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Original question:

TPS7A05: Maximum EN input resistance

TPS7A05: Maximum EN input Pull down resistance

MattLin
Intellectual 1050 points
Part Number: TPS7A05

I recently created a thread regarding the input pull up resistance. By the looks of it, the TPS7A05 doesn't have an internal pull down resistor, so what should the pull down resistance on the EN pin be in relation to the pull up?

Thanks,

Matt

  • 0
    TI__Expert 4435 points
    Hi Matt,

    I don't recommend using a pull-down resistor on the TPS7A05 EN pin, unless you are trying to set a different enable voltage.

    2 most common configurations for EN pull-up:
    1) Pull-up resistor from EN to IN. Regulator will always be enabled with IN > 0.9 V.
    2) Pull-up resistor from EN to IN and a transistor to pull EN < 0.4 V to disable the regulator. If the transistor is OFF and IN = EN > 0.9 V, LDO will be enabled. If the transistor is ON and EN < 0.4 V, the LDO will be disabled.

    If you use a resistor pull-up from EN to IN and a pull-down resistor from EN to GND, you create a voltage divider that shifts the IN voltage range where the LDO will be enabled and disabled. This could be used to adjust the minimum IN voltage where the LDO will be enabled.
  • 0 in reply to Eric.Preiss
    TI__Intellectual 1050 points
    Hi Eric,

    Okay thanks. My only question would then be how does the device discharge the EN pin when the device is disabled?

    Matt
  • 0 in reply to MattLin
    TI__Expert 4435 points
    Hi Matt,

    In case #1, the charge in EN pin flows through the pull-up resistor to ground when IN = 0 V.
    In case #2, the charge in EN pin flows through the transistor to ground when the transistor is on and pulling EN low to disable the LDO.
  • 0 in reply to Eric.Preiss
    TI__Expert 4435 points
    Hi Matt,

    You can also omit the pull-up resistor on EN and enable the LDO in either of these ways:

    1. Pull-down resistor from EN to GND to disable the LDO. Use a switch (FET, BJT) to connect EN to IN to enable. You can also use a driver to apply > 0.9 V to EN to enable the LDO.

    2. Push-Pull driver to apply > 0.9 V to EN to enable and < 0.4 V to disable. Care must be taken to ensure a known voltage on EN when the driver is unpowered, otherwise the LDO could be inadvertently enabled.