TPS25940: Using a TPS25940A for Supercapacitor charger

Peter Bain

Part Number: TPS25940

Hi,

I am using the TPS25940A to charge a 5F super capacitor in a RTC backup scenario. I am having difficulty working out the set charge current for the capacitor (I would like less than 0.5A).
Using the TI excel spreadsheet tool for this device, it results in the following circuit.

CdVdT of 220nF for 92ms ramp time and Icharge or 0.27A
ILIM set at 1.89 with a RLIM of 47k.

Am I right in that the ILIM is the surge limit, but the 0.27A is the steady state charge current?

Is my circuit correct? (Still to add the super capacitor to the output.)

over 5 years ago

0 Praveen GD over 5 years ago

TI__Guru 57737 points

Hi Peter,

Before I help you with the calculations,

Can you let me know the maximum voltage rating of the Super Cap?
Can you share a system level block diagram ? This is to understand if the efuse is used just t charge the super cap or it has to serve pother load s as well.
Did you have a look at the app note Charging Super Capacitor With eFuse

0 Peter Bain over 5 years ago in reply to Praveen GD

Prodigy 60 points

Hi Praveen,

In reply to your questions:

The capacitor is rated at 5.4V (although we are looking at a 5V version as well).
The eFuse is only being used to charge the super capacitor. The 5V rail on the input is being used by other loads. The output is connected directly to the capacitor and the VBAT input of the RTC IC.
Yes I have read the app note waded through a number of posts on here using the search function.

Regards,

Peter

0 Praveen GD over 5 years ago in reply to Peter Bain

TI__Guru 57737 points

Hi Peter,

If super cap is the only load connected at the output of TPS25942, then the only current flowing through the efuse is to charge the capacitor.

So, you can actually use the current limit function of the efuse instead of dvdt cap to limit the inrush current/charging current of the capacitor.

You can refer to the following e2e post for more information,

https://e2e.ti.com/support/power-management/f/196/t/912333

0 Peter Bain over 5 years ago in reply to Praveen GD

Prodigy 60 points

Perfect, thank you!

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TPS25940: Using a TPS25940A for Supercapacitor charger