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RC net in the charger BQ24735

Myron Steinberg
Prodigy 145 points
Other Parts Discussed in Thread: BQ24735

What is a reason for RC net on the charger BQ24735 (see schematics below)? And how to select a proper RC for transistor BSC011N03LS?

  • 0
    Guru 22270 points

    Hello Myron,

    The main reason for the RC network is to prevent oscillation, spikes when switching at high frequency and supplying the current required to drive the MOSFET.

    Read through the following app note to get a little more technical information on how MOSFETS work.  Then you can download the Datasheet for the BSC011N03LS and use this information to calculate Rg.  There are also several tutorials on the web that show you how to drive a MOSFET http://www.ti.com/lit/ml/slup169/slup169.pdf

    Thanks,

  • 0 in reply to Greenja
    Prodigy 145 points

    Thanks a lot for a quick response.

    1. BSC011N03LS is a huge and low speed MOSFET with input capacitance > 6000pF (with big Miller capacitance additional to that).

    2. I know pretty well how MOSFETS work. But I do not have a BQ247345 Spice model or, at least, the BQ24735 output impedance for the pin which is driving that MOSFET (the BQ24735 DS does not say anything about that, just shows the schematic).

    Could you provide the Spice model for the BQ24735 or, at least, provide information about the output impedance of the pin BATDRV (pin 11)?

  • 0 in reply to Myron Steinberg
    Guru 22270 points

    Hello,

    I do not have the spice models for this device.

    The Datasheet does give some information on the BATFET gate driver on page 9.  The minimum load resistance is 500k and maximum turn-off resistance of 7.4K if that is any help.

    Someone from TI will be able to provide you with the missing information.

    Thanks,

  • 0 in reply to Greenja
    TI__Guru 56650 points

    Yes. On the page 9, the BATDRV turn-on current capability is around 60uA; the BATDRV turn-off resistance is 6.2 kohm typical  (7.4kohm maximum). We don't have spice model for this device.

    Also, we BATDRV RC has to match ACDRV RC. For example: When input source is plugged in,

    BATFET has to be turned off before input FETs are turned on. So, there is no input to battery shoot through.