The motherboards we commonly see usually come with a button battery, as shown below:
What is the purpose of this button battery?
For common computer motherboards, the button battery provides power to the system clock when the system is powered off, ensuring the accuracy of the system time, and also keeps the BIOS settings unchanged.
For system boards like STM32, the role of the button battery is also to ensure the accuracy of the system time when the system is powered off, providing power to the RTC.
Generally speaking, the capacity of button batteries is relatively small, mostly around one hundred mAh, so in circuit design, the main role of the button battery is to save some important data when the device is powered off. The typical lifespan is around several years.
How should the circuit for this button battery be designed?
The most commonly used solution is to use two diodes to achieve power switching, as shown in Figure 1 below:
The principle is actually very simple, mainly utilizing the unidirectional conduction characteristic of diodes. When the system power is off, the button battery will provide power to the RTC through D2. When the system power is normally supplied, the RTC power is provided by the system power. As shown in Figure 2 below:
The purpose of using two diodes is to ensure that when the button battery provides power to the RTC, D1 is cutoff, and the load on the system power side consumes almost no power from the button battery, thus enhancing the battery life.
The above circuit is sufficient for general products with low requirements, but in actual projects, it is also necessary to consider adding ESD protection devices and resistors, as the button battery comes into direct contact with the human body when first used or replaced, which carries a significant risk of introducing static electricity into the motherboard and damaging chips. This must be considered comprehensively!
What should be the most important consideration when selecting diodes?
It is the leakage current, which is a very important point that needs to be emphasized. A high leakage current will directly affect the battery life of the button battery. Generally, the manual of the diode will indicate this, as shown below:
It can be seen that leakage current is directly related to temperature.
Of course, an integrated diode can also be chosen, such as BAT54C, with the following leakage current:
Alright, I will stop here for today!
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