MAX931 Comparator Practical Circuit Example

The main power supply and backup battery are connected to the load through a simple diode "OR" logic circuit. However, when the battery voltage exceeds the main power supply voltage, the diode "OR" logic circuit will connect the battery power supply and cannot reasonably select the main power supply. The circuit in Figure 1 provides a solution to this problem. The voltage range of the main switching power supply is 7V to 30V, and the backup power supply is a 9V battery.

Figure 1. IC1 The MAX931 comparator monitors the main supply voltage. When the main supply voltage drops below 7.4V, it switches on the backup battery by connecting the negative terminal of the battery to ground.

The MAX931 is an ultra-low-power comparator with a 1.182V bandgap reference. In normal operation, the comparator output is low, the three parallel n-channel FETs are turned off, the negative terminal of the battery is floating, and the load is powered by the main power supply. When the main power supply voltage drops to 7.4V, the comparator output is high, which will turn on the n-channel FET, connect the negative terminal of the battery to ground, and the load is powered by the battery (Figure 2).

Figure 2. As the main supply voltage (Channel 3 in Figure 1) decreases, the gate voltage of the n-channel FET goes high (Channel 2). This turns on the battery, causing the output voltage (Channel 1) to reach 9V. When the main supply voltage reaches 8.4V, the n-channel FET turns off, restoring the main supply to power the output.

The gate drive circuit's D1, C1, and R6 provide a delay that eliminates transients that occur when the circuit switches from battery to main power. These transients can cause the system's microcontroller to reset, which is unacceptable for most systems. Figure 3 shows the circuit's behavior without transients. Note: R3 and R4 set the MAX931's hysteresis voltage to 800mV to ensure proper operation. Refer to the MAX931 data sheet to calculate the appropriate resistor values.

Figure 3. When the main power supply is quickly restored, the output of Figure 1 responds without transient disturbances.