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BUILD THE VOLTAGE
By Jim Stewart
Recently, I designed and prototyped a battery-powered piece of analog
equipment to run off two nine volt batteries. After the prototype was
finished, it was decided that the device needed to have the option of using
a wall-mount power supply, as well as batteries. Wall-mount supplies with
+9V and -9V outputs are not easy to find, but ones supplying 12 to 15 volts
DC are readily available.
Instead of redesigning the main printed circuit board
(PCB), I decided to use a small add-on board that would fit
inside the enclosure. On the board would be an LM317
regulator to bring the supply voltage down to nine volts.
Also on the board would be a TC962 charge-pump DC-to-DC converter. You supply +VX to the TC962 and it gives
back -VX. It’s a voltage mirror.
A glass mirror gives an exact copy of an image but
with left and right swapped. A voltage mirror gives a copy
of the input voltage but with the polarity swapped.
The output voltage of the LM317 regulator is given by
Figure 1 shows the schematic of the circuit. Jumpers J1
and J2 allow the circuit to be used with either a nine volt
battery or a DC wall-mount supply. The bars above the
jumpers (J1 and J2) show which ones to use for a wall
supply. For a battery, slide the J1 jumper left and the J2
jumper right. When jumpered for a battery, the LM317 is
bypassed. We’ll discuss J3 later. For the LM317, a 0.1 µF
capacitor at the input is recommended when the IC is more
than six inches from the DC source. Such would be the case
when using a wall supply, so we have C1 at the input.
Diode D1 is there because of my firm belief in
“Murphy’s Law.” In this case, Murphy says that if the input
polarity can be reversed, then eventually it will be reversed.
If you’re not a believer, you can replace D1 with a jumper.
VOUT = 1.25V ´ (1 + R2/R1) + R2 ´ IADJ
I wanted nine volts out, so I chose R1 = 200 Ω and R2
= 1.24 kΩ; both 1% resistors. A different value of VOUT
requires a different pair of values. Keep R2 low so that the
R2 x IADJ term is relatively small. IADJ is in the range of 40
µA to 100 µA, and changes with temperature.
Capacitors C2, C3, and C4 are 10 µF electrolytics. I
used tantalum, but a good aluminum cap should work fine.
C2 is the output filter for the LM317. C4 is the filter for the
negative output voltage of the TC962. C3 is the charge
pump capacitor for the TC962. (See the sidebar for a
description of how a charge pump works.) The TC962
charge pump runs at 12 kHz, but if you connect pin 6 to
ground, the speed doubles to 24 kHz. The higher frequency
allows a smaller value for the output filter capacitor (C4). J3
allows you to connect pin 6 to ground.
The Circuit Board
Figure 2A shows the component side of a small PCB
for the circuit. The board dimensions are 2. 8 by 1.3 inches.
The D-PAK surface-mount version of the LM317 is used; it’s
soldered to an area of copper to dissipate heat. (Note: A D-PAK is easy to solder, so don’t worry if you’ve
never used surface-mount devices.) Figure 2B
shows the solder side. There is a mounting hole
at each corner of the board big enough for a #4
screw. The mounting hole with the square
outline is connected to circuit ground, so
mounting the board to a metal chassis will
connect ground to the chassis. If that’s not
desirable, you can cut the etch as indicated.
Figure 3 is a photo of the assembled board.
I used terminal blocks for input and output
■ FIGURE 1.Voltage mirror schematic.