AN401 - Charging Methods for F-RAM RTC Backup Capacitor

Introduction

Using a capacitor to back up a real-time clock in the absence of system power is often preferable to using a battery, and is a practical option with the F-RAM family. When you use a capacitor backup source, the capacitor must be charged. The highest charged voltage yields the longest backup time period after power fails. In a 3 V part, VBAK can go up to VDD while in the 5 V part, the maximum allowed voltage on the VBAK pin is 3.75 V. This application note describes three simple methods that you can use to charge a capacitor with F-RAM RTC parts.