This application note describes a more maintenance-free way to sustain the data and clock in systems that only experience short breaks in the power supply (on the order of days). A Super Cap can be used as a type of secondary cell (a rechargeable battery) and can therefore provide an alternative solution to using a primary cell
This document explains how to use a super capacitor to sustain data and clock operation in systems using STMicroelectronics M41T56, M41T00, M41T11, M41T81, M41T94, and M41ST84W real-time clocks (RTCs). It presents this super capacitor solution as a more maintenance-free alternative to primary cell batteries, particularly for applications experiencing short power supply breaks.
| Supported RTC Models | M41T56, M41T00, M41T11, M41T81, M41T94, M41ST84W (16-pin) |
|---|---|
| Minimum Battery Voltage (M41T56) | 2.5 V |
| Maximum Battery Supply Voltage | 3.5 V |
| Maximum Power-Out Time (M41T56) | 181,818 seconds (approx. 2 days) |
| Maximum Power-Out Time (M41T00/11/81) | 250,000 seconds (approx. 2.9 days) |
| Maximum Power-Out Time (M41ST84W/94) | 400,000 seconds (approx. 4.63 days) |
The recommended starting value for R1 is 22 kΩ.
Using a 100,000 µF capacitor, the maximum power-down time is about 2.9 days.