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BQ2970: MOSFET Recommendation for Lifepo4 cell

Chaitanya Dasari
Prodigy 210 points
Part Number: BQ2970
Other Parts Discussed in Thread: CSD85302L, CSD83325L

Hi 

We are looking to use BQ29706 for a design powered with a single lifepo4 cell.

Design requirements

Continuous Charge current < 0.5A

Continuous Discharge current < 5A

As per the datasheet, OCC and OCD depend on the Rds(on) of the charge and discharge fets.

As per the calculations

Rds(on) = 40mohms and with that Pd = 1W in Discharge mode but  during SCD the power will be around 9W  and I am not able to find a MOSFET that certainly meets all the requirements 

what should be the pulsating current and all other MOSFET parameters what do we need to consider and recommend general specifications of the MOSFET that can work for my application? and what is the layout recommendations for the Shortciruit current?

  • 0
    TI__Genius 17320 points

    Hi Chaitanya,

    One thing I would note is that 40 mOhms is your total Rds(on) across the two MOSFETs, so each individual MOSFET will need to have an Rds(on) of 20 mOhms. The 9W will also be distributed between the two MOSFETs, so the FETs you're looking for only need to be rated for 4.5W, and since the SCD delay on the BQ29706 is 250 us, it only needs to handle the 4.5W for that long.

    If you still need assistance in finding a MOSFET with those parameters, I would be happy to loop in the MOSFET team to assist you.

    As for the layout recommendations, please see Section 12: Layout of the datasheet to find layout guidelines and an example.

    Regards,

    Max Verboncoeur

  • 0 in reply to Maxwell Verboncoeur
    Prodigy 210 points

    Hi Max

    Thanks for the details. One thing I missed mentioning was, I was looking for 2N Channel Common Drain Fet Array.

    Sure, I would be grateful, if you and your MOSFET team help me with a suggested MPN for my application.

    Regards

    Chaitanya 

  • 0 in reply to Chaitanya Dasari
    TI__Genius 17320 points

    Hi Chaitanya,

    I've added in the FET team in order to help recommend you a MOSFET that will suit your needs.

    Regards,

    Max Verboncoeur

  • 0 in reply to Maxwell Verboncoeur
    TI__Mastermind 31365 points

    Hello Chaitanya,

    Thanks for your interest in TI FETs. TI has a few common-drain dual N-channel FETs including the CSD85302L, 20V chip-scale device in 1.35mm x 1.35mm LGA package. Source-to-source on resistance is 24mΩ max at VGS = 4.5V & TC = 25°C and 36mΩ max at VGS = 2.5V & TC = 25°C. The BQ2970 provides 3.7V typical gate drive and I estimate Rs1s2 = 23mΩ typical and 28mΩ max. The estimated conduction loss at 5A:

    Pcond = 5A x 5A x 28mΩ x 1.25 (positive tempco assuming TC = 100°C) = 0.875W

    This is pushing the power dissipation capability of the package. The datasheet shows 1.7W max which is calculated assuming 1 in², 2 oz. copper pad (typical Rθja = 75°C/W) and TC = 25°C. Rule of thumb is this package can dissipate around 0.7W max. Using the transient thermal impedance curves in Figure 1 of the datasheet (Zθja = 0.035) it can handle 4.5W for 250μs: ΔTJ = PD x Zθja x Rθja = 4.5W x 0.035 x 75°C/W = 11.8°C.

    The other option is the CSD83325L, 12V chip-scale device in 2.2mm x 1.15mm LGA package. On resistance is much lower, Rs1s2 = 11.9mΩ max at VGS = 4.5V & 13mΩ max at VGS = 3.8V.

    Pcond = 5A x 5A x 13mΩ x 1.22 = 0.4W, well within the package capabilities

    I'm not sure how the lower on resistance affects the OCC and OCD thresholds.

    Best Regards,

    John Wallace

    TI FET Applications

  • 0 in reply to John Wallace1
    TI__Mastermind 31365 points

    Hello Chaitanya,

    Following up to see if your issue has been resolved. Please let us know.

    Thanks,

    John

  • 0 in reply to John Wallace1
    TI__Mastermind 31365 points

    Hi Chaitanya,

    Thanks again for your interest in TI FETs. Since I have not gotten a response, I am assuming your issue has been resolved and will close this thread.

    Best Regards,

    John