B RUEILDGAUSWLITACHTINOG R
We’re all familiar with linear
IC regulators, especially the
three-pin TO-220 package
types like the 7805 and the
LM317. They’re inexpensive,
and their low-noise and
fast transient response
make them ideal for many
applications. Their one
drawback is efficiency. For
example, a 7805 regulator
delivering one amp from BY JIM STEWART
an input of 12 volts will
dissipate seven watts of heat while delivering five watts to the load.
Plus, a large heatsink would be required to keep it cool enough to operate.
When efficiency is important — as with battery-operated designs — we have the option of using a
switching regulator. In fact, most modern equipment uses
switch-mode design in the form of off-line power supplies
and switching regulators. But many hobbyists I know shy
away from switchers. Using an older controller IC like the
LM3524 requires a fair amount of design, as well as an
external switching transistor. Then there’s the requirement
for an inductor. How do you choose the right one, and
where can you get them?
Fortunately, newer IC switching regulators such as
National Semiconductor’s LM2576 (one of National’s
“Simple Switcher®” chips) make it almost as easy as
using a 7805. The LM2576 is also available from ON
Semiconductors and Micrel. The chip is available in a
five-lead TO-220 package as shown in Figure 1, as well as
■ FIGURE 1
a TO-263 surface-mount package. The device is rated
for three amps maximum and is available in fixed-output versions ( 3.3V, 5V, 12V, 15V) and in an adjustable-output version.
When prototyping projects, a small printed circuit
board (PCB) with an adjustable regulator is handy to
have around. Here, we will build just such a board using
the LM2576T-ADJ (the adjustable version in a TO-220
package). The schematic is shown in Figure 2.
I wanted a design for currents up to one amp or
so. Since it’s always good practice to derate power
components, I picked the LM2576 because it’s rated for
three amps. Note that diode D1 is there as protection
against applying reverse polarity to the input. If you’re the
gambling type, you can omit it. But remember Murphy’s
Law: If a mistake can happen, then it will happen.
■ FIGURE 2