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BQ24780S: MOSFET minimum Vgs threshold level

Antony Lin
Genius 13480 points
Part Number: BQ24780S

Hi,

Do we have the parameter of “MOSFET minimum Vgs threshold level” for high side and low side respectively?  PLease check below (the RED circle) as an example from another chip.  THanks!

Antony

  • 0
    TI__Expert 5330 points

    Hi Anthony,

    We don't have maximum and minimum threshold as a parameter in the datasheet.

    The drivers are typically driving between 0V to 6V, although there are a couple of exceptions.  For very low duty cycle, the BTST cap can fall as low as 4.7V nominal, which is the refresh threshold.  This would be the turn-on for the high-side FET.  Also, if you have a 5V input, the REGN will be regulated at 4.6V typical, which would be used for high- and low-side FET turn on.  This would be the voltage to drive the high-side FET.  So the R_DSON at 4.5V is the key parameter.  To get a good R_DSON at 4.5V, you probably want a threshold that is 2.0V or less. 

    I would recommend a threshold in the range 1.2 - 2.0 V.  The FET on the EVM has a threshold at 1.3V.

    Regards,

    Steve 

  • 0 in reply to Steve Preissig
    TI__Genius 13480 points

    Hi Steve,

    Thanks for your comments.  I see we define the dead time is typically around 20ns.  May you provide explanation regarding how we control high side and low side FET to make sure the dead time is around 20ns?

    Thanks,

    Antony

  • 0 in reply to Antony Lin
    TI__Expert 5330 points

    Hi Anthony,

    The 20 nS typical dead time quoted on the datasheet for both high-to-low and low-to-high transitions is the dead time of the driver.  This does not take into account the characteristics of the FET such as threshold, gate capacitance, etc.  If the high side FET is turning off and the low side FET is turning on, then the high-side FET will turn off first (the driver will begin driving the HIGATE low) and 20 nS later, the low-side FET will begin turning on (the driver will begin driving the LOGATE high).

    The turn off is faster than the turn on because the drivers have a lower turn off resistance than turn on resistance.  For instance, the high-side driver has a turn-on resistance of 6 Ohm typical, but a turn-off resistance of 0.9 Ohm typical.  This means that any added gate capacitance will extend the dead time.  It is usually a small extension, though, relative to the device dead time.  If there is 500 pF Ciss input capacitance, then the time constant is 3nS for turn on and 0.45 nS for turn off.  So this would extend the 20 nS of the device by another ~2.5 nS.

    Regards,

    Steve