I am designing an ATtiny project that needs to operate for a specified amount of time on request, then power itself down completely. Although ATtiny MCUs are remarkable in terms of sleep mode power usage, and can be underclocked, the project needs to operate from a 12 V battery and the main culprit of power usage would be the LT1117 regulator. Plus the LT1117 has a low current limit of 2.7 mA so any low-power mode on the MCU would be negated by the need of some resistor in parallel with the MCU.
One way to bring quiescent current as close to 0 A as possible would seem to use a double latching relay. On pressing a spring loaded switch, the pulse would set the relay and connect the 12 V battery + terminal to the LT1117 regulator, the MCU would power on, do its job and then pulse the reset coil, powering itself down.
The other way would be a high-side FET switch instead. Seems to work well in the simulation attached.
- The S1 switch is the spring-loaded press button to power on the device, by discharging the added gate capacitor C3, the time constant of R3/C3 is long enough for the MCU to start and manage the gate itself. the button is pressed at t = 0.01 s
- The B1 source is the result of the digital write on the MCU pin that will manage the FET gate and keep it low. That state change happens as soon as it gets enough voltage to start, plus a 64 ms delay.
- The V3 source is used to model the power-off of the MCU at t = 1 s and used in the B1 source expression.
I would prefer to go for the FET option, besides the ATtiny I will have to supply 5 V for another relay, and that's all. Simulation gives me a quiescent current of around 2.5 µA from leakage. Is that realistic ? The most important feature for the project is that it reliably turns itself off, and to a lesser degree do not turn on spuriously.
Which method would you choose and why ?