4
DEMO MANUAL DC194
MICROPOWER BOOST REGULATOR
INTRODUCTION
The LT1317 and LT1317B are 600kHz PWM step-up
DC/DC converters. With a wide input voltage range (1.5V
to 12V) and high switch voltage (30V), these parts are
suitable for a wide variety of switching regulator circuits.
DC194 demonstrates their application in a simple boost
regulator with an output of either 3.3V or 5V.
DC194 is available in two versions. Version A uses the
LT1317CMS8. At light loads, this part switches automati-
cally into power-saving Burst Mode operation, resulting in
high efficiency over a broad range of load currents. Ver-
sion B uses the LT1317BCMS8, which operates at a fixed
frequency, regardless of load, eliminating low frequency
ripple on the output at the expense of light load efficiency.
You can confirm which version of the board you have by
checking the code on the IC; the LT1317 is marked with the
code LTHA, whereas the LT1317B is marked with LTHB. In
the comments below, “LT1317” will refer to both parts. In
cases where their characteristics result in different behav-
ior, the LT1317B will be mentioned explicitly.
This manual describes the operation of this demonstration
circuit, its performance and variations on the basic circuit.
For a thorough discussion of the LT1317 and its applica-
tion, please consult the part’s data sheet.
Hookup and Initial Tests
DC194 is a fairly simple low power switching regulator.
However, some precautions are necessary in order to test
the circuit safely. Proper hookup and accurate measure-
ments are necessary for meaningful evaluation of effi-
ciency and line and load regulation. Refer to Figure 2 for
proper connections.
OPERATIO
U
V
OUT
GND
V
IN
DC194 F02
DC194
BENCH
SUPPLY
1.5V TO 5.5V
1A
V
OUT
V
IN
I
OUT
V
V
A
I
IN
C
BULK
A
+
Select the desired output voltage by moving jumper JP1.
The input can safely accept a voltage up to 5.5V. A good
starting point is to apply 2.5V between the V
IN
and GND
terminals of the DC194, using a bench-top supply with a
1A current limit. Because the SHDN pin of the LT1317 has
been left floating, the LT1317 will begin operating as soon
as V
IN
is above 1.5V.
A load can be applied between the V
OUT
and GND termi-
nals, using either a fixed resistor, a decade resistor box
(provided that it is rated for the power) or an active load.
A simple initial load might be a 1/2W 100
Ω
resistor.
Warning: Because the basic boost circuit contains a DC
path between the input and output (through inductor L1
and diode D1), the circuit is not protected against a
shorted output. It is recommended that preliminary test-
ing of the circuit be performed using a current-limited
supply on the input.
With power applied to the DC194, the LT1317 should be
switching and regulating the output. Figure 3 shows some
of the circuit’s operating waveforms. The scope photos
show the output voltage, the current through inductor L1
and the voltage on the SW pin of the LT1317. The LT1317
is in Burst Mode operation in the first photo. The second
photo shows operation at a higher load current, where the
LT1317 is operating at a fixed frequency. LT1317B circuits
will operate in this mode at all load currents.
PERFORMANCE
Input Range and Power Capability
The LT1317 will operate from inputs above 1.5V. The
maximum allowable input voltage to this circuit is 5.5V,
which is based on the voltage ratings of the input and
output capacitors C1 and C2. The boost circuit will allow
the LT1317 to regulate the output only when the input
voltage is less than the desired output voltage plus one
diode drop. This means that the practical input range is
1.5V to 3.6V for a 3.3V output and 1.5V to 5.3V for a 5V
output.
The power capability of the DC194 is determined primarily
by the input voltage and the current limit of the LT1317’s
internal power switch and, to a lesser extent, by the value
of inductor L1. Therefore, the maximum load current that
this circuit can supply depends on the input voltage. A
Figure 2. Recommended Hookup for Proper Evaluation
of Efficiency and Regulation (See Text)
