BQ32002: super capacitor value suggestion

Kelvin Tee

Part Number: BQ32002

My customer is interested to adopt this device for RTC function. They plan to use super-capacitor at Vback. Do you have suggestion of what should be the supercap value in order to sustain the operation for a duration of 1 week and 1 month? It will be helpful if you have a chart to show other value options. Thanks

Rgds
Kelvin

over 5 years ago

0 Lane Boyd over 5 years ago

TI__Genius 16845 points

Hi Kelvin,

I would estimate the battery life using I=C*dV/Dt. You might use a fudge factor to account for unanticipated losses.

We can solve this equation by integrating it. The result is Vf = Vi - I*t/C, assuming the current is constant.

If we need to guarantee operation from the backup supply, then we can solve for capacitance at the minimum backup supply voltage, Vf =1.4V . Datasheet gives the max backup supply current Iback = 1.5uA. Let's assume Vback starts at Vi = 3.3V.

We can solve for C at t = 1 week and t = 1 month. The result is C = 0.477F for 1 week and C= 2.046F for 1 month. Using a 0.22F standard value, the estimation is 77 hours.

I hope this information helps.

Kind regards,
Lane

0 Kelvin Tee over 5 years ago in reply to Lane Boyd

TI__Genius 10035 points

Hi Lane

Customer just noticed their supply chain supercapacitor is already EOL. In the datasheet also suggests back up battery as well. Could you suggest a suitable rechargeable battery for this application?

Rgds
Kelvin

0 Lane Boyd over 5 years ago in reply to Kelvin Tee

TI__Genius 16845 points

Hi Kelvin,

CR2032 is a coin cell battery that many customers have used with this RTC. Make sure that the battery you select is specified as rechargeable.

Kind regards,
Lane

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BQ32002: super capacitor value suggestion