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About Boost output discharge function

GARAM AN
Intellectual 1990 points
Other Parts Discussed in Thread: LP8863-Q1

Hi all,

I know this function discharges boost output voltage as the discharge pin sinks few mA current when the boost is disabled.

I thought the boost output voltage will be reduced automatically, when there is no discharge function.

I want to ask about why boost output discharge function is needed?

Which situation this function is usually used?

Thank you

  • 0
    TI__Mastermind 21565 points

    Hi Garam:

    That's a good question. May I know which device you are looking?

    If the Boost converter doesn't work in Force PWM mode, the device won't sink too much current generally. After it disable with no load, the Vout remains high voltage for a long time. (Which mainly depends on Vout, Cout, Ileak, and the current through the feedback resistance net.)

    Below are some cases I just think of:

    1. Some customers prefer the initial status of the power rails are certain, to ensure the power on sequence or other purpose.  

    2. For some usage like PD charger, the Vout range is very wide, and can reach 20V. If the Vout remains 20V when load is plug out, and then connect it with another load with just 5V voltage rate. It's harmful.

    3. For some load, removing the voltage supply is only way to disable it. 

  • 0 in reply to Minqiu Xie
    TI__Intellectual 1990 points

    Hi Minqiu

    Thank you for the answer.

    The device I'm looking is LP8863-Q1 boost type LED Backlight Driver.

    I could not find some obvious reason in the automotive part(especially on the CID, cluster, display)

    Can I think like this?

    If output voltage keeps long time when the device turned off, LED keeps turning on even I want to turn off the LED, because there is current if there is voltage. 

    Thank you

  • 0 in reply to GARAM AN
    TI__Mastermind 21565 points

    Hi Garam:

    For simple LED usage, the LED won't turn off before Vout drops.

    But I'm not sure if LP8863 can stop the leakage current through the LED after disable. If it can, the LED would turn off even Vout still high. 

    I'll recommend you to check with LED team with a new post. 

  • 0 in reply to Minqiu Xie
    TI__Genius 11836 points

    Hi Garam,

    The output voltage of boost will discharge slowly through leakage current of output channel if discharge feature is unused. When the boost is disabled, the discharge pin typically sinks 30-mA current in order to discharge the boost output voltage. The boost voltage discharge time depends on output capacitance. The discharge function is enabled for a maximum of 400 ms or until the output voltage falls below 3.3 V.

                                                                                                

  • 0 in reply to Charles-Chen
    TI__Intellectual 1990 points

    Hi Charles,

    Which application and situation this function is usually used?

    If the output voltage reduced slowly, I think I does not matter in common situation.

    Thank you

  • 0 in reply to GARAM AN
    TI__Genius 11836 points

    Hi Garam,

    It is depended on the requirements of the customers' system.