Intro

If your device has separate RTC (Real Time Clock) chip you will definitely want to keep clock running continuously, even if the main power is not present or battery (if exist) is disconnected.

For this purposes on many electronic device boards (especially on old phones with removable battery) you can see small coin cell battery, that act as an independent backup powering for RTC IC.

Structural Diagram

ORing block have two power inputs and one power output, so it give a chance to connect backup coin cell with external power input and connect output voltage to the RTC chip.

Electrical Schematic (FET + SBD)

Main power pass through diode D1

Backup power pass through transistor Q1

R1=10k-470k

Schematic Simulation

Using Microcap 12 it's very easy to simulate schematic working:

When coin cell is not present and USB device is powered from USB 5V the RTC supplied from USB through diode D1, so the output voltage less by diode drop voltage.

NOTE: as you can see if R1=100k we have current leakage 50uA which is ok for USB powering, but if you use another Li-Ion battery for board powering 50uA in sleep mode is a big leakage, so you need to increase resistance of R1 or better replace MOSFET by the Schottky diode (SBD).

USB Only

Battery Only

When main power (USB 5V) is disconnected RTC supplied from secondary input (coin cell battery) through MOSFET Q1:

USB and Battery

When both power sources is connected (USB 5V and BAT 3V) RTC supplied from main power (USB 5V), so secondary power input (coin cell backup battery) not drained.

Electrical Schematic (SBD + SBD)

Li-Ion + Coin Cell

When both Li-Ion battery (main power source) and backup coin cell are connected the RTC powered only from the Li-Ion battery.

Li-Ion

When Li-Ion battery (main power source) disconnected the RTC powered from backup coin cell.

Which battery is suitable?

In case of using RTC chip (e.g. the most popular PCF8563)VIN supply voltage range very wide:

PCF8563T SOIC-8
PCF8563TS MSOP-8
Clock operating voltage:	1.0V-5.0V

Coin cell battery could be different chemical type, which means different voltages:

So, because minimum RTC operation voltage is 1.0V it's possible to use even low voltage 1.5V MnO₂ coin cell battery.

Board

Using SOD-323 package diode and SOT-23 transistor with 0402 components makes backup circuit pretty small.

In case of using just two diodes you can replace it by the diode array in SOT-23 package, but in my design I use two separate Schottky diodes SOD-323 for BOM optimization (because it's more common).

Real Testing of Backup Circuit

Testing performed on the development board.

Changing Coin Cell Battery without Clock Reset

If you need to change the RTC coin cell battery (no external supply connected) the clock will be reset.

If you want to save clock you can additionally have backup capacitor, that can act as a small reservoir of energy for supply RTC chip during battery replacement.

How much time can you have?

If RTC chip current I <= 1 uA:

Conclusions

• Battery reserve powering of independent RTC it's not difficult, all you need it's ORing (FET + SBD) | (SBD + SBD)
• If you use battery (e.g. Li-Ion) for device powering use (SBD + SBD) schematic to achieve low sleep current
• If device connected to the primary (main) power supply RTC chip doesn't drain battery, that give a chance to significantly increase device clock life
• Additional backup capacitor can be used for powering RTC chip during coin battery replacement

• Using internal MCU RTC can be more smart solution (e.g. some STM32 have dedicated VBAT pin for RTC and some register data backuping).