Figure 1. This circuit connects the load either to the battery voltage or to VDC IN, depending on which voltage is higher.

The Figure 1 circuit manages load switching without intervention from a microcontroller or system software. When operating on batteries and with VDC IN disconnected, the POK (Power OK) output for the MAX8814 is high. This condition connects load and battery by turning on Q4 and Q3. Node 1 is biased to the battery voltage through R2, which maintains Q1 and Q2 OFF. When VDC IN connects to a DC source, Q1 and Q2 are held OFF because C1 raises node 1 to VBATT + VDC.

High voltage at the gates of Q1 and Q2 is generated instantly when VDC IN is applied. To prevent damage at the POK input, Q5 is configured as a source follower (voltage buffer). With the gate biased to the battery voltage, POK sees no voltage higher than the battery voltage. When POK goes low, current flows through Q5 and pulls down the gates of Q1 and Q2, turning them ON. VDC IN services the load, and the charger IC (U1) charges the battery. C1 and R1 provide a short delay that allows Q3 time to turn off, preventing unregulated current from flowing to the battery.

When VDC IN is removed, POK becomes a high-impedance node, and battery current flows through the body diode of Q3. The load voltage is VBATT - VDIODE. Because Q5's gate is biased at the battery voltage, Q5 conducts until POK reaches a voltage level sufficient to to service the load by turning on Q4 and Q3. Figure 2 shows the response of the Figure 1 circuit (with battery installed), while VDC IN is applied and removed.

Figure 2. These waveforms illustrate behaviour of the Figure 1 circuit as the load is switched from DC power to battery and back to DC power. (CH1 is voltage across the load, CH2 is the DC supply voltage, CH3 is the active-low POK output, and CH4 is the battery current.)

A modified circuit (Figure 3) is suitable for battery-charging ICs such as the MAX1507, which does not provide a POK output. The comparator (U3) provides a POK output by comparing VDC IN to the battery voltage. Like Figure 2, Figure 4 shows voltage at the load when VDC IN is applied and removed, for the Figure 3 circuit with battery installed.
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Figure 3. For battery-charger ICs without a POK output, this circuit performs the same function as that of Figure 1.


Figure 4. These waveforms illustrate behaviour of the Figure 3 circuit as the load is switched from DC power to battery and back to DC power. (CH1 is voltage across the load, CH2 is the DC supply voltage, CH3 is the active-low POK output, and CH4 is the battery current.)