6
DEMO MANUAL DC194
MICROPOWER BOOST REGULATOR
voltage. Normally, the high resistance of the ammeter will
not be present, so its negative effect on efficiency mea-
surements is misleading. This measurement problem can
be avoided by adding a large (1000
µ
F to 10,000
µ
F) bypass
capacitor across V
IN
between the ammeter and the DC194.
Comments on Layout
The layout used for this demonstration circuit may be
transferred to your circuit board (Gerber files are avail-
able). Also, the data sheet includes layout guidelines.
A boost regulator contains a high frequency current loop
that includes the power switch (between the SW and GND
pins of the LT1317), the diode (D1) and the output
capacitor (C2). This loop should be kept small and an
unbroken ground plane should be placed below it. Con-
nect this local ground plane to the LT1317 near its ground
pin and to the system ground at just one point. The
feedback resistors and the components on the V
C
pin
should be as close as possible to the LT1317 and should
be returned to the LT1317’s ground pin.
Testing in Your System
You may want to paste this circuit into your system to test
compatibility. This should be done with care, since long
hookup wires and ground loops can introduce noise
sources and regulation problems that would not be present
if the LT1317 DC/DC converter was properly designed into
your PCB.
Treat the DC194 as a 3-terminal device with V
IN
, V
OUT
and
GND terminals. Wire the DC194 to your circuit board with
wires as short as practical, to points on the circuit board
that are close to each other. Also, add high frequency
bypass capacitors (0.1
µ
F ceramics) from V
IN
and V
OUT
to
ground on your circuit board.
If you are bringing power directly to the DC194, use two
wires from the input source to the V
IN
and GND terminals
of the DC194. The output power should be applied to your
system as described above, and either the input supply or
your circuit should be floating in order to avoid ground
loops.
OPERATIO
U
DESIGN ALTERNATIVES
Component Selection
The components used for the DC194 represent a compro-
mise in cost, performance and size. They are well matched
for the power capabilities of the LT1317 but there are many
options for the designer to optimize the circuit for his or
her application.
Diode D1 (Motorola MBR0520LT1) is a 0.5A, 20V Schot-
tky diode. It is a good choice for nearly any LT1317
application, unless the output voltage or the circuit topol-
ogy requires a diode rated for higher reverse voltages.
Motorola also offers 30V and 40V versions. Most 0.5A and
1A Schottky diodes are suitable and they are available
from many manufacturers. If you use a silicon diode, it
must be an ultrafast recovery type. Efficiency will be lower
due to the silicon diode’s higher forward voltage drop.
L1 is a 10
µ
H inductor rated for 1A of operating current. The
value of the inductor should be matched to the power
requirements and operating voltages of your application.
In most cases a value of 10
µ
H is suitable. The inductor
should be rated for ~0.75A peak without excessive satu-
ration—the current limit of the LT1317 internal power
switch allows the part to tolerate moderate inductance
loss. The Sumida CD43-100 used on the DC194 has a
relatively small footprint with low losses. The DO1608
series from Coilcraft offers a similar inductor. A smaller,
less expensive choice is the Murata LQH3C100K24, which
fits in a tiny 1210 footprint. Efficiency will be slightly lower
at higher operating currents. Finally, Coiltronics’ CTX10-1
is a surface mount toroidal inductor with good perfor-
mance; it will generate lower stray magnetic fields than the
drum-type inductors listed above.
Lower Ripple
The quality of the output capacitor is the greatest determi-
nant of the boost converter’s output voltage ripple. The
output capacitor performs two major functions. It must
have enough capacitance to satisfy the load under tran-
sient conditions, and it must shunt the AC component of
the current coming through the diode from the inductor.
The ripple on the output results when this AC current
