lt3460esc6-trpbf Linear Technology Corporation, lt3460esc6-trpbf Datasheet - Page 5

• Manufacturer Part Number: lt3460esc6-trpbf
• Description: 1.3 Mhz/650khz Step-up Dc/dc Converter In Sc70 And Thinsot
• Manufacturer: Linear Technology Corporation

BLOCK DIAGRAM
OPERATION
The LT3460/LT3460-1 uses a constant frequency, current
mode control scheme to provide excellent line and load
regulation. Operation can be best understood by referring to
the block diagram in Figure 1. 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; if it decreases, less current is delivered.
Feedback Loop Compensation
The LT3460/LT3460-1 has an internal feedback compensa-
tion network as shown in Figure 1 (R
because the small signal characteristics of a boost converter
change with operation conditions, the internal compensa-
tion network cannot satisfy all applications. A properly
designed external feed forward capacitor from V
SHUTDOWN
R
S
, C
S
OPTIONAL SOFT-START COMPONENTS
R
V
S
OUT
(EXTERNAL)
R1 (EXTERNAL)
R2 (EXTERNAL)
V
FB
IN
C
S
REFERENCE
(EXTERNAL)
SHDN
1.255V
FB
+
–
C
and C
A1
C
Figure 1. Block Diagram, LT3460
). However,
GENERATOR
OSCILLATOR
R
OUT
1.3MHz
C
RAMP
C
C
to
–
+
COMPARATOR
A2
FB (C
most applications.
The LT3460/LT3460-1 uses peak current mode control.
The current feedback makes the inductor very similar
to a current source in the medium frequency range. The
power stage transfer function in the medium frequency
range can be approximated as:
where C2 is the output capacitance, and K1 is a constant
based on the operating point of the converter. In continuous
current mode, K1 increases as the duty cycle decreases.
The internal compensation network R
proximated as follows in medium frequency range:
The zero
is about 70kHz.
G
G
f
Z
P(s)
C(s)
F
=
in Figure 2) will correct the loop compensation for
2 • • R
R
=
=K2 •
s • C2
K1
S
1
s • R
C
• C
,
Q
s • C
C
C
• C
DRIVER
C
LT3460/LT3460-1
C
+
–
+ 1
Q1
0.1Ω
SW
GND
C
, C
C
can be ap-
3460 BD
3460fb
5