Power Backup Solutions for Microcontrollers

A microcontroller is a highly integrated chip that contains all the components of a controller. Typically, this includes a CPU, RAM, ROM, I/O ports, communications and a real time clock (RTC). In many applications, it is desirable to have a backup mode which retains RAM data and keeps the RTC running in the event of a system power failure.

Cymbet's EnerChip, thin-film, rechargeable batteries are backup power alternatives to traditional bulkier power sources, such as coin cell batteries and super capacitors. EnerChip is specifically designed for a range of currently available microcontrollers including:

• TI MSP 430
• Freescale HC
• Atmel PicoPower AVR
• MicroChip PIC
• ST Micro ST62
• EM Microelectronics EM

The backup power current of the microcontroller may be of the order of 1 µA with the RTC oscillator on, but the actual current will depend on the specific device and supply voltage. The backup current will be approximately proportional to the battery backup supply voltage less 1.4V. In case of a button-cell lithium battery, the supply voltage will be 2.7V. In case of a super capacitor, the supply voltage will decrease linearly with charge until SRAM and RTC reach their operating cutoff, which will typically be between around 2.0V.

Microcontroller Backup Power Guidelines
Electrically isolate the microcontroller in the event of system power loss, so that the backup power sources do not have to power the rest of the circuit. This will limit the required backup current.

Select a low power CMOS microcontroller. Many microcontrollers are now available with low current drain in mind.

Determine the true backup current. The backup current specified by chip manufacturers is often the worst-case maximum. The typical current may lower by a factor of 2, doubling the backup time.

Determine the true required backup period. This may be hours or days to satisfy most applications. In most of the civilized world, over 99% of power outages are less than 1 hour in duration. The backup power period in hours will be the capacity of the backup energy source in µAh divided by the expected data retention current in µA.