Electronic Design

Get +5 V/+3.6 V From 4 AA Cells

Today's 386 SX processors require more power than two AA cells can deliver for reasonable lifetimes. Thus, for optimal results, four AA cells are needed.

With this unique logic-power-converter design (see the figure), a switchable 3.6 or 5 V at 200 mA can be attained using four AA cells. The supply incorporates a MOSFET switch that can switch to a lithium backup battery, providing a 3.4-V output when the main battery is dead or removed. The supply consumes only 380 µA under no-load conditions.

The circuit operates in a somewhat novel mode as a step-up/step-down converter. When the cells are fresh (from about 6 V to about 5.2 V), the LT1173's gain block drives the p-channel MOSFET, turning the circuit into a linear voltage regulator. This may seem inefficient, but the batteries are quick to drop from 6 V to 5 V. At a 5-V input, the efficiency (for the 3.6-V output) is 3.6/5 or 72%, which is reasonable. As the battery-pack drops in voltage, efficiency increases, reaching greater than 90% at 4.2-V input.

At a point below 4-V input, the circuit switches to step-up mode. This mode squeezes the batteries for all of their available energy. In this case, efficiency runs between 83% at approximately 4-V input to 73% at 2.5-V input.

The supply can deliver 200 mA over its entire operational range. In its linear mode of operation, the supply has no current spikes that, because of the fairly high internal resistance of the alkaline cells, can reduce battery life. The topology delivers over 9.3 hours of 3.6-V, 200-mA output power, compared to just 7 hours using the traditional flyback topology employed in other designs.

The supply has a backup function implemented with another LT1173. The power for this part is "stolen" from the main logic output. The quiescent current for the backup LT1173, approximately 110 µA, is taken from the four AA batteries. The lithium "backup" battery is drained only by the 10-µF capacitor's leakage, the switch leakage, and about 1.5 µA from the 910-k/1-M resistor divider. The total load current equals about 5 µA. The LT1173's current, which comes from the main logic supply, is about 110 µA.

The backup supply is designed so that when the Backup/Normal input to the VN2222 switch goes high, the feedback string is connected. However, the converter won't operate because the output is held high (>3.4 V). When the main AA cells are removed or discharged, the backup supply will automatically kick in. Consequently, approximately 10 mA at 3.6 V is provided to back up system parameters and/or data in memory.

An additional feature of the backup supply is the low-battery detector inside the LT1173. It is implemented by connecting the 910-k/1-M resistor divider. The resistor divider sets the BL4 output to go to a logic low when the voltage on the lithium cell(s) goes below 2.4 V

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