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BQ77915: RCHG and RDSG calculations

Shivani Ghorpade
Prodigy 190 points
Part Number: BQ77915

Hello,

The typical application in datasheet states system designers should select the RCHG and RDSG values to meet the desired FET rise and fall time.

The recommended values are RCHG -> 1kohm and RDSG -> 4.5kohm. And since we are using additional components to protect CHG FET, RCHG2 -> 1Mohm.

We are using FET IRFS3207TRLPBF for our application.Can you give some references to calculate the RCHG and RDSG values. And can we go ahead with these recommended values for experimenting.

Regards,

Shivani Ghorpade

  • 0
    TI__Guru 54715 points

    Hi Shivani,

    RCHG2 in figure 29 is used to limit current into the CHG pin during short circuit or other discharge fault with load present. 1M is typically a good value.

    RCHG will limit current into the gate when a charger is present and CHG turns on.  Ciss for the IRFS3207 is 7.6 nF, a moderate amount.  You might start with 1-3 k ohm.

    RDSG limits current into the DSG pin both on turn on and turn off. Turn off is typically the more critical condition as it will help determine if the FET stays in its safe operating area on one extreme, or if the inductive spike from fast switching is large on the other.  Since RDSGOFF is small, you might start with 4 to 5k ohm.  The 4.5 k value in the example is not typically available, 4.7k or similar should be a good start.