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Electronic Design

Supply circuitry selects main or backup voltage and disconnects load

A complete portable-equipment power supply, suitable for systems that can be plugged into a docking station, is presented in this circuit idea. When the main supply is removed or falls out of regulation, selector circuitry automatically switches the load to a regulated switch-mode supply powered by a backup battery. The system signals a controlling processor when this switchover occurs, and it also issues a warning when the backup-battery voltage falls below a programmable threshold.

Two discharged alkaline or NiCd cells are sufficient to operate the dc-dc backup converter (IC1 and associated components) (see the figure). This converter produces a pin-selectable output of 5 V or 3.3 V with an output current of 200 mA. The presence of the main supply voltage (5 V) deactivates the backup supply: The output of an ultra-low-power comparator/reference device (IC2) is low in the presence of 5 V, and connects load and supply by turning on the p-channel MOSFET Q3. IC2’s low output also places IC1 in shutdown and turns off the nchannel MOSFET Q2. C3 is charged by the main supply via the parasitic diode in Q1, so R5 pulls the Q1 gate high, turning off that device as well.

If the 5-V supply fails or falls out of regulation (as defined by a 4.75-V threshold determined by R3 and R4), the IC2 output goes high, disconnects the main supply by turning off Q3, turns on the backup supply by pulling IC1 out of shutdown, and connects backup voltage and load by turning on Q1 and Q2. Q1 is chosen for low RDS(on) to minimize power dissipation, and Q2 is chosen for its ultra-low VGS threshold, to ensure a reliable switchover to Vbattwhen the main supply fails. The charge on C3 (present at all times, as mentioned previously) ensures a quick turn-on of the backup supply, and the charge on C4 supports the output voltage during switchovers between 5 V and Vbatt.

When the system is off (the backup converter is shut down; 5 V is absent), you can turn on the backup supply by momentarily depressing the pushbutton. A connection from this switch to an I/O line gives on/off control to the processor a well, and allows the supply to send on/off signals to the processor. Depressing the pushbutton turns on Q1, Q2, and IC1, enabling C4 to charge. Then R6’s pull-up/latching effect takes over when the push button is released.

Q3’s connection makes the drain more positive than the source. This unusual orientation allows the internal parasitic diode to conduct current when 5 V is connected, quickly charging C4 and providing power to IC2 (when IC2’s output goes low, Q3 turns on and its RDS(on) shunts the parasitic diode). Q3’s low forward drop has a negligible effect on the main supply voltage tolerance.

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