Supercapacitor charging

I have a circuit that uses an ATTINY84V - the "V" means it works with the voltage as low as 1.8V. I am using all the power saving features possible, so when in sleep mode (which it is in most of the time) it draws only a microamp or so. I am powering it from a 1F 2.5V supercapacitor with a low ESR. (This one: http://www.digikey.co.uk/product-detail/en/B0810-2R5105-R/283-2776-ND/1026744)

I need the supercap to be connected to the circuit permanently, with no on-off switch. I also want it to carry on delivering power as long as possible (in sleep mode).

It mostly works fine. But sometimes the whole thing hangs, and even a hardware reset won't sort it out. Only disconnecting the power (which as I say will not actually be possible in the end product) or reprogramming it (with the supercap still attached) will deal with the problem. This does not happen if the circuit is run from two AA batteries.

I have a feeling that the issue may to do with the way the supercap is charged or possibly discharged. At the moment I am charging it simply by connecting it directly to a 2.5V power supply, via a voltage regulator. There is no current limiter. Do I need one?

You need to look into the "Brownout" function in the micro. This shuts it down when the supply voltage gets too low, this overcomes the problem of a microcontroller that locks up under low power conditions.

// Per.

Yes I think I probably do need the brownout but unfortunately it increases the power consumption.

However I didn't want to raise the brownout issue here because I mostly wanted to sort out the supercap issue first.

Following the link you provided and Googling super capacitor, I see there is a "B" series capacitor designed specifically for powering telecom equipment during small power failures.

Also, have you measured the current draw going back through your regulator circuit when the power feeding it has been removed? Could that be reducing the lifetime of the capacitor charge?

Paul

Yes it's the b series one I'm using I think. The charger circuit (ie voltage regulator) is disconnected when not charging so the current it draws is not an issue

robertjenkins: I am charging it simply by connecting it directly to a 2.5V power supply, via a voltage regulator. There is no current limiter. Do I need one?

Voltage regulator simultaneously is a current limiter. You can choose a small regulator with current limit of 100...150 mA, such as TPS782xx.