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.
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
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.