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BQ76200: Do the CHG and DSG Pins Ever Float and is this the reason for using 10MOhm RGS Resistors

Airy
Prodigy 100 points
Part Number: BQ76200

The bq76200 datasheet seems to imply that the CHG and DSG pins are always driven given the FET driver on and off resistances in the electrical characteristics. If the CHG and DSG pins are indeed always driven, what is the purpose of the 1mOhm RGS resistors?

  • 0
    TI__Mastermind 25955 points

    Hi Richard,

    Could you please point out which figure you're referring to?

    The lowest valued resistor I see are 100 ohms in the layout example - they're used as RC filters.

    For more information on different circuit example, please see: http://www.ti.com/lit/an/slva729a/slva729a.pdf


    Sincerely,
    Bryan Kahler

  • 0 in reply to Bryan Kahler
    Prodigy 100 points
    Bryan,

    The bq76200 datasheet (SLUSC16) simplified schematic on page 1 shows 10MOhm resistor across the FETs gate and source (RGS). I assume the resistor is there to keep the FETs off because the CHG and DSG pins float at certain times?
  • 0 in reply to Airy
    TI__Guru 54715 points

    Hi Richard & Bryan,

    The 10M ohm gate-source resistors are most important to keep the FETs off when there is no power applied.  Once the part  has adequate power the outputs will be driven.  Since the gate drives are resistive and will decrease with output voltage, the pull down does help dissipate any residual voltage near 0V.  When the FETs are on, while each resistor may draw only approximately 1 uA, due to the charge pump efficiency together they may add 10-12 uA load to the system.  The 1 Mohm mentioned in the  original post's title should be in the capability of the charge pump but together would consume much of its capacity and add significantly to the system load.  To avoid potential confusion I will attempt to edit the title.

    Keep in mind that the simple schematic is simplified.  While the EVM demonstrates that the part can function with a simple implementation, many times a system will be more satisfactory with additional components.  It may be helpful to review the apnote bq76200 Beyond the Simple Application Schematic.

  • 0 in reply to WM5295
    Prodigy 100 points

    Thank you for the response and thanks for changing the title from 1M to 10M, which was a typo on my part. It sounds like the CHG and DSG pins never float (I wasn't sure if they float when VDDCP is below UVLO and CHG_EN and DSG_EN are high) but it sounds as though the CHG pin is always driven through an internal resistor R(CHGFETOFF) to BAT when off and CHG pin is always driven through an internal resistor R(CHGFETON) to VDDCP when on. Likewise, the DSG pin is always driven through an internal resistor R(DSGFETOFF) to PACK when off and DSG pin is always driven through an internal resistor R(DSGFETON) to VDDCP when on. Can you confirm that the CHG and DSG pins never float?

  • 0 in reply to Airy
    TI__Guru 54715 points
    Hi Richard,
    If power is applied CHG and DSG do not float.