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Delayed EN with a capacitor

Mark Pieper
Expert 4630 points
Part Number: TPS709-Q1

Hi,

Can I implement a delayed turn-on by connecting the EN to a capacitor to GND? The internal 300nA current source will charge the cap up and when it reaches 0.4-0.9V or so the LDO will turn on. I understand that this is not a precision circuit and that the turn on threshold varies. I also appreciate that during rapid power cycling the delay may not occur.

Is there a path internal to the device when Vin is removed for this EN capacitor to decay? If not I'll add a discharge resistor.

Thanks,

Mark

  • 0
    TI__Guru 55940 points
    Hi Mark,

    This is an interesting idea. As you mentioned, the enable circuitry is not a precision circuit and the delay may not occur with rapid power cycling. If you were looking for those, we would recommend a voltage supervisor to drive the enable pin. As long as you are not using an external pullup and relying only on the internal pullup, it should work; however, it should be noted that we have not benched this application and recommend prototyping.

    EN is clamped by a 6.5 V Zener diode. This diode will provide a discharge path for your capacitor. If Vin were to discharge faster than the enable capacitor can discharge, the capacitor would then discharge into the input again through diodes in the internal pullup path.

    Very Respectfully,
    Ryan