I’m Darshan Hiranandani, working on designing a circuit to manage RTC backup power since my RTC IC lacks a dedicated pin for this. Here’s the situation:
Normal Conditions: The RTC is powered by a 3.3V LDO, which can be sourced from either a battery or USB.
Backup Conditions: The backup power source is a LiPo battery (3.5V - 4.2V).
I need a circuit to switch off the battery power to the RTC when the LDO is providing 3.3V. Here’s what I’ve tried:
Diodes: Using two diodes doesn’t work because they pass the highest voltage to the RTC, and I need it to pass the lower 3.3V when available.
P-MOSFET: I attempted to use a P-MOSFET to switch off the battery power when a voltage from the 3.3V LDO is applied to the gate. However, the MOSFET only switches off when the gate voltage is equal to or higher than the drain-source voltage, which is not suitable since the battery voltage (3.5V - 4.2V) is higher.
Given this, I’m looking for alternative approaches. Could anyone provide suggestions on the following?
Alternative Switching Methods: What are some effective methods to switch off the battery power and prioritize the 3.3V LDO for the RTC?
Circuit Design: What circuit designs or components can be used to achieve automatic switching between the 3.3V LDO and the LiPo battery?
MOSFET Configuration: Is there a different way to configure the MOSFET or another MOSFET type that could work in this scenario?
Power Management: How can I design a power management circuit to ensure that the RTC receives 3.3V from the LDO and switches to the battery only when the LDO power is not available?
Switching Solution: What are the best practices for creating a reliable voltage source switcher for an RTC that handles different voltage levels?
Your insights or suggestions on resolving this issue would be greatly appreciated!
I’d start with simplest options. 1) get the right RTC and this won’t be an issue. 2) use an ldo to 2.8 or 3v and then the diodes method you wanted to do. 3) use a zener or higher Vf diode so the output is lower than your 3.3 supply(but you may lose some.of.your battery operating range).
I agree too that the better approach is to take a RTC that is designed from the start to do power failure switching. Read the design notes carefully. There are requirements on how quickly the power rails can rise and fall to have it switch over reliably (at least on a few I’ve worked with over my career).
Some RTCs will even handle charging the battery if you need that.
There are power prioritizing ICs out there. But you’d probably pay way more to do that than to get the better RTC.
Another possibility is to use a diode feeding a large capacitor. Supercaps have been used for this as well. Just make sure that the RTC operating from the supercap is electronically compatible with the powered off MCU. You don’t want to bleed power through protection diodes into the power rails.
Maybe not oscillate, but I need to fix it. Thanks for pointing that out!
EDIT: The circuit simulates okay, though a lot depends on the behavior of the LDO when the output is “driven/back-fed” by the battery. A Schottky diode can be added as shown here, but the RTC has to be able to operate at 3.3V - Vf(Schottky).
Is there an absolute need to use a battery for backup of the RTC, as that seems to complicate things significantly. If you can use a super-capacitor instead then you just need a series schottkey diode and possibly a current-limiting resistor from the supply to the RTC (and super-cap) and ensure that all the control pins are in a state that minimizes current draw. Given the safety concerns about Li-Po batteries, not using one would also be a big plus.
