Required suggestion for Super capacitor IC Selection

[Aditech]

Hello Team,

We are evaluating the use of a supercapacitor as a backup power source for our LoRaWAN module.

Our requirement is to keep the LoRaWAN module operational for approximately 5 seconds during a power interruption. The System required near by 250mA & operating voltage is 5V. We would appreciate your guidance on the most suitable solution for this application.

Specifically, we would like your recommendation on the following:

1. Considering the short backup duration requirement of only 5 seconds, can the supercapacitor be connected directly without a dedicated management IC?

2. If a supercapacitor management/charging IC is mandatory, could you please explain why it is required for this application? What are the risks or limitations of connecting the supercapacitor directly without a dedicated IC, and under what conditions would direct connection not be recommended?

We would appreciate any reference designs, application notes, or design recommendations related to this use case.

We look forward to your valuable suggestions.

[Aditech]

Hello Team,

Look forward to your valuable response.

[Peter BENSCH]

Well, this Community is a forum for users of devices manufactured by STMicroelectronics. Since you did not mention that the LoRaWAN module is from ST, and ST also does not manufacture supercapacitors, your question is, at least initially, quite general and would probably be better suited to a general electronics forum.

But let us look at a few points, even though you have not stated the type of module or other properties such as peak current, current consumption in receive mode, sleep current, minimum supply voltage, or the timing profile:

• a dedicated power management device is certainly overkill for 5 seconds, so you should consider operating the supercap directly at the module (incidentally, with the supply line to the supercaps kept as low in resistance as possible)
• assuming a constant current of 250mA, we can calculate a minimum capacitance value for the following discharge cut-off voltages:
  • 4.5V: approx. 2.5F
  • 4.0V: approx. 1.25F
  • 3.5V: approx. 0.83F
  • 3.0V: approx. 0.63F
    
    
• for LoRaWAN modules, not only the buffer capacitance is decisive, but also the ESR of the supercap, since a non-negligible voltage drop occurs there during current peaks

In practice, a supercap of 3...5F could therefore be sensible, possibly with several MLCC of around 100µF each connected in parallel.

Does that already help a little?

Good luck!
/Peter

[Andrew Neil]

As @Peter BENSCH said, this doesn't seem to have anything to do with ST ?

The key thing to remember is that SuperCaps are not like batteries:

• A battery givesa (nominally) constant voltage, until it dies;
• A capacitor gives a constantly-decreasing voltage as it discharges.

Your system design will have to take that into consideration...

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@Aditech wrote:

operating voltage is 5V

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Is it really ?

Most things nowadays actually operate on around 3V or less ...