AN-1042: EnerChip™ CC Backup Power for Epson RX-8564 RTC

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Doc AN-72-1042 Rev B

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Circuit Description and Operation

Under normal operation, the RTC 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. 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 needs no more than one hour of charging time per month. Such

a low charging duty cycle would present a negligible parasitic load to the main power source, as the EnerChip

charges quickly and its charging current decays to just a few tens of nanoamperes when fully charged. The

EnerChip CC controls the EnerChip charging function internally.

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 operation of the

real-time clock. In applications where the RTC is connected to a CPU or MCU, it might be necessary to connect

the CBC31xx RESET/ line to the gate of an n-channel FET placed between the CPU/MCU ground pin and system

ground. This configuration ensures that no parasitic ESD I/O structures in the CPU/MCU are biased with the

interface bus lines, and thus draining the EnerChip.

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 and the

switchover threshold voltage is adjustable. A block diagram of the EnerChip CC is shown in Figure 2.

Figure 2. Block diagram of the EnerChip CC Thin Film Battery with Integrated Battery Management

EnerChip CC includes the following functional elements:

Internal charge control circuit that converts a wide range of input voltages to a tightly

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regulated battery charging voltage;

Battery cutoff circuit that disconnects the load from the embedded EnerChip when the

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discharge voltage reaches a preset limit;

Output pin (RESET/) that can be used to drive an interrupt line on an MCU to indicate that the

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system is operating in battery-backed mode;

A voltage detection circuit allowing a user-selectable threshold trip voltage;

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A control line that enables and disables the internal charge pump that can be controlled

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externally to reduce power consumption when the internal EnerChip does not need to

be charged.