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BQ21061: Reverse voltage on LS/LDO output

Part Number: BQ21061
Other Parts Discussed in Thread: TPS22916

Dear,

I have a question as in https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1104591/bq21061-reverse-voltage-to-the-ldo-of-bq21061?tisearch=e2e-sitesearch&keymatch=bq21061#, i.e. I need to connect an external voltage (~3V) to the OUTPUT of the LDO(LS/LDO pin) and not the input to program the MCU. Reading the answers to this previous post I understand that possible damage could happen if an external voltage was applied to the LDO output when no input was applied since the body diode of the mosfet between VINLS and LDO conducts and this current path is not a normal behaviour.
To solve this issue I would like to know if it is possible to connect a schottky diode between VINLS and LDO with the anode connected to the LS/LDO pin (in our configuration VINLS is connected to PMID). In this manner when we will apply a voltage to the LS/LDO pin the schottky diode will conduct and bypass the electronic components inside BQ21061 between VINLS and LS/LDO pins avoiding possible damage of this electronics. During normal behavior the schottky diode will not conduct since the VINLS pin will be greater than LDO output.

Please not that, in our application, we cannot insert a schottky diode in series with LS/LDO output to prevent current to flow in reverse direction since at power up the BQ21061 has the LS/LDO output by default to 1.8V and the voltage drop with this schottky series diode will lead to a voltage less than 1.7V that is the minimum voltage for power up our microcontroller.

Thanks

Francesco

  • Hi Francesco,

    What value is the shotkey diode? This could be one way to protect the LDO fet, however having current flowing into PMID while the device is in ship mode could potentially have unexpected behavior, this is not something we typically recommend, particularly because many of the internal circuits are disabled to minimize ship mode power consumption.

    A potential way to circumvent this would be to have the shotkey to ground in series with a FET that is driven only when your external voltage is applied. Would that be a viable solution?

    Best Regards,

    Juan Ospina

  • Dear Juan,

    the schottky diode that we intend to use has a forward voltage of about 0.25V for 10mA current. Aniway the solution that I proposed, in addition to your controindications, has the problem that, in our configuration we have VIO pin connected to the LS/LDO pin and therefore appling voltage on LS/LDO pin inject a voltage also in VIO pin that could damage some internal circuits of BQ21061.

    To solve all problems above, another solution, could be to connect at the LS/LDO output a load switch with reverse current blocking (as TPS22916). In this way when we will applied an external voltage to the output of the TPS22916 load switch (the ON pin of the TPS22916 will be connected to the VIN pin of the TPS22916) this voltage will not applied to the input of the TPS22916 (that is the output of the BQ21061 LDO). I have only one doubt using this solution, i.e. when we will applied a voltage to the output of the TPS22916, the output of the LDO of BQ21061 (input of the TPS22916) will be floating (BQ21061 will be completely turn off since we have a battery pack connected but with battery protection circuit open that disconnect battery) but I think that the smart pull-down of the TPS22916 is active and therefore the load switch will be disabled and reverse current blocking will be active without need to reach a minimum reverse current activation of 500mA (IRCB).

    Using this load switch (TPS22916) would be a good solution for us since it has a very low power consumption behaviour.

    Which is your opinion for this solution?

  • Hi Francesco,

    I believe this would be a good solution as the TPS22916 reverse current would prevent the flow of current through the LDO body diode. I will have to read more about the TPS22916 and get back to you as that concern could be valid. The LDO also has a drain resistor while disabled but I am not sure it is active while in ship mode, I will have to check.

    Best Regards,

    Juan Ospina.

  • Hi Francesco,

    After some testing, the LDO output has a pull down drain resistor that is enabled both when the LDO is disabled and also when the device enters shipmode. Its a resistor of about 150 Ohms so there shouldn't be too much of a concern that the enable of the TPS22916 might be floating.

    Best Regards,

    Juan Ospina

  • Dear Juan,

    thanks for your answer. We have a BQ21061 evaluation board and we are waiting also for the TPS22916 evalauation board. We will test the solution with TPS22916 connected to the LDO output of the BQ210161 to understand better if the load switch will block the reverse current and then I come back to you with the result of our test, but, as you suggested in your last message, I think that the pull down drain resistor of the LDO keeps in off-state the TPS22916 when external voltage was applied at its output. 

    Best Regards

    Francesco