How to drop the voltage slightly (~100mV) for a MCU mostly in deep sleep?

Question

How to drop the voltage by a low value (let's say <150mV) without wasting current to supply a microcontroller that spend most of its time in deep sleep mode?

I'm working on a board with a microcontroller that will accept up to 3.6V as supply voltage but LiFePO₄ and Li/SOCl₂ can go slightly higher when fully charged (or during charge for LiFePO₄). The microcontroller uses ~10μA during deep sleep and can use up to 500mA (mostly spikes) when active and transmitting over WiFi (ESP32).

I know that I can use an LDO with a low quiescent current but I am curious if there are better/cheaper alternatives. For example, I thought about using the drop voltage of a Schottky diode.

Answer 1

Another possibility would be to have two independent connections to the supply voltage, one being used for low power mode and another one for a high power mode according to the following:

Right before driving the MCU into deep sleep mode, the active low pin _ACTIVE can be disabled (high), thus turning the PFET \$M1\$ off. This means, that the MCU will be supplied by the highly ohmic path formed by \$R_2\$ and \$R_1\$. This voltage devider must be designed to simultaneously provide a voltage smaller or equal than \$3.6V\$, and a current of approximately \$20\mu A\$ for example (just enough to keep the MCU up and running). Once you want to wake up the MCU, the pin _ACTIVE can be enabled (low) and the PFET will be turned on. Since it has a very low \$R_{DS,ON}\$, the highly ohmic path will be bypassed. The additional voltage drop that you require can be achieved by selecting a schottky diode with an appropriate forward voltage.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

Both of those batteries (more the LiFePO₄ than the Li/SOCl₂) have useful charge below the 3.3V required for a microcontroller. If you were hoping to squeeze the absolute maximum amount of battery life possible, you could use a buck-boost converter. They exist in small form factors and high efficiencies. Otherwise, use an LDO. You probably could use a schottky diode but you probably shouldn't. The forward drop of a Schottky is dependent on temperature and current, and is not tightly controlled like an LDO. They're also about the same price as an LDO (@ qty 1 on digikey, the cheapest Schottky diode is $.13 and the cheapest LDO is $.11).