BQ25895: control of BATFET, buck mode, power on output

Hi Jeff

Thank you for the replies.

I assume for point 2, if the SYS was the source to charge the supercapacitor (and it was current limited appropriately), the maximum supply voltage would be defined by MINSYS i.e. 3.7V? or would it be possible to supply greater than this?

Further to this, I have three more questions:

1/ Are there any other devices with similar VinDPM ranges? Minimum charge + supply current 2A. Ideally the VinDPM range should be up to 8V. I don't mind this being set with resistors, provided the charge voltage is programmable (or settable) via I2C or using resistors.

The BQ25895 has a VinDPM 2.6 - 15.3V. Most other devices has a much lower range (up to 5.08V or similar).

2/ What is the transient response time of VinDPM i.e. if the input voltage changes, how quickly should it react. I have some indicators of this from some of the traces, but no hard figures or scope shots with the timescales set appropriately. 

3/ Maximum voltage input for BQ24895 - The recommended operating range is 3.9 - 14V. The absolute maximum is 22V. I feel this is unusual compared to other similar parts e.g.

- BQ24163 = 10.5V max operating voltage, 18V input voltage (non operating), 20V absolute maximum.
- BQ24193 = 17V max operating voltage, 22V absolute maximum.

If I wanted to run 17V DC into the BQ24895 indefinitely, would this be acceptable?

Thanking you again in advance.

BQ24163

With no battery and charge disabled, the maximum regulated voltage on SYS is MINSYS max = 3.7V.

Regarding 1, bq24650 controller with external FETs and no power path has external resistors for setting VINDPM. bq24261/1M have VINDPM up to 11V. The bq2426x family also works a bit differently at SYS in that when charge is disabled and no battery is attached, SYS regulates to BATREG not MINSYS. However, bq2426x SHIPMODE requires VBUS removal which is not the same as bq25895 turn off the BATFET. You could add an external PMOS between bq2426x BAT pin and the battery to disconnect the battery.

Regarding 2, bq25895 VINDPM has a 1-10ms response time.

Regarding 3, the bq25895 VBUS OVP falling threshold is 13.5V so it will not function above that voltage. The max operating voltage is function of the low side FET from SW pin to ground and/or a duty cycle limitation. The higher abs max voltage is to protect against spikes at hot plug. You could implement external VINDPM higher than 5V on bq24193 using an op-amp and reference that monitors VBUS and drives a different value into ILIM.

Thanks Jeff, very helpful.

Finally, relating to 3/ again, I am comfortable with it not functioning above 13.5V. What I would like to know is if it is safe to run 17V into this input indefinitely (but I understand it will not charge unless < 13.5V). i.e. will there be damage caused by this. Ideally a figure for the maximum recommended input voltage (non-operating) for the bq25895.

My suspicion is that it would be fine (given specification from bq24163) but I would like some confirmation - could this be checked with the design team?

Thanks again

Thank you Jeff - perfect.