LT3467/LT3467A
6
3467afe
APPLICATIONS INFORMATION
OPERATION
The LT3467 uses a constant frequency, current-mode con-
trol scheme to provide excellent line and load regulation.
Refer to the Block Diagram. 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 comparator 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 amplifi er A1,
and is simply an amplifi ed version of the difference between
the feedback voltage and the reference voltage of 1.255V.
In this manner, the error amplifi er sets the correct peak
current level to keep the output in regulation. If the error
amplifi er’s output increases, more current is delivered to
the output. Similarly, if the error decreases, less current
is delivered. The soft-start feature 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 components connected to
the LT3467.
The Block Diagram for the LT3467A (not shown) is identical
except that the oscillator frequency is 2.1MHz.
Duty Cycle
The typical maximum duty cycle of the LT3467 is 94%
(88% for the LT3467A). The duty cycle for a given ap-
plication is given by:
DC =
|V
OUT
|+|V
D
|–|V
IN
|
|V
OUT
|+|V
D
|–|V
CESAT
|
where V
D
is the diode forward voltage drop and V
CESAT
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 conduction
mode so that the actual duty cycle is reduced.
Setting Output Voltage
R1 and R2 determine the output voltage.
V
OUT
= 1.255V (1+ R1/R2)
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 suffi ce.
The LT3467A switches at 2.1MHz, allowing for even smaller
valued inductors to be used. 0.9μH to 6.8μH will usually
suffi ce. 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
2
R power
losses. Note that in some applications, the current handling
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 effi ciency, 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 1.3MHz,
(2.1MHz for the LT3467A) such as ferrite core.
