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BQ2973: Select MOSFET

Michael Jeong1
Expert 2610 points
Part Number: BQ2973
Other Parts Discussed in Thread: CSD85301Q2

Hello Expert,

Our customer have a plan to use BQ29728 or BQ29737.

As you know, there is lead time issue recently.

So our customer want to use CSD85301Q2 which is stock in TI.COM.

Below is their target cell spec. and will use 2 type of Cells.

Is there some issue or no problem if they use a combination "BQ29728 or BQ29737 + CSD85301Q2" to protect 2 type of Cells?

If there is some issue, which electrical spec. is the problem in MOSFET?

Also, could you please recommend right part of MOSFET to me?

I'm looking forward to receiving your good comment and guide.

* Battery cells spec.

1. type 1

2. type 2

Best regards,

Michael

  • 0
    TI__Mastermind 33620 points

    Hello Michael,

    The MOSFET selection wouldn't be limited what type of cell you are using, but more by the operating conditions you expect. I would recommend you read the Design Example Procedure shown in Sections 10.2.1 and 10.2.2 of the datasheet. Choose the FET following these steps and your conditions.

    Based on the Rdson of the MOSFET they chose (~25-mOhm @ ~3.5-Vgs) and following the design example (which assumes a OCD of 100-mV, same as the BQ29737). Your part would not be able to discharge at even 1C without triggering the protection (100-mV/50-mOhm = 2-A) OCD would trigger at 2-A. Using the BQ29728 would discharges of up-to 3-A (As OCD in this part is 150-mV).

    Again, I would recommend your customer to read these sections in the datasheet, and decide on what they would like to do based on their intended operating conditions.

    Best Regards,

    Luis Hernandez Salomon

  • 0 in reply to Luis H. S.
    Expert 2610 points

    Hello Luis,

    Thanks for your kind support.

    I have a two questions.

    Could you please give your reply to me about each questions?

    First,

    Our customer have a plan to set "Max. Charging Current 402mA, Max. Discharging Current 300mA".

    So they are going to use "BQ29737 + CSD85301Q2" combination.

    Is there no problem?

    Give me your opinion.

    Second,

    In case of BQ29737, Is the voltage of COUT, DOUT pin the same as the BAT voltage?

    Best regards,

    Michael

  • 0 in reply to Michael Jeong1
    TI__Mastermind 33620 points

    Hello Michael,

    1) Our customer have a plan to set "Max. Charging Current 402mA, Max. Discharging Current 300mA". So they are going to use "BQ29737 + CSD85301Q2" combination. Is there no problem?
    Again, MOSFET selection would depend on their system. For those charge and discharge currents there should be no issues. But there are many factors they have to take into account when selecting a FET. What protection feature thresholds do they have, when do they want them to be triggered, etc. Depending on what their needs and protection worries, they can select the FET based on its gate-source threshold and RDSON resistance. See your FET's RDSON vs VGS graph for your operating ranges.

    2) In case of BQ29737, Is the voltage of COUT, DOUT pin the same as the BAT voltage?
    See Section 7.5 - FET Output, DOUT and COUT of the datasheet. But yes, essentially it will be around ~BAT voltage. This is why it is important to see the FET's RDSON vs VGS graph, as it could greatly increase the FET resistance if the BAT voltage is outside the intended ranges. For your particular FET, I don believe it is too much of an issue, but ideally you want a RDSON that would remain more constant with your varying cell voltage. At 3-V, your FET would have a RDSON closer to 30-mOhms per FET, at 2-V your RDSON  may reach ~50-mOhms per FET. So just keep these in mind when selecting a FET.

    I hope these helped.

    Best Regards,
    Luis Hernandez Salomon