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TPS7B87-Q1: External pull-up on PG pin

Naveen Bevara
Intellectual 1970 points
Part Number: TPS7B87-Q1

Hi team, 

One of my customer would also like to know if a pull-up resistor can be connected externally between the PG pin and Vo, since there is a requirement from the microcontroller side. What would be the impact on PG behavior due to this addition?

Regards,
Naveen

  • 0
    TI__Expert 6825 points

    Hi Naveen,

    It is valid and you can do that the issue is that the switch inside the LDO has a resistance of about 360ohms in the worst case, so you will just need to decide if that voltage that is created between the 4k pull up and the 360ohm voltage divider is going to be low enough to trigger all your logic thresholds.

    Regards,

    Mark

  • 0
    Prodigy 60 points

    Hello Mark,

    I would just like to confirm my understanding. Does this mean the circuitry internal to the LDO at the PG pin can be considered as 360 Ohm between PG and GND?

  • 0 in reply to User
    TI__Expert 6825 points

    Hi Manvitha,

    Yes that is correct that is what the impedance of the FET that is used to pull down looks like. So if you model it as a 360 ohm you should be fine in your simulations.

    Regards,
    Mark

  • 0 in reply to Mark Sellers
    Prodigy 60 points

    Hello Mark,

    In continuation to the above question, we would be seeing this scenario during the programming of micro-controller. The LDO would not be supplied, VDD5 is directly supplied externally. Since PG is pulled up internally, does the 30K resistor still come into picture?

  • 0 in reply to User
    TI__Mastermind 37925 points

    Hi Manvitha,

    Are you saying that the input to the LDO is not powered while VDD5 is powered by 5V? If so, this will probably damage the device. 

    I'm not sure what you mean about the 30k resistor. Are you expecting PG to work in this scenario? Or are you just asking if the 30k resistor will contribute to the node impedance?

    Thanks,

    Nick

  • 0 in reply to Nick Butts
    Prodigy 60 points

    Hi Nick,

    1. Regarding applying 5V at output of the un-powered LDO, are we expecting damage due to the flow of reverse current? Could you please provide the reverse current capability of the device?

    2. We do not need the PG to work during the programming scenario, should we consider the 30k in this case?

  • 0 in reply to User
    TI__Expert 6825 points

    Hi Manvitha,

    There is no specified reverse current capability of this part, but in general we would recommend to keep it to less than 5% of the rated current for the LDO. Whether or not this will cause damage will depend on what the impedance of the input is. If you have a low impedance source connected at the input the diode will source a lot of reverse current and this will cause damage to the LDO. If you have a high impedance source at the input the diode will charge the input capacitors and then there will no longer be any reverse current. The LDO will most likely survive this situation as long as the input capacitance is small enough (A couple hundred microfarads or less).

    Regards,

    Mark