Microcontroller Back-up

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 solid state rechargeable batteries are backup power alternatives to traditional bulkier power sources, such as coin cell batteries and super capacitors. EnerChip is specifically designed to support a range of currently available microcontrollers including the following MCUs. Application Note AN-1027 for MCU power backup is here >>

  • 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.

EnerChip CBC-EVAL-05 Evaluation Kit for MCU Power Backup

The EVAL-05 kit is ideal for designers who want to experiment with using EnerChip solid state batteries for small footprint, life-of-product power backup. The details and datasheet for the EVAL-05 can be found here >>

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.

Calculating Back-up Time

  1. Identify the primary source of power to the unit. This is normally AC Power through an AC/DC converter or a Primary Battery.
  2. Determine the primary power source outage characteristics. For AC power, this is usually commercial AC power interruptions ranging from brown-outs to disruptions in the utility grid. For Primary batteries this is usually the time it takes to change out a failing battery or the time a primary battery has failed before user change out.
  3. Determine power outage duration. In the case of Primary AC interruption, studies have shown that 99.5% of all these outages are 4 hours or less. The outlier data ranges up to 14 hours. Please click on the RTC Backup table an the RTC Backup webpage for the power outage study details.
  4. Calculate the amount of back-up power required using the MCU sleep current specifications.
  5. Implement a simple EnerChip CC single chip power back-solution. It's like a micro Uninterruptible Power Supply on your board.

 

MCU Devices Supported

- TI MSP 430

-Microchip PIC

-Freescale HC

-Atmel PicoPower AVR

-ST Micro ST62

-EM Microelectronics

- Many other popular MCUs are supported. Please contact Cymbet for additional information.

EVAL-05 EnerChip CC power backup Schematics

The following schematic is shows the simple EnerChip CC CBC3112 or CBC3150 to MCU connections. Additional details can be found in Applications Note AN-1027

EnerChip MCU power backup schmatic

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