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TLC5947: What is the purpose of the Zener diode in PMOS line scan circuit?

Winter Lin19
Prodigy 40 points
Part Number: TLC5947
Other Parts Discussed in Thread: TLC5957

Hi,

  I want to design a 15x15 LED matrix by TLC5947 in Vled 3.3V.

My question is what is the purpose of the zener diode (in TIDA-01615 reference design and TLC5957 datasheet)?

and how to decide the spec in different Vled? in my case is 3.3V.

Thank you.

  • 0
    Guru 283620 points

    Section 2.3.2 of the reference design manual says:

    […] Another cause of the ghost is that the line voltage is not discharged quickly due to parasitic capacitance of the line. Thus, a Zener diode is added to discharge each line when the line MOSFET is off.

  • 0 in reply to Clemens Ladisch
    Prodigy 40 points
    But how to decide the power and the voltage of the Zener?
  • 0 in reply to Winter Lin19
    TI__Expert 3190 points

    Hi Winter,

    it's hard to give a specific value, since different LED panels may have different parameters.

    If the Zener voltage is set too high, the upperside ghosting would not be eliminated, if the Zener voltage is set too low, the LED will stand too high reverse voltage, which may decrease the life span of LEDs.

    I will suggest to set the voltage at the 0.6*VLED, with the LED voltage at about 4V~5V, and LED Vf at about 2~3V.

    Regards,

    Shawn.