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TLC5929: Headroom questions

Hardy Xue
Expert 3640 points
Part Number: TLC5929


Hi Team,

We design TLC5929 to drive 3S8P IR LED and LED power source is 5.5V.

We would like to clarify some question.

  1. How to define the headroom on each channel? Like VF_LED= 1.6V. the headroom on chip would be 5.5-1.6*3=0.7V
  2. If the headroom is dynamic, could you use the same LED power 5.5V to drive 2S LED on some channel? That means the headroom would be 5.5-1.6*2=2.3V

Best regards,

Hardy

  • +1
    TI__Genius 12915 points

    Hi Hardy Xue,

    1. Yes the output channels are current sinks to provide constant current but not voltage. The voltage dropped on output are determined by external circuit. To provide precise and constant current, the voltage need to be above the knee voltage in below image. Considering different bins of LEDs, 0.7V may be marginal for TLC5929.

    2. Yes it is OK that some channel series with 2 LEDs while some with 3 LEDs. The key point is leaving enough voltage for current sinks normal operation.

  • 0 in reply to Hardy Wu
    TI__Expert 3640 points

    Hi Hardy Wu,

    Thanks for your comments.

    By the way, I have one more question for EVM.  The LED is directly lit when we add 5V on EVM. Does it normal? Because I checked the datasheet, the default setting of OUT0 -OUT15 are off when device is powered on.

    Best regards,

    Hardy

  • 0 in reply to Hardy Xue
    TI__Genius 12915 points

    Hi Hardy Xue,

    No the LEDs should be off. You mean all LEDs are illuminated?