BQ32000: use super cap at Vback

question 1 the difference between bq32000 and bq32002 is clear, 

The BQ32000 integrates a trickle charger for VBACK, BQ32002 does not.

https://e2e.ti.com/support/clock-and-timing/f/48/t/735057?BQ32002-What-differences-are-there-between-BQ32000-and-BQ32002-

Hi Mingmo,

I believe that the super cap needs to be charged by external supply, because Vback simply serves as a supply pin and there's no setting that switches this pin between sink only and souce/sink.

The charging time depends on the RC time constant of the series resistor connected to supply (plus the internal resistance of voltage source) and the super cap. 

According to datasheet table 6.5, current consumption using Vback is 0.9uA typical, so that is how fast the super cap will be discharged when backup supply is used. 

Also according to datasheet, lowest voltage with Vback is 1.4V, meaning that (Vcc - 1.4V)/(0.9uA/C) is how long the supercap can support the device, where 0.9uA/C is the slew rate and Vcc-1.4V is the voltage difference.

Regards,

Hao

Mingmo,

I was talking about BQ32002 in the previous post. There's detailed description of trickle charger in BQ32000 datasheet.

Regards,
Hao