To step a
DC voltage up
or down, or to
convert +V to
-V, it is common
to use circuits that
be bulky and
requirements are low, it often makes more sense to use
capacitor-based charge pumps. Charge pumps are available as
inexpensive integrated circuits such as the TC962.
Figure SB1 shows the block diagram of a charge pump used
to convert a positive DC voltage to a negative DC voltage. An
internal oscillator drives transistor-switches that control the
charging and discharging of the two external capacitors CF and
CS. CF is the so-called flying capacitor, while CS is the charge-storing capacitor for the output.
Operation is as follows:
• Step 1: Switches SW1 and SW3 are closed which connects
the top of CF to +Vin and the bottom of CF to ground. So,
CF charges up to +V. Switches SW2 and SW4 are open.
• Step 2: Switches SW1 and SW3 open, and switches SW2
and SW4 close. SW2 connects the top of CF (the positive
end) to ground, while SW4 connects the bottom of CF to
• Step 3: While SW2 and SW4 are closed, charge is
transferred from CF to CS.
• Step 4: SW2 and SW4 open, while SW1 and SW3 close.
The cycle repeats.
A charge pump can also be used as a voltage doubler as
shown in Figure SB2. The value of the output voltage is actually
Figure SB1. Charge Pump Block Diagram.
Vout = 2V - 2Vd,
where Vd is the
voltage drop across
can also generate 3V
and V/2, as well as
other voltage ratios
such as 3V/2, 4V/3,
2V/3, and so on.
A charge pump can be modeled as a voltage (V) in series
with an impedance (ZS) as shown in Figure SB3. ZS is composed
of the reactance of CF (XC) and the resistance of the switch (RS). A high switching frequency (ƒ) keeps XC low since XC = 1/( 2 π ƒ
C). RS depends on the size of the MOSFET switching transistors;
in an IC, the transistors are relatively small. So, the value of ZS is
mostly determined by RS.
For the TC962, RS is about 35
A value of 35 ohms may
not sound like much, but at a
load current of 20 mA it
causes a 0.7 volt drop in
output voltage. The bottom
line is that devices like the
TC962 work best in low
Figure SB2. Voltage Doubler.
Figure SB3. Source Impedance.
Charge pumps have two specifications for efficiency:
voltage conversion efficiency (ηV) and power conversion
efficiency (ηP). Voltage conversion efficiency is the measure of how well the magnitude of Vout matches the magnitude of Vin: ηV = |Vout / Vin| x 100%. Power conversion efficiency is ηP = (Pout
/ Pin) x 100%.
For the TC962, voltage conversion efficiency is 99.9%, so the
two magnitudes would differ by no more than 0.1% with no load.
The power conversion efficiency is 97%, so 3% of the input
power is lost in the process of converting +V to -V.
vertically. Make sure the cathode (the banded end) is in the
hole with a square outline (refer to Figure 2A). On the
solder side, bend the leads out to hold the diode in place.
Now, solder them and cut off the excess lead lengths. Next,
insert the two resistors, being careful not to mix them up.
The smaller value (200Ω) should be closer to the LM317.
Solder them in as you did with the diode.
Next, insert the capacitors. Verify that
C2, C3, and C4 are mounted with the correct
polarity. The positive end for each cap is
indicated by a plus sign (+) on the board.
Solder them in as you did with the resistors.
Next, insert jumpers at J1 and J2. On the
board, a line of copper is above each set of
holes showing the connections for a wall-mount supply. To use a battery, insert a
jumper between the middle hole and the left
hole at J1, and a jumper between the middle
hole and the right hole at J2. If you’re using
terminal blocks for VIN and VOUT, mount
them now. Okay, you’re finished building it.
rubbing alcohol and a scrub brush. Then, before applying
power, give the assembled board a good visual inspection.
Fix any problems you find. Apply VIN and measure +VOUT
and -VOUT with your multimeter. You should have a working
unit. Now, find a project where you can put it to good use.
ITEM DESCRIPTION SOURCE
IC1 LM317 D-PAK package Jameco part #838006
IC2 TC962 eight-pin DIP
D1 1N4001 or equivalent
R1 200 Ω, 1/4W, 1%
R2 1.24 kΩ , 1/4W, 1%
C1 0.1 µF, ceramic, 50V
C2 10 µF, electrolytic, 35V
C3 10 µF, electrolytic, 35V
C4 10 µF, electrolytic, 35V
Printed circuit board SV-2-DV
Terminal block: two-position 5 mm spacing
Terminal block: three-position 5 mm spacing
Shorting block: three pieces 0.1 in spacing
Male header strip, eight-pin, 0.1 in spacing
Jameco part #2094485
Clean off the solder side with some
30 August 2012