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BQ76940: External Balancing FET RDS

Daniel Herold
Prodigy 170 points
Part Number: BQ76940
Other Parts Discussed in Thread: CSD13381F4, TIDA-00449, CSD13383F4

Hi,

in slua749a, section 4: Cell Balance, it says "A FET with a defined RDS(ON) at approximately ½ the cell voltage is desired." 

What does this mean? Should the value of RDS(ON) be half the cell voltage, e.g. 4.2 V -> 2.1 Ohm? Or should half the voltage drop at the FET? I am confused, because I looked at FETs of other designs and the RDS(ON) seemed randomly selected.

  • 0
    TI__Guru 54715 points
    Hi Daniel,
    It is half the voltage drop at the FET, specifically at the Vgs.
    When balancing the VCx pins for that cell will pull together. The cell voltage will divide approximately between the 2 input filter resistors neglecting the internal resistance of the IC. The available gate voltage for the external FET will be about 1/2 of the cell voltage, see figure 4 or 5 of slua749. So with a 4.2V cell the gate voltage available to turn on the FET is at most 2.1V. When selecting the FET it would be desirable to have the RDSON specified at or below this voltage for Vgs. Until considering the power dissipation of the FET it may not be as much of a concern what the resistance value is as that it has a specified maximum.
    On the TIDA-00449 the CSD13381F4 has RDSON specified at 3 different voltages including 400 mOhm max at VGS 1.8V. This gives good confidence it will perform predictably with the 2.1V Vgs in the balancing circuit.
    Considering the CSD13383F4, it also has RDSON specified at 3 different voltages but the lowest is 65 mOhm at Vgs of 2.5V. This is a lower resistance value than the '381 FET, and the figure 7 curve in its data sheet shows it is typically still conducting well at a voltage below 2V, so it may work very well as a balance FET, but the numerical specification does not give a value so more testing may be desired before selecting this FET.
    The EVM used P-channel balancing FETs and the board could be used with LiFePO4 cells, so a FET with a defined RDSON at a lower voltage was desired. The selected part had a 1.5V Vgs condition for RDSON. Again the resistance was not as much of a concern as that it had a defined performance.