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Purpose of Gate Resistors on bq29700 EVM

Charles Gervasi
Expert 2270 points

What is the purpose of the 5.1M resistors between the gate and the source of the transistors on the bq29700 EVM?

I don't see them in the typical application schematic in the bq29700 datasheet.

  • 0
    TI__Guru 54715 points
    The gate pull down resistors help keep the FETs off when the part is in its low power state. Without the resistors leakage of the IC pins, board or FETs could allow a gate voltage to develop on the FET gate capacitance and increase power consumption of the system. The resistors are mentioned in sections 7.1.4 and 7.1.5 of the datasheet.
  • 0 in reply to WM5295
    Expert 2270 points
    In 7.1.4, it says "A high impedance resistor of 5 MΩ is connected from DOUT to VSS for gate capacitance discharge
    when the FET is turned OFF." Does this mean there are internal resistors or that the board designer should add them?

    In 11.2, it says "The 5-M resistor for an external gate-source is optional." If in undervoltage mode it let's DOUT and COUT and float, don't we need something to set the gate voltage? If so, the resistors should not be optional.
  • 0 in reply to Charles Gervasi
    TI__Guru 54715 points

    The resistors are not internal and should be added externally if desired. The author might have written that more clearly.  The board designer has the option to implement the resistors and trade off active mode current to keep the resistors biased vs. the chance of leakage waking the part again after shutdown