I want to find the most efficient way to power an STM32 microcontroller from a lipo (3.7V nominal).
I have calculated that @ 3.3V, the current consumption is approximately:
- in run mode:1.75mA
- in idle mode: 200uA
- in shutdown mode: 0.6uA
One of the smallest buck boost converters I could find, LTC3532, has the following efficiency curve:
and its quiescent current is at best 50uA.
As a result, it would not be very efficient (~75%) to power the uC even from this small converter when the uC is in idle, or even worse completely inefficient in shutdown mode.
I also considered using an LDO, but again, at best I could find quiescent currents in the range of several microamps, thus not being efficient for the shutdown mode.
Is there any way to provide such low powers efficiently and not directly from the battery?
I also found this quote on a forum
The biggest impact for increasing efficiency to an LDO, apart from Iq, will be to reduce your input voltage to the LDO. Often this is done by using a switching regulator before the LDO.
What is the point of this? I think I understand the theory, where with the switching regulator you can get Vin of LDO to be as close to Vout+Vdrop as possible to have maximum efficiency, but why not use a switching regulator directly then?
EDIT: As mentioned by W5VO in the comment, I could find a lower Iq LDO than the several microamps mentioned above. So is the best way to go to use an LDO at 3.3V with the lowest possible quiescent current and dropout voltage, and only exploit the energy of the LiPo from ~4.2V down to 3.3V+Vdo?