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TPS92515-Q1: Constant Current LED Driver RGB LED color problem due to output deviation when outputting low current.

Grady Lee
Genius 3015 points
Part Number: TPS92515-Q1
Other Parts Discussed in Thread: TPS92515

Hi, TI expert

Constant Current LED Driver Inquiry about RGB LED color problems due to output deviation when outputting low current.

Using three TPS92515-Q1, RGB LED is currently being used in 3S68P structure.

At the lowest brightness, the current of about one channel is controlled around 30mA, but the green brightness value is large compared to Red/Blue, so the color difference is felt even with small deviations.

The color changed after replacing only the corresponding channel driver chip of the product where the green was strong.

It would be nice if there is no current control deviation, but if not, please consider whether there is a way to reduce the deviation with the peripheral circuit configuration.

Please provide detailed answers to the above issues.

  • 0
    TI__Mastermind 28655 points

    Hello Leo,

    What is your current sense resistor value?  What is Viadj when current is 30 mA?

    If you are dimming to low Iadj levels you may want to look at the TPS92515A.  The TPS92515-Q1 has a turn on calibration that can change the current sense error amplifier offset on power cycle.  The TPS92515A does not have the calibration circuit though it's set-point accuracy is a little less.

    Best Regards,

    Irwin

  • 0 in reply to Irwin Nederbragt
    Genius 3015 points

    Hi, Irwin

    I will reply again after checking.


    Also, I would like to show you the circuit diagram and inquire, but the customer is reluctant to disclose the circuit diagram.


    Give me your personal email address, and I'll send you a schematic.


    Please provide your personal email address.

  • 0 in reply to Grady Lee
    Genius 3015 points

    Hi, Irwin

    VIadj voltage is 236mV for one board and 240mV for one board when measuring two boards.

    However, it is a problem with color differences.

    I don't understand exactly what function to describe the calibration function, so please check which part of the schematic can be modified in what direction.

    Give me your personal email address, and I'll send you a schematic.

    Please reply.

    Thank you.

  • 0 in reply to Grady Lee
    TI__Mastermind 28655 points

    Hello Leo,

    At Viadj of 236 mV or 240 mV puts Vcs at 23.6 mV or 24 mV.  The TPS92515 or TPS92515A are not designed to be accurate analog dimming parts.  The turn-on calibration can change the error amplifier offset by 5 mV increments which can happen during power cycling (this is for the TPS92515).  The TPS92515A does not do this but its set-point accuracy is less, worst case +/- 15 mV.  To get more accurate dimming PWM dimming can be used.

    Also note that green LEDs tend to be much more efficient, more lumen output at a given current over red and blue.  It would be best to look at the current and see if it is higher on the green LEDs to make sure it is a current regulation issue.

    I also sent you an email with my email address.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Genius 3015 points

    Hi, Irwin

    The schematic for the question below was sent by mail.

    Please check the attached circuit diagram, and also review any areas on the circuit diagram to be modified.

    Thank you.

  • 0 in reply to Grady Lee
    TI__Mastermind 28655 points

    Hello,

    I looked at the schematic and have a few other questions:

    What is the input voltage and input voltage range?

    What are the LED string votlages, R, G, and B will be different?

    What is your maximum output current?

    What is your desired switching frequency?

    Using PWM dimming will make the light output more accurate when dimming low and they have output capacitors.

    Best Regards,