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BQ24780S: LODRV Capability

Part Number: BQ24780S
Other Parts Discussed in Thread: BQ24780, CSD17308Q3, CSD17309Q3

Hi Experts,

When using BQ24780S LODRV to drive MOSFET, it is found that this pin's incapability.

Original: Target Mosfet is csd17308Q3, BQ24780 works normal with proper driving.

Currently: csd17309Q3, it seem LODRV is not able to drive this mosfet because Qg is larger than CSD17308Q3.

Refer to datasheet, there is not spec expect Rdson about LODRV.  It is hard to decide which MOSFET to use when important specs are vague.

Could you please clarify LODRV's ability so that to facilitate MOSFET selection? Thanks!

  • Hi Wenhao,

    Datasheet specifies turn-on and turn-off resistances of Low-side driver as:

    These can be used to estimate turn-on and turn-off times using

    t_on = (Qg * Ron) / (V_REGN - V_PLAT)

    t_off = (Qg * Roff) / (V_PLAT)

    Using a typical plateau voltage of 2.6V,

    t_on = Qg * (3.53 A^-1)

    t_off = Qg * (0.48 A^-1)

    Note: these are using the worst-case numbers. 

    Generally you want total FET turn on/off time less than or equal to 2% of the switching regulator period, which produces about 1% efficiency loss (for switching loss only.)  For the fastest switching frequency of 1 MHz, this would be 10^-8 S.  This would give maximum Qg = 4.98 nanoCoulombs.

    The EVM uses a FET with Qg = 3.9 nC. The csd17309Q3 has Qg = 7.5 nC.  This is a little higher than I would generally recommend, but the low-side driver should still be able to drive the FET.  It will lead to higher switching frequency, but I would still expect that your total siwthcing time would be about 4% of the regulator period, and this is assuming that you are running at 1 MHz. 

    What do you mean by "LODRV is not able to drive this MOSFET?"  Can you describe in a little more detail?  Are you not seeing the FET turn on at all?  I would not expect that to be the case, and if that is what you are seeing, then perhaps there is another root cause.




  •  (Datasheet picture did not paste correctly in above post)

  • Hi Steve,

    Please see attached pics. 

    Below is 17308 ON&OFF Waveforms

    Below is 17309 ON&OFF Waveforms. 

    Experimental results show that with 17309, 23780S needs more time to drive it into platinum, thus, unfortunately, will lead to some state shifting issues. I will get to further testing results after evaluation of driving capabilities of LODRV. After all, 17309 or 17308 both are OK to use.



  • Hi Steve,

    Another question, Could you please explain how you calculate 3.53A and 0.48A, respectively. My expected the worst case calculating is showed as below:

    Vregn is between 5.7-6.3V.

    Providing Plateau voltage is 2.6V, then



    Please give a comment to 3.87 and your answer 3.53. Thanks!


  • Hi Wenhao,

    I used the V_REGN nominal value of 6.0 V whereas you are using the minimum value of 5.7V.  I agree that using the minimum value, as you did in your calculation, is more robust.