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AN-1028 データシート(PDF) 3 Page - Cymbet Corporation

部品番号 AN-1028
部品情報  Ultra Low Power MSP430 Backup Using the EnerChip??CC
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メーカー  CYMBET [Cymbet Corporation]
ホームページ  http://www.cymbet.com
Logo CYMBET - Cymbet Corporation

AN-1028 データシート(HTML) 3 Page - Cymbet Corporation

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AN-1028: Ultra Low Power MSP430 Backup Using the EnerChip™ CC
©2008 Cymbet Corporation • Tel: +1-763-633-1780 • www.cymbet.com
Doc AN-72-1028 Rev B
Page 3 of 3
Under normal operation, the MCU 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. In that mode,
the EnerChip CC is isolated from the MCU by means of transistor Q1 being turned off when its gate is forced
high using an I/O from the MCU. Transistors Q2A and Q2B allow the main supply voltage to pass to the MCU,
their respective gates being forced low by another MCU I/O. 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
would need no more than one hour of charging time per month. This would present a negligible parasitic
load to the main power source, as the EnerChip charges quickly and its charging current decays to a small
fraction of a microAmpere when fully charged. Therefore, the EnerChip CC supply (VDD) and ENABLE pins may
be forced to the low state the vast majority of the time using an MCU I/O. In this mode, the EnerChip CC will
draw essentially zero current. The l/O line can be forced high periodically as needed to replenish the charge
in the internal EnerChip. The MCU can be programmed to automatically do this periodically or for an hour or
two automatically upon returning to the normal operating condition after a power loss condition has occurred.
Output drive current of the MCU I/O must be on the order of 0.5 mA minimum to ensure the EnerChip CC
internal charge pump operates properly.
During a main power supply interruption, the MCU is programmed to isolate (by turning off Q2A and Q2B) the
main supply from the MCU and EnerChip CC to prevent the EnerChip from inadvertently powering whatever
output stage is connected to the main supply line. Simultaneously, Q1 is turned on, allowing current to flow
from the EnerChip CC output to the power supply pin of the MCU, which is programmed to go automatically
into a low power mode when the threshold voltage limit is reached. The purpose of the dual transistors in the
VDD line is to prevent leakage current from passing through the transistor body diodes in any of the operating
modes.
Q1 can be replaced by diode D1, eliminating the need for using the MCU I/O that is otherwise controlling
the gate of Q1. Ensure that during power-up, the gates of Q2A and Q2B are low so that power to the MCU is
applied through the two transistors. If that condition does not occur naturally, it can be forced low using a filter
capacitor on the gates of Q2A and Q2B, or with a pull-down resistor (R1) as shown.
The implementation described herein has been demonstrated with the MSP430 low power microcontroller.
Embedded source code written for this application is available from Cymbet upon request.
Conclusion
Traditional backup power sources have long charge times, high self-discharge, high steady state parasitic
charging current, and often require external components for charge control and discharge cutoff. The EnerChip
- with its low self-discharge, low parasitic current draw when charged, and fast recharge time - is a superior
solution to supercapacitors and coin cell batteries in backup power applications. When implemented in MCU-
based systems, the EnerChip CC delivers a robust backup power source with high cycle life and integrated
battery management while drawing no current under normal operating conditions. This approach can lead to a
significant extension in the operating life of the main battery.


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