I've got a battery powered project I've got a rather successful design for at the moment. It's the
Crazy Clock - I think I've talked about it before here. The design works, but I have somewhat of an academic question that may lead to a design improvement perhaps.
In a nutshell, it's an ATTiny85 powered by a NCP1402 based 3.3 volt boost converter driven by a single AA battery. The controller sleeps most of the time, and most of that time it draws less than a microamp - closer, in fact, to 100 nA. I've used the microCurrent Gold and established that the average battery draw is around 230 micro-amps, which means the battery will last easily longer than a year, which is just fine.
Since the circuit works, I haven't really paid much attention beyond that, but I happened to take a look at the output voltage of the boost converter and saw that the ripple was actually quite high - something like 150 mV P-P. Hmm.
One of the things I have in the works is that I'm going to swap out the MBRA140 Schottky for a BAT54. My motivation for that was to reduce the BOM cost given that I obviously don't need a 2A rated part for this. I'm also going to swap out the output filter cap for a cheaper one, but I do note that the ESR has gone up to 1 ohm, which is higher than the NCP datasheet recommends (~.25 ohm).
In any event... I suppose the ripple isn't impacting anything because the ATTiny has such a wide supply voltage tolerance, and this isn't in any way an analog circuit. But in the interest of better manufacturing reliability, should I try and reduce that ripple? The only way I would think it would be possible would be to increase the inductance. I suppose the up-tick in the output filter cap ESR isn't going to be helping any.
Is it just too much to ask for the boost converter to keep the ripple under control when the current load is so light?
Or is the NCP1402 perhaps swatting a fly with a jackhammer? The single AA battery is non-negotiable. What else can I do to get 3-3.3 volts regulated out of one?