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Using the Atmel® picoPower™ AVR® Microcontroller with Cymbet™ EnerChips™

Introduction

The Cymbet EnerChip and Atmel picoPower AVR are combined in a demonstration board designed to show the advantages of Cymbet EnerChip rechargeable batteries for embedded processor applications. Cymbet EnerChips provide virtually unlimited system life. They are smaller, lighter and environmentally friendlier than primary lithium coin cells for backup power. The demo board consists of an Atmel Atmega169P picoPower AVR microcontroller with integrated LCD controller, Cymbet EnerChip cells, and a Cymbet charge control circuit.

System Considerations

Cymbet EnerChips are ideal for applications requiring a high number of charge/discharge cycles, such as backing up microcontrollers during intermittent power outages. Such systems will need a charge controller to keep the EnerChip at a full state of charge without overcharging the battery and thereby reducing its useful life. The charge circuit will need a source of voltage higher than the EnerChip charging voltage of 4.1V. This may require a charge pump. The charge circuit must regulate the charge voltage and have a means of isolating itself from the EnerChip (current blocking) to prevent the EnerChip from discharging through the charge circuit when system power is off. Isolation from the main power supply and voltage reduction from the battery output to the load may also be necessary.

Consideration needs to be given to the minimum voltage needed by the system to run off backup power, on how to switch between main power and backup power, and on how to signal the microcontroller that it will be running off backup power. Special algorithms may be used so that the microcontroller sheds loads and puts itself into a low-power sleep mode during backup.

The power-fail sensing circuit must not feed power back into the main power bus when the system switches over to the EnerChip for backup power. In the Demo Board, a 4.7 MOhm resistor is used to limit current on the VREF line to ≈500 nA. A diode would have eliminated all current leakage and have been an even better choice.

The charge circuit in Figure 4 meets all key system requirements. It is powered by two lithium coin cells, so that no boost charge pump is needed. Charge voltage regulation utilizes a Zetex ZR40401F41TA shunt voltage regulator, but could also have used a low IQ LDO, such as a TI TPS715XX regulator.

Current blocking to prevent the EnerChip from being discharged when main power is off is implemented with transistors Q2, Q3, and Q5. Transistors Q3 and Q5 are dual packaged devices, with the two transistors wired in series so that the off-state leakage current is negligible for a μAh-rated EnerChip.

PNP transistor Q2 is used as a comparator to shut off the charge circuit when the input voltage falls below ≈ 4.8V (4.1V plus the VBE of the transistor). When Q2 turns off, N-channel FET Q3 is turned off, which then turns off P-channel FET Q5.

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