Datasheet > AN-1014

AN-1014

更新时间： 2024-10-28 12:54:11

品牌	Logo	应用领域
CYMBET		微控制器
页数	文件大小	规格书
4页	829K
描述
Using the AtmelÂ® picoPower AVR Microcontroller with Cymbet EnerChips

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AN-1014

Application Note

Using the Atmel® picoPower™ AVR® Microcontroller

with Cymbet™ EnerChips™

A Demonstration Vehicle for Cymbet Solid State Thin Film Rechargeable Batteries

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 V_REFline to ≈500 nA. A diode would

have eliminated all current leakage and have been an even better

choice.

Figure 1. Cymbet Atmel picoPower AVR Demo Board.

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.

Figure 2. Charge Controller Block Diagram.

Charge Circuit Description

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 I_QLDO, 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.

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.

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

V_BEof the transistor). When Q2 turns off, N-channel FET Q3 is

turned off, which then turns off P-channel FET Q5.

DOC-111014 RevA

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