STM32 with dual supply low power

[sgt_johnny]

I want to redesign a current product to make it better and more efficient.

The device is battery powered with a lithium battery (2,7 to 4.2V). A NFC reader in the circuit needs 3.3V to operate its transmitter at an optimate performance.
A STM32 will be used at 8Mhz to control the device. Since a motor, which is directly connected to the battery, will pull the battery way down, a buck-boost converter is used to created 3.3V for MCU and the rest. These Buck-Boost regulators have terrible efficiency at low power (like 50uA consumtion to put out 1uA for Sleep) i would like to avoid to power the mcu only by the buck boost.

I would like to shut down the regulator when entering sleep, and only power the STM32 at a LDO at 1.8V in sleep mode.

How can i create a dual supply concept for the STM32?

[technix]

BAT54C to VCC

[SiliconWizard]

Don't know the max current your circuit requires.
If 300 mA max is OK, you could also consider using an LTC3106 which has very low quiescent current in burst mode. Additionally it has a secondary input for a backup supply.

[Mr. Scram]

BAT54C to VCC

I'm not sure I understand your suggestion.

[technix]

BAT54C to VCC

I'm not sure I understand your suggestion.

BAT54C is a common cathode dual Schottky diode. By connecting two power sources to the two anodes and draw power from the common cathode you create what is known as a power OR circuitry. Now the voltage at the common cathode would be 0.3V below whichever power source that has a higher voltage.

BAT54C has a 300mA current limit. If your chip can draw more current under all circumstances you might want to use some more powerful diodes for a similar setup.

[Mr. Scram]

There are ICs with a boost converter and a linear regulator for when the voltage is above the desired voltage for exactly this reason. Maybe that's an option? I know Maxim has some, but I wouldn't recommend using Maxim if you can help it.

A more intelligent power controller might be another option.