed the notification mail.

    Rset is 39kohm - which I can see is not 20mA but 30mA

    Vf is typically 3.2V per led, or 32V for the 10 leds in series. The LED is Cree C512-WNN

    I can of course test with Rset at 60.4kohm instead, but 30mA is still within spec.

  • 0 in reply to Torben Brendstrup
    TI__Expert 4550 points
    Hi Torben,
    can you describe more about your failure mode? your application looks good to me, have you checked operation waveforms while it's working? And your input cable looks pretty long, have you checked the resistor of this cable?
  • 0 in reply to Caryss Li
    Prodigy 70 points

    Hi,

    It stops regulating when enabled. And thus Vin ~ Vout = 12V.

    I have taken some waveforms before it stops working that may help you.

    VIN transient when power is applied, there is a bit of overshoot to 15.4V:

    VOUT Transient, a smooth ramp to 16.8V

    Then, PWM is applied, which ramps the voltage from 12.4V to 31.6V over ~30ms:

    But wait, we started at 16.8V so why is it 12.4V now? So i looked at a longer time frame:

    So at power-on, the output voltage goes to 16.8V then ramps down to 12V and when PWM is applied, it rises to 31.6 and holds steady at 30.8.

    There is a barely audible tick when it's powered on.

    Is this any help to you?

  • 0 in reply to Torben Brendstrup
    TI__Intellectual 1780 points
    Hi Torben,

    Thank you for sharing the details. Please give us some time to review your measurements and one of us will get back to you next business day (Monday, Mar 19th).

    Thank you,
    -Harini
  • 0 in reply to Harini Sankabattula
    TI__Intellectual 1780 points
    Hi Torben,

    Could you please share more details about the failure mechanism?

    For load, you've 10 LEDs (Vf = ~3.2V) in series, so ~31V is expected. You said "Stops working after a while", is it a few seconds of leaving the LEDs on? You also said "Is there anything that comes to mind that would make the TPS61187 fail over time (ranging from DoA to weeks or months)?"

    Your conditions (30mA and 32Vboost) are within DS spec, so you shouldn't hit thermal limit. However, did you try with 20mA/channel load setting?

    Also does your input supply have enough current limit?

    Best,
    -Harini
  • 0 in reply to Harini Sankabattula
    Prodigy 70 points

    Yes, stops after a while is the timeframe i gave:ranging from DoA to weeks or months.

    We will try with 20mA/channel as soon as I can.

    I'm not sure exactly what you mean by your last question. The power supply is rated at 12V 7.5A, with 4A for another pcb, and 1.75A each for two of these.