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The proliferation of portable equipment in everyday life presents unique challenges in the design of power-conversion and power-management circuitry. These include the accommodation of multiple power sources and output voltages, size and weight constraints and, most importantly, the conservation of battery energy.

Critical in the design of portable equipment is provision for the switchover between battery voltage and an ac source. This changeover should be accomplished with minimum user intervention and power loss. Care must be taken, for example, to avoid inserting components in series with the battery voltage. When battery voltage is low any additional voltage drops in the current-delivery path (due to series components) will have a devastating effect on the efficiency.

When considering the cost of a design, any investment that reduces resistance in the battery-current path brings dividends in the form of longer battery life. When you plug in the ac wall cube, for example, any dc-dc circuitry powered by the battery should draw minimal (preferably zero) current. When the ac power source is operating the system should avoid back-charging the battery unless that operation is incorporated as a controlled charging scheme.

The following six circuits illustrate, with generally increasing complexity, some practical ideas for accomplishing the switchover between a battery and an ac-power source.

Diode isolation (Method #1)

The simplest method for switching between two power sources employs two Schottky diodes for isolating the sources (see Fig. 1.) The wall cube’s output must be higher than that of the battery. Diode D2 is reverse-biased when ac power is applied, prohibiting the flow of current from battery to load. When the ac source is removed, diode D1 prevents the flow of battery current to the wall cube.

Fig. 1: Two isolating Schottky diodes offer the simplest means
for switchover between two power sources.

This scheme requires minimum design effort and board space, but it carries two disadvantages. D1’s forward voltage drop (~0.4 V) lowers the battery voltage, which can be unacceptable if the battery voltage is low to begin with. The drop also wastes power with an amount equal to load current times forward voltage dissipated in D1.

MOSFET switch (Method #2)

This scheme replaces D2 in Method #1 with a p-channel MOSFET (see Fig. 2.) The MOSFET turns on when wall-cube voltage is absent, allowing the battery to power the load. The presence of wall-cube voltage turns off the MOSFET by biasing its gate above its source disconnecting the battery. For 100-mA load currents, a p-channel MOSFET with 50 milliohm on-resistance drops 5 mV and dissipates only 0.5 mW. (Compare with the diode configuration of Method 1 which drops 400 mV and dissipates 40 mW.)

Fig. 2: To save power in the Fig. 1 circuit, replace one diode with a MOSFET.

Note that MOSFET on-resistance depends on its gate bias. With ac power removed (Fig. 2, again) the MOSFET gate is at ground and its source is at the battery voltage. This V_GS voltage must produce an on-resistance low enough to support the desired output voltage at maximum load current, so low-threshold MOSFETs are preferred for these applications.

Dual outputs with ac/dc switchover (Method #3)

Method 3 is a handy design (see Fig. 3) for power supplies in which two output voltages (in this case, 5 V at 600 mA and 3.3 V at 200 mA) are derived from an ac source or a 2-cell, 3-V battery. This scheme addresses the disadvantages of Methods 1 and 2. In particular, it places no diode or MOSFET in the path of load current from the battery.

Fig. 3: This switchover circuit accommodates two output voltages.
Click here for larger image.

A switch-mode, dc-dc boost converter (U1) accepts the unregulated dc input voltage (2 V to 5 V) and generates a regulated output of 5.1 V. Another switch-mode boost converter (U2) includes a linear regulator powered by its own boosted output. Setting U1’s output at 5.1 V ensures that it will be higher than that of U2 ensuring that U1 will power the U2 linear regulator when ac power is applied.

U2 delivers 300 mA of load current with 95% efficiency, and it includes a linear regulator that delivers 200 mA at 3.3 V. To prevent battery current from flowing into U1, a comparator in U2 (LBN/LBO) shuts down U1 in the absence of a dc voltage from the ac source.

When both ac and battery power are applied, U2’s feedback senses that the output voltage (5.1 V) is above the set-point for regulation and U2 remains in idle mode (no switching) as long as the output is above its set-point. Those conditions guarantee that the ac source maintains U2’s linear regulator and the 5V output as well, with a minimal current drain of less than 1 µA from the battery into U2.

Switchover between unregulated dc and battery (Method #4)

Method 4 (see Fig. 4) generates a single 3.3-V output from the wide-ranging dc (5 V to 16 V) produced by an ac wall cube. It provides a regulated 3.3 V with 200-mA output current, and uninterrupted switchover to a battery backup consisting of two AAA alkaline cells.

Fig. 4: This switchover circuit accommodates a single output voltage
and a wide-ranging ac-power output of 5 V to 16 V.

U1 is a switch-mode step-down converter programmed by the 218 k/122 k resistor divider to produce an output of 3.4 V. To prevent the flow of battery current into U1, another divider (R1/R2) monitors the input voltage and shuts down U1 when the wall-cube voltage is lost.

U2 is a switch-mode step-up converter that boosts a 2-cell-battery voltage (about 3 V) to 3.3 V achieving 85% efficiency at a 100-mA load. U2’s output is pulled to 3.4 V when the wall-cube voltage is present, causing U2 to stop switching because its feedback senses an output above the regulation set-point. U1 then supplies U2’s quiescent current (via its OUT terminal) as well as the load current. For these conditions the battery drain is less than 1 µA.

Regulated dc and battery inputs (Method #5)

When the output is 3.3-V at 200 mA and the wall-cube output is a reasonably well-regulated 5 V to 9 V the use of a linear regulator in addition to a switching regulator can save costs (see Fig. 5.)

