Backup lithium battery with delay

　　Author ：Iflowpower – Portable Power Station Supplier

This application note describes the main power supply and backup battery through diode "or" logic circuit and load connection. This architecture is easy to understand, but when the battery voltage exceeds the main supply voltage, the diode "or" logic circuit will connect the battery, and the main power supply cannot be reasonably selectable. This article gives a method of solving this problem.

MAX931 comparator, comparator built-in 2% base. Main power and spare batteries connected to load through a simple diode "or" logic circuit. However, when the battery voltage exceeds the main power supply voltage, the diode "or" logic circuit powers the battery, and cannot reasonably select the main power supply.

Figure 1 is given a method of solving this problem, the voltage range of the main switching power supply is 7V to 30V, the spare power supply is 9V battery. Figure 1.IC1MAX931 comparator is used to monitor main power supply voltage.

When the main supply voltage drops below 7.4V, it can be grounded back to the backup battery by grounding the battery negative. The MAX931 is an ultra-low power comparator with a 1.

182V band gap. When working properly, the comparator output is low, three parallel N-channel FETs are turned off, and the battery negative is empty, by the main power supply Power supply for load. When the main supply voltage drops to 7.

4V, the comparator outputs a high level. It will turn on the N-channel FET, ground the battery negative, powered by the battery (Figure 2). Figure 2.

Main power supply voltage (channel 3 in Fig. 1) gradually decreases, the gate voltage of the N-channel FET becomes high (channel 2). This will turn on the battery so that the output voltage (channel 1) reaches 9V.

When the main supply voltage reaches 8.4V, the N-channel FET is turned off, and the main power supply is output. The D1, C1, and R6 of the gate drive circuit appear a certain delay, which eliminates transient interference that the circuit appears from the battery to the main power source, and these transient interference can cause the system's microcontroller to reset, this One point about most systems is unacceptable.

Figure 3 shows the characteristics when the circuit does not exist transient interference. Note: R3 and R4 set the hysteresis voltage of the MAX931 to 800mV to ensure the correct working state. Please refer to the corresponding resistance value for the MAX931 data data.

Figure 3. When the power supply is quickly restored, the output response does not exist in transient interference.