AN-1028: Ultra Low Power MSP430 Backup Using the EnerChip™ CC

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Doc AN-72-1028 Rev B

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Circuit Description and Operation

Many systems utilizing microcontrollers require non-volatile date retention and/or real-time clock operation in

the event of main power interruption. Normally, this is accomplished with the use of a supply supervisory circuit

coupled with a backup power source such as a coin cell or supercapacitor. When main power is interrupted -

as during line power outages or when the main battery is removed for recharging or replacement, the backup

power source provides enough energy to maintain the data in the embedded SRAM or keep the internal real-

time clock operational. Many MCUs have an internal supply supervisor circuit that can be used to control the

switchover function from main power to auxiliary power. When the supply voltage threshold level is reached

during power droop or complete power loss, the MCU can be programmed to generate an internal interrupt that

places the MCU into a low power state for maximum run time from the backup power source. The EnerChip

CC is a surface mount device that contains a rechargeable battery with integrated battery management that

performs the charge control, discharge control, threshold voltage detection, and supply supervisory functions

all in one low profile package. It operates over the range of 2.5V to 5.5V.

The EnerChip CC quiescent current of ~ 2 µA will in some cases be greater than the current drawn from the

MSP430 microcontroller while in low power standby mode. Therefore, it is desirable in those cases to eliminate

the EnerChip CC parasitic current consumption whenever possible. In other words, when the EnerChip internal

to the EnerChip CC is not being charged, the EnerChip CC ought to be disabled without compromising its utility

or performance.

This can be done by disabling the EnerChip CC most of the time by taking advantage of the internal supply

supervisory feature of the MSP430 and, as a result, dramatically reduce overall system power consumption.

This is especially important when the main system power is a battery that will need to be recharged or replaced

during the operational life of the system.

The circuit of Figure 1 relies on the internal supply supervisor feature of the MCU to signal the system when

main power has fallen below the threshold voltage necessary for normal circuit operation. While the EnerChip

CC also has this threshold detection feature, its operation contributes to the quiescent current draw. In MCUs

where that function is already embedded, it is logical to remove the redundant feature if doing so results in

lower operating current and does not compromise other essential attributes of the system. The remaining

features of the EnerChip CC that may continue to be utilized with the threshold voltage detection and

switchover features disabled are:

Internal battery charge control circuit that tightly regulates the charging voltage on the internal

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EnerChip;

Battery cutoff circuit that disconnects the load from the EnerChip when the discharge voltage

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reaches a preset limit.

Both of these features are essential elements to maximizing the service life of the EnerChip and should be

utilized to ensure proper operation of the embedded EnerChip as the backup power source.