L1022 battery.Power-down protection circuit in single-chip microcomputer system

　　We should be grateful to the engineering and technical designers of the microcontroller chip, who first provided the internal conditions for the microcontroller system to successfully implement "power-down protection". This is: the microcontroller is allowed to be powered at a voltage as low as 2V or even smaller. Its most basic operation can still be guaranteed (the external input and output functions will be disabled or stopped).

　　When the main power supply is lost, the battery supplies energy for the continued operation of the microcontroller. The battery energy at this time is very precious and is often calculated at the "uA" level. And there is another unavoidable result, that is, as the protection time is extended, the battery power will be used up. Therefore, the protection circuit has a maximum protection time parameter, which cannot be exceeded during use, otherwise the protection will fail.

　　After the battery has been used for the protection period, it is necessary to replenish the electric energy so that it can be fully engaged in protection work during the next protection time. So, there is another question of how to charge the battery.

　　Low temperature lithium iron phosphate battery 3.2V 20A -20℃ charging, -40℃ 3C discharge capacity ≥70%

　　Charging temperature: -20~45℃ -Discharge temperature: -40~+55℃ -40℃ Support maximum discharge rate: 3C -40℃ 3C discharge capacity retention rate ≥70%

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　　To summarize: when the battery is powered by the main power supply normally, it must be charged by the main power supply; when the main power supply is lost for a while, the battery is discharged to maintain the operation of the microcontroller system.

　　The following is a standard power-down protection circuit. (VCC=6V).

　　To ensure sufficient battery power. Properly selecting the size of R1 can ensure that the charging current and charging time are reasonable.

　　For example: To charge a 3V6*60mAH battery and the charging time is about 8 hours, we choose the charging current to be 8mA, R1 = (6V-0.6)/8 (0.6 is the conduction voltage drop of the series connected diode).

　　The Zener diode connected in parallel with the battery is used to prevent the battery from overcharging.

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　　The discharge path is: the battery supplies power to the power supply port of the microcontroller through R1+R2. After the power supply current passes through R1+R2, there will be a voltage drop. When it reaches the VCC port of the microcontroller, the voltage will be lower than 3V6, usually at 2V-- Around 2V5, do not try to increase the power supply voltage of the microcontroller at this time. This will be counterproductive and make the microcontroller still work in the normal power supply state. There will be some differences in this low supply voltage for various microcontrollers from various microcontroller manufacturing companies. Adjust the resistor R2 to ensure that the current consumption is as small as possible while ensuring that the microcontroller can keep running.

　　Note: The current size of the power-down protection is also related to the crystal frequency of the microcontroller and the writing of the program software, because when the battery supplies power to the microcontroller, although the battery power cannot be reversely conducted through D2 to the public power supply line of other devices, However, other output ports of the microcontroller will discharge current through other devices, resulting in a large protection current and greatly shortening the working time of battery protection. As far as the microcontroller is concerned, the higher the crystal frequency, the greater the required power-down protection current.

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