BQ25895: VSYS and VBUS during system reset

Nicolas Venturo

Part Number: BQ25895

Hello,

We're developing a battery-based product where power for the whole system comes from VSYS. We'd like to be able to perform a system-wide reset by using the BATFET reset feature (enabling BATFET_RST_EN), and connect QON to the output of our watchdog. However, when a charger is connected on VBUS, VSYS isnt disconnected after a reset, since it only turns off BATFET and not the RBFET. Is there a way (ideally, using the BQ25895's pins and not I2C commands) to turn off the RBFET and therefore create a true disconnection of VSYS, both from the battery and VBUS?

Thanks!

over 8 years ago

0 Jeff F over 8 years ago

TI__Guru**** 166466 points

Nicolas,

Will the device be in default mode (watchdog timer expired) when you want to do the reset?

0 Nicolas Venturo over 8 years ago in reply to Jeff F

Intellectual 810 points

Hey Jeff,

Probably: I want the watchdog timer to make a system reset, which takes 15s of QON being pulled low, so if the BQ watchdog timer is 15s or less, it will be in default mode. What is the difference in each case?

0 Jeff F over 8 years ago in reply to Nicolas Venturo

TI__Guru**** 166466 points

Nicolas,

Default and minimum watchdog timer is 40s. The reason I asked is we are currently testing a full system reset circuit using a NFET from ILIM resistor to GND, with gate tied to /QON. When ILIM is allowed to float high to its 0.8V clamp, Q1 essentially turns off, which means SYS will collapse. This will only work if the EN_ILIM bit is enabled, which is the default. We haven't fully tested on the bench yet. Would this be of interest?

0 Jeff F over 8 years ago in reply to Jeff F

TI__Guru**** 166466 points

Nicolas,

Bench testing has revealed that /QON does not toggle the BATFET when VBUS is powered. Therefore, my idea above will not work.

0 Nicolas Venturo over 8 years ago in reply to Jeff F

Intellectual 810 points

Yes, I believe I tried letting ILIM go high a couple days ago too, with the same results. So the only option is pretty much disconnecting the input voltage from the BQ using a transistor? I'd like to avoid that, since that transistor would need to have very low Rdson or waste a lot of power.

0 Jeff F over 8 years ago in reply to Nicolas Venturo

TI__Guru**** 166466 points

Nicolas,

Unfortunately, yes. Alternatively, you could place between SYS and the system load.

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BQ25895: VSYS and VBUS during system reset