4
DEMO MANUAL DC242
DC/DC CONVERTER
OPERATIO
U
Introduction
The schematic in Figure 1 highlights the capabilities of the
LT1777. The application circuit is set up for an output
voltage of either 3.3V or 5V, set by jumper JP1.The demo
board comes equipped with input, output, GND, SYNC and
SHDN terminals to make bench testing convenient.
Since the LT1777 allows such a wide input range, from
7.4V to 48V, the internal control circuitry draws power
from the V
CC
pin, which is normally connected to the
output supply. During start-up, the LT1777 draws power
from V
IN
. However, after the switching supply output
voltage reaches 2.9V, the LT1777 uses the output to
supply its internal control circuitry, thereby reducing
quiescent power by hundreds of milliwatts when operat-
ing at high input voltage. This helps maximize efficiency
at high line voltages.
Theory of Operation
During normal operation, the internal power transistor is
turned on during each cycle when the oscillator sets a latch
and turned off when the main current comparator resets
the latch. While the internal switch is off, Schottky diode
(D1) carries the inductor current until it tries to reverse or
until the beginning of the next cycle. The voltage on the V
C
pin, which is the output of the error amplifier, controls the
peak inductor current. The FB pin provides the error
amplifier an output feedback voltage, V
FB,
from an external
resistor divider. When the load current increases, it causes
a slight decrease in V
FB
relative to the 1.24V reference,
which, in turn, causes the voltage on the V
C
pin to increase
until the average inductor current matches the new load
current.
Sense Inductor
The LT1777 uses an external sense inductor to program
the dI/dt limiting circuitry. The minimum usefull induc-
tance value for the sense inductor is 470nH, which pro-
duces a dI/dt of approximately 2.2A/
µ
s. Larger sense
inductors yield lower current slew rates, resulting in
reduced high frequency RFI emissions at the expense of
slightly reduced efficiency. Refer to the “Selecting Sense
Inductor” section in the LT1777 data sheet for information
on programming a different dI/dt rate through the main
power inductor.
SYNC Pin
This pin is used to synchronize the internal oscillator to an
external clock with a frequency between 130kHz and
250kHz. If a switching frequency higher than nominal is
desired, remove jumper JP2 and tie an external oscillator
of the desired frequency between the SYNC terminal and
the input GND terminal. The external oscillator can be a
TTL compatible level or 3.3V, with a duty cycle between
10% and 90%. Refer to the “Selecting Main Inductor”
section in the LT1777 data sheet for information on
optimizing the inductor value for running at a higher
frequency.
How to Measure Voltage Regulation and Efficiency
When trying to measure load regulation or efficiency,
voltage measurements should be made directly across the
V
OUT
and GND terminals rather than at the end of test leads
at the load. Similarly, input voltage should be measured
directly at the V
IN
and GND terminals of the DC242. Input
and output current should be measured by placing an
ammeter in series with the input supply and load. Figure 3
shows the typical efficiency curve for L2 (sense inductor)
= 470nH, V
IN
= 12V, V
OUT
= 5V.
Figure 3. LT1777 Output Efficiency
LOAD CURRENT (mA)
0
EFFICIENCY (%)
75
80
85
400
DC242 F03
70
65
60
100
200
300
500
V
IN
= 12V
V
OUT
= 5V
L2 = 470nH
