Just For Starters
tial means less heat and a more reliable
system. Of course, you must provide a
low enough input voltage to take advantage of low drop-out power savings.
Power distribution schemes vary
based on the circuit assembly technology at your disposal. The best solution is
to distribute power on a printed circuit
board (PCB) with continuous copper
planes for each voltage rail. This is the
lowest inductance and resistance
method. Power connections between
regulators and planes should be with
multiple vias to keep inductance and
resistance low. If you are wiring a circuit
by hand on a breadboard, try to minimize the distance that power must travel from the regulators to the other components. Longer wires increase inductance, which increases the circuit's sus-
ceptibility to higher frequency problems. Whether using a PCB or
breadboard, adequate power supply
decoupling is essential to minimize
disruptive electrical transients that
result from switching digital circuitry. A minimum decoupling provision
is to place a high-frequency ceramic
capacitor at the power leads of each
integrated circuit (IC) on your board.
A small capacitance — such as 0.1
µF — should be used so that the
capacitor can be effective at the
high frequencies generated by
switching logic. It is equally important to minimize the wire lengths
between each capacitor and its
associated IC to reduce the inductance of the capacitor circuit.
Longer wire lengths can nullify the
beneficial effects of decoupling capacitors. Figure 4 shows a basic decoupling
scheme for a small system. Each IC has
Mark Balch is the author of
Complete Digital Design (see www.
works in the Silicon Valley high-tech
industry. His responsibilities have
included PCB, FPGA, and ASIC design.
Mark has designed products in the
fields of telecommunications, HDTV,
consumer electronics, and industrial
computers. In addition to his work in
product design, Mark has actively
participated in industry standards
committees and has presented work
at technical conferences. Mark holds
a bachelor's degree in electrical
engineering from The Cooper Union in
New York City. He can be reached
via Email at mark@completedigital
Figure 4. Power supply decoupling
a 0.1 µF decoupling capacitor. There
are also a couple of higher-value "bulk"
capacitors ( 100 µF) that provide lower-frequency decoupling. Voltage regulators have a finite response time to
changes in current demand. The bulk
capacitors help with these lower-frequency transient events. Exact
values for the bulk capacitors are
usually imprecise, especially given
wide tolerances in components.
Recommendations may be available
from regulator data sheets.
NUTS & VOLTS
Safety considerations cannot be
over-emphasized when dealing with
power circuits. Proper fusing, insulation,
cooling, spacing of components, and
component selection should not be
neglected in the haste to build your creation. It is always wise to conservatively
de-rate components based on their
power specifications. For example, if a
component is to dissipate one watt,
select a component that is rated for two
watts. Building a safe, reliable power circuit is not difficult for small systems
and will give you confidence in your
An attentive reader pointed out a mistake
in last month's column on PCB design.
The thickness of half-ounce copper was
incorrectly listed as 0.006 inches, while it is
actually one tenth of that, 0.0006 inches.
Circle #89 on the Reader Service Card.