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BQ25173-Q1: Management of the CE pin

Jose
Intellectual 305 points
Part Number: BQ25173-Q1
Other Parts Discussed in Thread: BQ25173

Hi,

We are designing a circuit that will use the BQ25173, but we have a peculiarity that is not common.

So, when the power is turned on, and the BQ25173 is powered, the microcontroller may or may not be powered.

We want the BQ25173 to always have the charge activated, regardless of whether the microcontroller is powered or not.


1 - The question in this case is whether the internal pull-down on the CE pin ensures that the BQ25173 is always in charge enabled mode, or whether it would be better to add an external pull-down.

2 - When the microcontroller is activated, and we want the BQ25173 to continue with the charge active, the microcontroller I/O is forced to LOW, in which case the voltage can reach 0.8V.

Since we need to ensure that the voltage on the CE pin is lower than 0.4V, is it better to add a diode?

3 - Is it necessary to add a TVS and/or inversely polarized schottky diode to the output of the BQ25173 to suppress voltage peaks when the supercapacitor is inserted or removed?

In our application, the supercapacitor can be added or removed while the system power is on.


4 - Does the BQ25173 have output protection against connecting the supercapacitor with the wrong polarity (if it is charged)?

Thank you in advance for your comments.

regards

Jose

  • 0
    TI__Guru 70796 points

    Hi Jose

    Jose said:
    1 - The question in this case is whether the internal pull-down on the CE pin ensures that the BQ25173 is always in charge enabled mode, or whether it would be better to add an external pull-down.

    Bill J -- The CE pin has a 3.3M pull down resistor and V-Low is 0.4V max.  The leakage current from microcontroller I/O could be large enough to require external pull down.  

    Jose said:
    2 - When the microcontroller is activated, and we want the BQ25173 to continue with the charge active, the microcontroller I/O is forced to LOW, in which case the voltage can reach 0.8V.

    Bill J -- yes this may require a diode or possible external mosfet make sure CE pin remains below 0.4V.

    3 - Is it necessary to add a TVS and/or inversely polarized schottky diode to the output of the BQ25173 to suppress voltage peaks when the supercapacitor is inserted or removed?

    Bill J -- TVS and output capacitor would be a good idea.  The concern would be ESD damage also.

    Jose said:
    4 - Does the BQ25173 have output protection against connecting the supercapacitor with the wrong polarity (if it is charged)?

    Bill J- Sorry but no.

  • 0 in reply to Bill Johns
    Intellectual 305 points

    Hi Bill,


    Thank you for your clarifications, which were very objective and very useful.

    We are completing the project and have an additional question regarding the charge current.

    The datasheet says

    For charge currents below 50 mA, an extra RC circuit is recommended on the ISET pin to achieve a more stable
    current signal.

    We found that the BQ25173-Q1EVM circuit allows charge currents between 0 and 800mA and has no additional RC circuit.
    We couldn't find a more specific reference elsewhere to the type of RC circuit we should adopt.


    Since we're going to have low charge currents, what do you suggest we do?

    Regards

    Jose

  • 0 in reply to Jose
    TI__Guru 70796 points

    Hi Jose

    The standard RC network is 4700pF / 2k. But with the large output C this may not be needed, recommend evaluating performance of your circuit.