power backup for CMOS ICs during mains power failures

7 power backup for CMOS ICs during mains power failures Even very brief mains power failures cause problems forelectronic circuits. Stored data are lost and the oper- ating statuses are no longer what they were before the power failure. Mains power failures cannot be prevented, but methods can be employed to provide a voltage backup for the duration of the fault. For this reason, mains-powered equip- ment is often fitted with backup batteries (nicad or lithium cells) to maintain operation during a mains power failure. In view of the low currents (microamperes) required for data storage with modern RAMs, there is an alternative backup method which is well worth consider- ing: power backup with an electro- lytic capacitor for energy storage! The circuit diagram shows just such an application. The "power stand-by" capacitor Cl is 4700μF and with a maximum load current of 10μA, the discharge time at an output voltage of 5 V is approxi- mately 53 minutes. The operating voltage of the...