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BQ29700EVM-610: How to choose the MOSFET for the BQ29700?

Jacking Jin
Expert 1145 points
Part Number: BQ29700EVM-610
Other Parts Discussed in Thread: CSD16301Q2, , CSD16406Q3, CSD

Dears,

The BQ29700EVM-610 can support charge current about 5A, and the EVM choose the CSD16301Q2, the MOSFET's Rdson is 23mΩ, If I according the design theory that the max current(VOCD/2*Rdson=100mv/(2*23mΩ)≈2A)，I don't konw why can support 5A. Meanwhile, If I want use the BQ29700 to reached 10A discharge current, which MOSFET is the better choice(small size)?

Best Regards

Jacking

  • 0
    Expert 1145 points
    Dears,
    Who can help me to solve the confusion? Many thanks!
  • 0 in reply to Jacking Jin
    TI__Expert 6085 points
    in your design requirement with a OCD current of 10A and OCD threshold of 100mV
    2*Rdson = 100mv/10A Rdson = 5 milli-ohm. also you need to know the max charge current. lets assume 2A. hence the Overcurrent threshold would be 10mOhm * 2A = 20mV. now to pick the proper FET consider, worst case Short circuit that will be allowed ~for the SCC time of 250uS...@MAX PACK voltage. I SCD = VPACK/( Rcell+ 2*Rdson_+ parastic line resistance).... Look in the TI CSD family of logic or pwer FET for the appropriate VDSS @ calculated SCD. Then refer to FET's Vgs vs Rds_On and pick the needed operating Vgs. with the appropriate FET VDSS. Also closely pay attention to the FETs package type for the theta ja coefficient. As a starting point, start off the design exercise with CSD16406Q3

    hope this helps...
    vish
  • 0 in reply to Vish Nadarajah
    Expert 1145 points
    Dear Vish Nadarajah,
    Thanks a lot for you reply.
    I can understanding your means, but I still have some questions need to verify from you. As you said, why the BQ29700 EVM can support the max 5A? According the formula, the max current only sustain about 2A. Thanks!
    Best regards
    Jacking Jin