Fig. 5: A better-regulated ac-power source enables use of a linear regulator (U1.)

Linear regulators are easy to use and require fewer components than switching regulators. (Most linears have internal pass transistors, so the only external components required are bypass capacitors.) Their drawback is power dissipation, equal to I_LOAD(V_IN - V_OUT). Power wasted in the regulator is considerable when the input is much higher than the output. Even if efficiency is not important the heat generated by a linear regulator is generally unacceptable in portable systems.

The linear regulator’s output is set at 3.4 V and the switch-mode step-up converter’s output is set at 3.3 V. Tying these outputs together as shown enables a novel shutdown scheme. U2 contains a low-battery comparator whose input (LBI) monitors the wall-cube voltage via a divider formed by the 100 kilo-ohm resistor and three diodes. Loss of this voltage causes the comparator output (LBO) to go high, shutting down U1 by turning on Q1.

U2 then takes over and maintains the output by boosting the battery input to a regulated 3.3 V (with 85% efficiency at 100 mA). If the wall-cube voltage is reinstated, U1 comes out of shutdown and generates 3.4 V, causing U2 to stop switching because it senses an output above its regulation set-point. Under that condition, U2 draws less than 1 µA (through its OUT pin.)

A complete medium-power system (Method #6)

A complete power supply for medium-power portable systems such as a PDA, hand-held inventory computer, or point-of-sale computer (see Fig. 6) accepts either unregulated dc or the output of two AA batteries, and delivers 500 mA at 3.3 V (3.3 V MAIN.) The circuit includes a lithium backup battery for powering the 3.3 V RAMs when neither ac source nor other battery source is present but this backup battery is never allowed to power the 3.3 V MAIN output. A boost circuit that generates -20 V at 20 mA for biasing LCDs can take power from either the main battery or the ac source.

Fig. 6: This power supply for medium-power portable devices provides
three regulated outputs with all necessary switchover capabilities.
Click here for larger image.

U1 is a switch-mode step-down converter whose wide input range (5 V to 16 V) enables it to operate from a wall cube or auto battery. It maintains a 3.4 V output while monitoring the wall-cube voltage, and shuts itself down when that voltage disappears, preventing current flow into U1 from the main battery. U2 is a switch-mode step-up converter that boosts the output of two AA cells to 3.3 V. (Alkaline cells typically produce 1.8 V to 3.2 V; NiCd or NiMH cells typically produce 1.8 V to 2.9 V.) It delivers 3.3 V at 500 mA, with 95% efficiency.

The outputs of U1 and U2 are connected together at 3.3 V MAIN. When the wall cube is active, U2 senses (as in Method #5) that the output voltage (3.4 V) is above regulation. U2 therefore goes into idle mode with U1 supplying it (less than 1 µA) and supporting the main output.

U3 is a boost converter based on a regulated charge pump, that converts an input of 1.8-3.6 V to a fixed output of 3.3 V (3.3 V RAM.) When ac power is absent and the main battery is low or missing, U3 converts the output of a lithium coin battery (as low as 2.7 V) to a RAM-backup voltage. Housed in a small micro-8 package, U3 remains shut down, drawing less than 1 µA, as long as the ac source or the main battery power is present.

U4 is a switch-mode boost converter configured in the inverting mode, powered by the wall cube or the 2-cell battery to produce -20 V at 20mA with 75% efficiency for biasing LCDs. U4 is never powered by the lithium backup battery.

U5 is a comparator with an internal 1.25 V reference, powered by the 3.3-V RAM output. (During startup, Q1’s body diode allows U5 to be powered by 3.3 V MAIN.) U5’s output goes low after startup, which turns on Q1 and allows the 3.3-V RAM output to be supplied by 3.3 V MAIN. U5’s low output also turns off U3 by holding its SHDN pin low.

If the ac source and battery power are both removed, U5 senses this condition at its input and drives its output high, releasing U3 from shutdown and allowing it to supply the 3.3-V RAM output. U5 is sustained during this transition by the charge on its bypass capacitor at the V+ terminal. While U3 is operating, it powers itself and U5 in addition to the 3.3-V RAM output. U5’s high output also turns off Q1, which prevents 3.3 V RAM from feeding 3.3 V MAIN. The p-channel MOSFET, Q1, is configured so that current flows into its drain terminal.

The LCD power converter (U4) derives power from the wall cube or the battery pack while powering itself from 3.3 V MAIN. When the wall cube is active it pulls Q2’s gate high, which turns off Q2 and thereby disallows any flow of battery current to L3. Unplugging the wall cube grounds the gate of Q2, turning it on and allowing the battery to power L3.

Design notes

• A switching regulator with internal power switch is generally satisfactory for generating low output currents. The lower component count reduces overall size, which in turn simplifies the pc layout -- especially for applications in which space is at a premium.
• To minimize battery drain, choose a regulator that includes a shutdown or idle function.
• An internal comparator that activates the shutdown in response to external changes offers further advantages. Make sure the output voltage is lower than the IC’s maximum voltage rating.
• Before connecting the outputs of any two voltage converters together, first make an independent test on the one programmed for lower output voltage: while monitoring its supply current, apply (force) the output with a voltage ~0.5V higher than that programmed. The supply current should drop to the level of battery current when the ac supply is operating (about 1 µA), but it should not become negative. If it goes negative or remains too high the converter should not be used. Other switch-mode regulators should be rejected if this test causes their output device to turn on, allowing current flow to drain the battery or flow back into it (back charging.)

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