BQ25895: using it with external adapter

Hi,

Will get back to you on the next business day.

Thanks,

Ning.

Dear Ning,

as a side question I kindly ask you about the WEBENCH Power Designer tool, because it seems not longer available for the BQ25895, hence in order to optimize the inductor do you have some related guidelines other than the information reported into the datasheet section 9.2.2.2?

Thanks and best regards!

F.

Thank Ning,

one question, with these setup the maximum input current is set to 3.25 A.

With this settings the maximum available current for the load applied to the SYS pin should be estimated as:

Isys_max=Vin*Iin*eta/Vo=5*3.25*0.93/3=5.04A (1)

assuming a discharged battery with a voltage equal to 3V.

So the available current for the load will be equal to this value reduced by the Fast Charging Current Limit written inside the REG04 register.

Now suppose to have a final load that require to properly work 5.1V@3.0 A.

I'm planning to put a boost stage (TPS61236P)  with the input connected the BQ25895 output SYS pin in order to step-up the battery voltage to the required one. Suppose that this boost converter efficiency is equal to 91.5% then the boost input current will be equal to:

Iin_boost = Vo*Io/(eta_boost*Vin)=5.1*3.0 / (0.915*3.0)=5.57 A (2)

bacause this value is greater than the maximum available current as calculated with relation (1) the load, at these conditions and during the CC battery charging stage, can not be powered with the requested current.

Then with a Fast Charging Current Limit set to 1.0A, the maximum current available for the boost stage will be equal to approx 4A hence for the boost load the maximum current should be equal to:

Iout_boost=eta_boost*Vin_boost*Iin_boost/Vo_load=0.915*3*4/5.1 = 2.15 A

So at the end of the story with the following data:

- Vin_charger = 5V

- Vout_boost = 5.1V

- Icharge_battery = 1A

during the CC battery charging stage this system will be able to power the load with a maximum current equal to 2.15 A.

Make sense?

Thanks and best regards!

F.

Hi,

We will check and get back to you.

Thanks,

Ning.

Dear @Ning,

do you have some update about this matter?

Thanks and best regards.

F.

Dear @Ning,
thanks for the links provided, I've read them, but I've not found any relevant information related to my specific question; anyway I think that if the above calculations are correct the chip is not suitable for this application due to the limited output current at VSYS pin and the requested output current and battery charging current. Simply these requirements is too strong to be satisfied by using this (very nice and attractive) integrated solution. From this point of view may be better use another approach, hence a charging path (classical LiPo charger) followed by a boost stage like the TPS61236P. The BQ25895 is certainly a nice solution because the chip is able to automatically perform the managing of the power paths between main power source and the battery path. Not using this chip require a dedicated managing strategy of the power path with particular reference to the commutation task between the main one to the backup one when the main power supply is lost... or check if is possible use as specification a lower current value on the system output.

Thanks for any help you can give on this matter.

Best regards.

F.

Dear @Ning,

thank you for your support, in my application I do not need to use the OTG mode, hence I'm thinking to charge the battery through the VBUS line as input power and SYS/BAT will be the output power lines, in that sense my calculation would address the maximum available current on the SYS output pin when the battery is charged with a fixed current so I think that this should be the critical situation from the overall current available on the SYS pin for the load.

Just to perform a roughly calculation I think that if the maximum deliverable current on the SYS pin, during the charging stage, is equal to 5A (because is the max charging current for the battery with a input VBUS = 5V and max input current 3.25A) using a max charging current 1 A let me room for 4 A of available current on the SYS line, this value is of course only when I'm into the battery charging stage because when the main power will lost the discharging current from the battery can be up to 9 A through the BATFET.

I'm right?

Thanks and best regards!

F.

Dear Ning,

after looking through the forum I've found this thread on the forum that answer to my question (here below for reference):

LINK

so I think that my calculation was correct.

Thanks and best regards.

F.