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
REF
line 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
Q
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
V
BE
of the transistor). When Q2 turns off, N-channel FET Q3 is
turned off, which then turns off P-channel FET Q5.
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.
© Cymbet Corporation
•
18326 Joplin Street, Elk River, MN 55330
•
763-633-1780
•
www.cymbet.com
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