BQ24172: Vsys Drop issue

nick lin

Part Number: BQ24172

We are using Bq24172 as our charger.

We noticed that when VBAT < 5.8V, and we plug adaptor (15V) for charging, we saw Vsys will drop from 5.8V to 5.0V, then pull up to adaptor voltage 15V.

Is there any possibile reason for this voltage dropping?

CH1(Yellow): VSYS

CH2(Blue): Gate of Q18

CH4(Green): VIN

over 1 year ago

0 Jeff F over 1 year ago

TI__Guru**** 166996 points

Hi Nick,

It appears that Q18 is turning off very slowly compared to input FET turn on. The dip is when the FET is off and FET's diode is on. Can you retake the plot above but add ACDRV or gate of input FETs?

Regards,

Jeff

0 nick lin over 1 year ago in reply to Jeff F

Prodigy 60 points

HI Jeff,

Thanks for reply,

please refer to the capture as below.

CH1(Yellow): VSYS

CH2(Blue): Gate of Q18

CH3(Purple): ACDRV

CH4(Green): VIN

0 Jeff F over 1 year ago in reply to nick lin

TI__Guru**** 166996 points

Hi Nick,

That doesn't look too bad. What is the turn off time spec for the batttery fet?

Regards,

Jeff

0 nick lin over 1 year ago in reply to Jeff F

Prodigy 60 points

HI Jeff,

I am attaching the time spec of the Battery FET.

0 Jeff F over 1 year ago in reply to nick lin

TI__Guru**** 166996 points

Hi Nick,

That looks reasonable. I haven't looked at the EVM for this device in a while but I seem to recall that a small dip as the input FETs turn on and the battery FET turns off is expected.

Regards,

Jeff

0 nick lin over 1 year ago in reply to Jeff F

Prodigy 60 points

Hi Jeff,

I notice that there are Rgs=499K/Cgs=47nf/Cgd=4.7nf components for tuning the turn-on speed of Input Fet.

Is there any side effect to increase Cgs/Cgd with high value? such as 1uF.

thanks

Nick

0 Jeff F over 1 year ago in reply to nick lin

TI__Guru**** 166996 points

Hi Nick,

yes, You can modify the turn on/off time with those components.

Regards,

Jeff

0 nick lin over 1 year ago in reply to Jeff F

Prodigy 60 points

Hi Jeff,

Sorry for my late reply.

Is it possible to modify the turn of/off time of BATFET? Could it be modified by serial resistor of BATDRV(R576)?

thanks

Nick

0 Jeff F over 1 year ago in reply to nick lin

TI__Guru**** 166996 points

Hi Nick,

Yes.

Regards,

Jeff

Power management

Power management forum

BQ24172: Vsys Drop issue