A number of companies are developing products that will allow utility providers (water, gas, and electric) to read the customers' meters remotely. Data is transmitted from the meter to a nearby data collection unit (DCU) via an RF link. The gas and water meters are typically powered by a small lithium battery. The battery is supposed to last for 20 years. This is difficult to achieve with the typical power-supply topology.
That RF data link transmits usage data at intervals, which could be once every 20 minutes, or once every two hours, depending on what the utility needs to know. The current draw for one of these RF modules powered from 3.6V might be a few hundred mA — but of course it is just a short burst (very short duty cycle), so overall power draw is low enough that the cell should last many years. But it probably won't.
These small lithium cells, like all cells and batteries, have A-hr (ampere-hour) or mA-hr ratings that indicate that (if they were perfect devices) you could draw a certain current for a specific number of hours, at which point, the cell would be completely depleted. A 1A-hr cell could supply 1A for an hour; or 500mA for 2 hours; or 5A for 12 minutes.
With the very small cells, besides the mA-hr rating, you need to be aware of their peak current draw — and not exceed the manufacturer's specs. If you do, you will shorten the cell's life appreciably.
Here is the way to work around this. Use that small cell to power a high-efficiency boost switcher. You can reasonably expect to boost from 3V to 15V with efficiency above 90 percent. Charge a very large capacitor through a resistor to 15V. That should keep the input current to the boost switcher quite low. Use that charged capacitor as the DC source to power a high efficiency buck regulator. Set up the buck switcher to provide 3.6V. You'll get 3.6V for as long as you need while the big capacitor runs down. And the lithium will never be taxed with high current draw.
To improve overall performance, we can add a very low power microcontroller (MCU) with a built in real time clock (RTC). The RTC can wake up the MCU at the appropriate time. The MCU can enable the boost switcher for the right amount of time. Then it can enable the buck switcher and the transmitter module and send the data. And then everyone goes back to sleep.
The whole thing should look like this:
It should be possible to integrate everything except the power cell, the two inductors, the big capacitor, and the RF module onto an IC.
Have you ever designed and built a power supply like this one?