AN-1027: Ultra Low Power MCU Backup Using the EnerChip™ CC

©2008 Cymbet Corporation • Tel: +1-763-633-1780 • www.cymbet.com

Doc AN-72-1027 Rev01

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Under normal operation, the MCU derives its power from the main system supply, which might be a large coin

cell, prismatic cell, cylindrical batteries, or indirectly from wall power through a voltage regulator. In that mode,

the EnerChip CC is isolated from the MCU by means of transistor Q1B being turned off when its gate is forced

high using an I/O line from the MCU. Transistor Q1A allows the main supply voltage to pass to the MCU, its

gate being forced low by another MCU I/O line. Because the EnerChip has very low self-discharge, it needs to

be charged only after it has been used in the performance of its duty as a backup power source or on a very

infrequent basis to keep it fully charged when not being used. To maintain a charge of 90% or greater on the

EnerChip when it has not been used as a backup power source for prolonged periods, the EnerChip would

need no more than one hour of charging time per month. This would present a negligible parasitic load to the

main power source, as the EnerChip charges quickly and its charging current decays to a small fraction of a

to the low state the vast majority of the time using an MCU I/O line. In this mode, the EnerChip CC will draw

essentially zero current. The l/O line can be forced high periodically as needed to replenish the charge in the

internal EnerChip. The MCU can be programmed to automatically do this periodically or for an hour or two

automatically upon returning to the normal operating condition after a power loss condition has occurred.

Output drive current of the MCU I/O line must be on the order of 0.5 mA minimum to ensure the EnerChip CC

internal charge pump performs properly.

During a main power supply interruption, the MCU is programmed to isolate (by turning off Q1A) the main

supply from the MCU and EnerChip CC to prevent the EnerChip from inadvertenly powering whatever output

stage is connected to the main supply line. Simultaneously, Q1B is turned on, allowing current to flow from the

EnerChip CC output to the power supply pin of the MCU, which is programmed to go automatically into a low

power mode when the threshold voltage limit is reached.

Q1A and Q1B can be combined and implemented in one low cost, small footprint 6-pin package such as an

SC70-6. Ensure that during power-up, the gate of Q1A is low so that power to the MCU is applied through Q1A.

If that condition does not occur naturally, it can be forced low using a filter capacitor on the gate of Q1A.

The implementation described herein has been demonstrated with a commercially available low power

microcontroller. Embedded source code written for this application is available from Cymbet upon request.

Conclusion

Traditional backup power sources have long charge times, high self-discharge, high steady state parasitic

charging current, and often require external components for charge control and discharge cutoff. The EnerChip

- with its low self-discharge, low parasitic current draw when charged, and fast recharge time - is a superior

solution to supercapacitors and coin cell batteries in backup power applications. When implemented in MCU-

based systems, the EnerChip CC delivers a robust backup power source with high cycle life and integrated

battery management while drawing no current under normal operating conditions. This approach can lead to a

significant extension in the operating life of the main battery.