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LP3855: Enable pull up resistor

Jeff Coletti
Expert 8755 points
Part Number: LP3855

Hi Support team, 

I need to ask this question because the data sheet is specifying that there be a 10k pull up on the enable pin. This values is specified multiple places in the data sheet. Why do i need a pull up and can i connect the enable pin directly to the VIN. Customer is getting ready for a large build and needs to be 100% sure.

Thanks

Jeff Coletti

  • 0
    TI__Mastermind 37005 points

    Hi Jeff,

    I assume you mean the ERROR pin?

    The ERROR pin is the output of an open-drain comparator.
    A comparator will try to pull the output to ground until a threshold is reached, then it will release and the resistor will pull the output to a high voltage.

    If you tied the comparator to Vin, the ERROR pin will effectively sink 1mA of current during the initial stages of startup, and whenever it detects the output voltage is outside of the regulation point.

    If you do not need the ERROR pin then the datasheet recommends that it be tied to RTN.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    TI__Expert 8755 points

    Hi Stephen, 

    Nope the enable pin, send you a direct email

    JC

  • 0 in reply to Jeff Coletti
    TI__Mastermind 37005 points

    Hi Jeff,

    I see it now, you mean the "shutdown" pin.
    I'll respond to your email but I also wanted to add the update below for future users.

    The 10k resistor is being used as part of an RC filter with whatever input capacitance is on the chip.
    This is to clean up the transition from enabled to disabled, and vice versa.

    The datasheet shows a lack of hysteresis on this pin in the electrical characteristics table, and it explicitly states this hysteresis does not exist in the "shutdown operation" section. Furthermore the datasheet explains a fast rising or falling edge from a digital device will be acceptable.

    If the customer can ensure that the device driving the shutdown (enable) pin is clean, the 10k resistor is not necessary.
    If there is noise on the signal driving the shutdown (enable) pin, the 10k resistor is needed.

    Thanks,

    - Stephen