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BQ25703A: FET Design Choice

Tony DiSanto
Prodigy 10 points
Part Number: BQ25703A
Other Parts Discussed in Thread: CSD87588N, , CSD17551Q3A, BQ25703

Hello, 

I'm currently reviewing a design that has used TI part # CSD87588N as the buck and boost NexFET for switching. This device combines both the low and high pair of FETs Q1 and Q2 into a single LGA package.

 

The xxx87588N, Q1 specifies CiSS @ 566 pF (typ) and Qg @ 3.2 nC (typ) whereas Q2 is spec’d at 2310 pF (typ) along with a much larger gate charge Qg 13.7 nC   After reviewing the TI reference design it appears that the suggested FET as a much lower CiSS and Qg.  

 

  1. Do you believe the larger values for Ciss and Qg for Q2 are acceptable?
  2. Are they any potential using this dual style of FET with the BQ25703A device?

 

Design: Vin= 5V,  2S Li-Ion battery, 1A system load.  Inductor is 3.3uH,  No OTG

Thanks

  • 0
    TI__Genius 11850 points

    Hi Tony,

     

    Higher Ciss and Qg generally causes slower turn-on time and increased switching losses. I think the CSD87588N's Ciss and Qg specs are close enough to the CSD17551Q3A FETs (used on the EVM) that they should not cause problems, but you will need to validate this on your own board to be sure.

     

    By the way, I think the BQ2588x family of boost chargers would be a better fit for the design requirements that you listed. The BQ2588x would be a simpler solution since the switching FETs, blocking FET, and BATFET are all integrated inside the IC.

     

    Best regards,

    Angelo

  • 0 in reply to Angelo Zhang87
    Prodigy 10 points

    Thank-you,  I've dug into the bq25703 and did a few calculations for FOM.   I also believe the differences between the gate charge for the Q1 and Q2  of the CSD87588N Dual FET Power Block II  is related to stacked die.  I believe TI  tried to balance conduction loss and switching loss when using these devices.   Digging into design the layout is just crap!