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BQ25173: BQ25173 supercap connection

Jose
Intellectual 305 points
Part Number: BQ25173

Hi,

We are evaluating the BQ25173 to charge a supercapacitor, but we have some doubts due to our type of application.
In our case, we can't have the supercapacitor on the electronic board.

The main board has the BQ25173 connected to the BACKUP pin of a device to ensure that the device is powered in the event of a power failure.
The same device BACKUP pin has a connector on the PCB where the supercapacitor is connected via a 15 cm long cable.

We intend to use a supercap of 2.5F to 7.5F depending on the version of the equipment.

The supercap has a Vmax of 5.4V, so we set the BQ25173 to 5.2V.
The charging current will be around 130mA

In this application, here are the questions:

- Will the BQ25173 work normally when the supercapacitor is connected to it with a 15 cm cable?

- When there is no supercap in the circuit, is the output voltage of the BQ25173 the same as it was programmed for?

Please note that the BACKUP device pin is connected to BQ25173

- In Power-On without a supercap inserted, is the BQ25173's output voltage stable or does it spike above the programmed voltage?

- Can the supercap be inserted and removed while the BQ25173 is active?

Thank you in advance for your comments

regards

Jose

  • 0
    TI__Guru 70636 points

    Hello Jose

    Jose said:
    Will the BQ25173 work normally when the supercapacitor is connected to it with a 15 cm cable?

    Bill J -- Yes this would be OK.  The lower charge current reduces risk of any problems.

    Jose said:
    When there is no supercap in the circuit, is the output voltage of the BQ25173 the same as it was programmed for?

    Bill J -- Yes, unit will go into voltage regulation.

    Jose said:
    In Power-On without a supercap inserted, is the BQ25173's output voltage stable or does it spike above the programmed voltage?

    Bill J -- No does not overshoot

    Jose said:
    Can the supercap be inserted and removed while the BQ25173 is active?

    Bill J -- yes.  Adding ESD protection to exposed connections would be a good practice. 

  • +1 in reply to Bill Johns
    Intellectual 305 points

    Hello Bill,

    Thank you for your quick and very informative reply.

    I'd like to ask you one more question that is very important to us.

    We have to disconnect the voltage divider from the output when there is a power cut, in order to maintain as much autonomy as possible.

    We don't have any Mosfet P in the BOM, so we'd have to add another component to the already long BOM.

    As we have Mosfet N (BSS123) in the BOM, is there a suggested circuit for this purpose but with a Mosfet N ?

    Thanks.

    regards

    Jose

  • +1 in reply to Jose
    TI__Guru 70636 points

    Hi Jose

    I have a design for this type of circuit based on a P-Type MOSFET see BQ2173-Q1 Evaluation Board User's Guide.

    The Q1 circuit controlled by /PG pin.

    Do not have an N-type design.