lt3467aes67 Linear Technology Corporation, lt3467aes67 Datasheet - Page 6

• Manufacturer Part Number: lt3467aes67
• Description: 1.1a Step-up Dc/dc Converter With Integrated Soft-start
• Manufacturer: Linear Technology Corporation

APPLICATIONS
LT3467/LT3467A
OPERATIO
The LT3467 uses a constant frequency, current-mode
control scheme to provide excellent line and load regula-
tion. Refer to the Block Diagram above. At the start of each
oscillator cycle, the SR latch is set which turns on the
power switch Q1. A voltage proportional to the switch
current is added to a stabilizing ramp and the resulting
sum is fed into the positive terminal of the PWM compara-
tor A2. When this voltage exceeds the level at the negative
input of A2, the SR latch is reset, turning off the power
switch. The level at the negative input of A2 is set by the
error amplifier A1, and is simply an amplified version of the
difference between the feedback voltage and the reference
voltage of 1.255V. In this manner, the error amplifier sets
the correct peak current level to keep the output in regu-
lation. If the error amplifier’s output increases, more
current is delivered to the output. Similarly, if the error
decreases, less current is delivered. The soft-start feature
Duty Cycle
The typical maximum duty cycle of the LT3467 is 94%
(88% for the LT3467A). The duty cycle for a given
application is given by:
Where V
is in the worst case 330mV (at 1.1A)
The LT3467 and LT3467A can be used at higher duty
cycles, but must be operated in the discontinuous conduc-
tion mode so that the actual duty cycle is reduced.
Setting Output Voltage
R1 and R2 determine the output voltage.
6
V
DC
OUT
=
= 1.255V (1+ R1/R2)
D
|
V
is the diode forward voltage drop and V
|
OUT
V
OUT
| |
+
| |
U
+
U
V
D
V
D
| – |
INFORMATION
| – |
U
V
CESAT
V
IN
|
|
W
U
CESAT
of the LT3467 allows for clean start-up conditions by
limiting the rate of voltage rise at the output of comparator
A1 which, in turn, limits the peak switch current. The soft-
start pin is connected to a reference voltage of 1.255V
through a 250k resistor, providing 4μA of current to
charge the soft-start capacitor. Typical values for the soft-
start capacitor range from 10nF to 200nF. The LT3467 has
a current limit circuit not shown in the Block Diagram. The
switch current is constantly monitored and not allowed to
exceed the maximum switch current (typically 1.4A). If the
switch current reaches this value, the SR latch is reset
regardless of the state of comparator A2. This current limit
protects the power switch as well as the external compo-
nents connected to the LT3467.
The Block Diagram for the LT3467A (not shown) is
identical except that the oscillator frequency is 2.1MHz.
Switching Frequency and Inductor Selection
The LT3467 switches at 1.3 MHz, allowing for small valued
inductors to be used. 4.7μH or 10μH will usually suffice.
The LT3467A switches at 2.1MHz, allowing for even
smaller valued inductors to be used. 0.9μH to 6.8μH will
usually suffice. Choose an inductor that can handle at least
1.2A without saturating, and ensure that the inductor has
a low DCR (copper-wire resistance) to minimize I
losses. Note that in some applications, the current han-
dling requirements of the inductor can be lower, such as
in the SEPIC topology where each inductor only carries
one half of the total switch current. For better efficiency,
use similar valued inductors with a larger volume. Many
different sizes and shapes are available from various
manufacturers. Choose a core material that has low losses
at 1.3 MHz, (2.1MHz for the LT3467A) such as ferrite core.
2
R power
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