Supercap For Emergency Shutdown

[Ryan Reschan]

Hello,

I’m working on the 2nd generation of a product where the C-SIP will place two processors (and thus PCBs). We will have wall power, will not be using a battery, and will not have an available USB connection for power.

So we would like to connect a supercap to the VIN_USB since from what I’ve read about the TPS65217C ship, after VIN_AC goes down, VIN_USB is next inline.

We plan on monitoring our main power into the product through the C-SIP ADC to detect a potential failure and thus trigger the shutdown process while relying on the supercap to give us enough time to properly shutdown our Linux OS.

Is such a connection going to be okay?

Thanks,

Ryan

• This topic was modified 1 year, 6 months ago by Ryan Reschan. Reason: adding picture as an attachment

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[Neeraj Dantu]

Ryan,

This should be possible as PMIC is able to switch from AC to USB input when VIN_AC goes below 4.1V(See 8.3.9.1 in PMIC datasheet: https://www.ti.com/lit/ds/symlink/tps65217.pdf).

However, note that valid voltage on VIN_USB is 4.3V or higher. So, you will have to make sure the voltage at the USB input stays above that range until shutdown occurs.

Please also check out https://octavosystems.com/app_notes/am335x-battery-applications-with-osd335x-sip/ and https://www.ti.com/lit/an/slva901/slva901.pdf.

Best,

Neeraj

[Ryan Reschan]

Thank you for the replay Neeraj.

I have not come across the brownout article yet so I will have to give that a read.

It’s good to know that it should be possible.

-Ryan

[Ryan Reschan]

VIN_USB having to be 4.3V or higher has me concerned if we go with the supercap on that line. At least a light load resistor on VIN_BAT takes care of that pin not holding any charge.

Would putting the supercap on VIN_BAT be a better option? Obviously the charging would need to be turned off since it’s not a Li-ion battery on that connection.

It would then be a similar setup to Solution Circuit Number 3 in the Brownout document except we would not have a supervisor or a load switch.

Thoughts on that setup?

Thanks,

Ryan

[Neeraj Dantu]

Ryan,

Although VIN_BAT operating voltage range is lower(2.75V – https://www.ti.com/lit/ds/symlink/tps65217.pdf), note that the input should be able to support boot device voltage(3.3V), so actual required voltage on VIN_BAT is higher.  Having said that, VBAT does offer wider voltage input than USB.

We recommend prototyping to understand what the implications are and how much capacitance you will need for your application to support safe shutdown.

Best,

Neeraj

[Ryan Reschan]

Yes, I agree with your last sentence. I’m going to design the first PCB to allow us to wire up either option.

Thanks for the information,

Ryan

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