July 17, 1997

Op amp makes precise 9V-battery splitter

Steve Hageman, Hewlett-Packard, Santa Rosa, CA

Alkaline 9V batteries are ideal for many portable, handheld applications, because they have 650 mAhrs of capacity, operate as low as approximately 6V at the end of their lives, and are available everywhere.

The trouble starts when you want to power 5V analog parts, and you need a true swing from ground to 5V. Although many op amps advertise single-supply operation, many only swing 100 mV or so to ground on the output. To generate a negative bias usually requires that you use a negative voltage converter, such as an ICL7660 charge pump.

As an alternative, you can generate a bipolar supply of 5V and ­2.3 to ­4V using a precision 5V reference and one op amp as a supply splitter (Figure 1). This circuit provides a precision-regulated 5V output and an unregulated minus bias voltage from one 9V battery. The circuit operates from an input of 6.3 to 9V (or more) and precisely regulates the 5V output (±10 mV) over this input range. The minus bias voltage swings from ­2.3 to ­4V over the same input, enough to provide plenty of negative swing for nearly any single-supply op amp.

This circuit can provide 10 mA on the 5V output over the full input-voltage range because of the LT1006 op amp's unique high-current output stage. The idling current is 1.7 mA with a 9V input and 800 µA with an input of 6.3V. The LT1029 reference consumes most of the current. Although this current is higher than the current of an ICL7660-based design, this op-amp supply splitter uses fewer parts for the same functionality.

Add bypassing capacitors only from the 5V output to the minus bias output. Adding capacitance from the 5V or minus supply to the output common may make the op amp unstable. If you must add capacitance, a 10 ohms resistor in series with the capacitor ensures stability. As an alternative, you can add a 10 ohms resistor from Pin 6 of the LT1006 inside the feedback loop to the output common, but this addition will increase the dropout voltage at high-output-current levels. (DI #2058)

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