In my application I want to give power to an ATmega328p and an MMA8451 accelerometer. When active, these devices require 5.2mA and 24uA respectively, and when in power saving mode, they require 4.20uA and 1.8uA respectively.
I only need to be taking a measurement once every 100ms, so as a result, both components will be in power saving mode most of the time. Also, my main requirement is to make the system as energy efficient as possible. My question is, how can I provide power to these components efficiently? What kind of power supply should I use?
From what I have seen, dc/dc converters' efficiency, even the ultra-low power ones, give an efficiency of 20-40% at 100uA, and give no info about my system's sleeping current level (around 10uA).
Due to application specific reasons, powering the devices directly from a battery is not an option.
EDIT: The project will be powered by a LiPo (3V-4.2V). As a result the accelerometer, which takes voltages up to 3.6V, cannot be connected directly to the battery. Furthermore, the reason behind my idea of using the LiPo and not another kind of battery, is because the application also has an SD card. I will be required to write to the SD card once every few minutes, requiring bursts of 100mA or so, at 3.3V. The reason I did not mention it initially was that I planned to use a different DC converter just for the SD card, because its current requirements are well above the rest of the circuit, and because obviously there is no converter which is efficient from 10uA until 200mA.