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BQ76930: REGSRC source follower configuration

Julián Cid
Prodigy 30 points
Part Number: BQ76930

Hi,

I'm trying to select a suitable MOSFET to power the REGSRC pin. I don't understand the source follower topology proposed in the bq760x0 Family Top 10 Design Considerations section 9. Since the voltage at the drain (BAT) is going to be significantly bigger than the voltage at the gate (VC5X), the moment the MOSFET closes and starts conductiong then the voltage at the source will be around the same as at the battery. This will cause the Vgs to go below the Vgs(th) of the MOSFET, since Vsource is now bigger than Vgate, and the circuit opens. The process then repeats, causing a switching effect.

Is there something I'm missing? Is something wrong with my analysis?

Thanks in advance,

Julián.

  • +1
    TI__Mastermind 25605 points

    Hi Julián,

    It may do that if the components were ideal and the switching was instantaneous. In reality what the FET will do is as the voltage at the source increases and the Vgs decreases, the Rds of the FET will increase until the FET is operating in the linear region. The voltage at the source will settle out at around (Vg - Vgsth).

    Regards,

    Max Verboncoeur