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Never buy another 9V battery

Don't buy expensive, short-lived 9V batteries. This little DC-DC converter is adjustable and lets you use two AA, C or D-size cells instead.

By Peter Smith

Say you want a 9V battery to supply 40mA to a circuit. That’s a pretty modest current but if you use a PP3 style 9V battery it won’t last long at all. In fact, if you’re using a typical "heavy duty" 9V battery, it will last less than 20 minutes before the voltage drops to 7.8V. That may be enough to stop your circuit working. Or maybe you are using an alkaline type. Depending on the brand and price, you might get about two hours life. Not good.

By comparison, two AA alkaline cells driving this DC-DC Converter circuit to give 9V at 40mA will last about 7 hours. And rechargeable AA cells can be even better. Table 1 shows the comparisons.

Click for larger image
Fig.1: the functional block diagram of the TL499A. It's housed in an 8-pin DIL package and contains both series (linear) and step-up switching regulators.

This circuit can deliver up to 90mA at 9V (with less life from the cells) or can be set to deliver anywhere between 4.5V and 20V. You might never have to buy another 9V battery ever again.

Back in the November 1990 edition of SILICON CHIP, we described a single cell to 9V DC converter suitable for replacing 9V batteries. That design proved very popular and was subsequently updated in August 1992. Unfortunately, the TL496 power supply IC used in both of these projects is now obsolete.

This project is based around the Texas Instruments TL499A, a similar but more versatile variant of the TL496. Most notably, its output voltage is programmable, making it suitable for use in a variety of low-power applications.

Unlike the original TL496 designs, this new design is specified for use with two cells. This enables the converter to produce more realistic output current levels. For low-power applications, two cells are also more cost effective, as more of their energy is extracted before the terminal voltage falls below the converter’s minimum input voltage.

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