LT3460ES5 LINER [Linear Technology], LT3460ES5 Datasheet - Page 5

• Manufacturer Part Number: LT3460ES5
• Description: 1.3MHz Step-Up DC/DC Converter in SC70 and ThinSOT
• Manufacturer: LINER [Linear Technology]

OPERATIO
Feedback Loop Compensation
The LT3460 has an internal feedback compensation net-
work as shown in Figure 1 (R
the small signal characteristics of a boost converter change
with operation conditions, the internal compensation net-
work cannot satisfy all applications. A properly designed
external feed forward capacitor from V
Figure 2) will correct the loop compensation for most
applications.
The LT3460 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 continu-
ous current mode, K1 increases as the duty cycle de-
creases.
The internal compensation network R
mated as follows in medium frequency range:
The zero
G
G
f
V
5V
Z
C1: TAIYO YUDEN X5R JMK212BJ475KG
C2: TAIYO YUDEN X5R EMK316BJ105
D1: CENTRAL SEMICONDUCTOR CMDSH2-3
L1: MURATA LQH32CN-220 OR EQUIVALENT
IN
P s ( )
C s
( )
2 • •
OFF ON
K
C1
4.7 F
s C
Figure 2. 5V to 12V Step-Up Converter
2
K
•
•
1
1
R
4
2
s R
C
SHDN
,
•
U
V
•
IN
C
5
s C
C
LT3460
C
•
22 H
GND
•
L1
C
2
C
C
SW
1
FB
1
C
D1
and C
3
R2
130k
R1
15k
C
). However, because
C
, C
OUT
C
can be approxi-
C
22pF
F
to FB (C
C2
1 F
3460 F02
V
12V
70mA
OUT
F
in
is about 70kHz.
The feedback loop gain T(s) = K3 • G
crosses over 0dB far before f
small. Figure 3 is the Bode plot of the feedback loop gain
measured from the converter shown in Figure 2 without
the feedforward capacitor C
the previous discussion: Phase margin of about 20 is
insufficient.
In order to improve the phase margin, a feed-forward
capacitor C
Without the feed-forward capacitor, the transfer function
from V
With the feed-forward capacitor C
becomes:
The feed-forward capacitor C
The zero always appears before the pole. The frequency
distance between the zero and the pole is determined only
by the ratio between V
V
V
FB
FB
OUT
OUT
OUT
–10
–20
–30
–40
60
50
40
30
20
10
to FB is:
F
R
R
0
in Figure 2 can be used.
1
1
1
R
R
1
1
PHASE
GAIN
R
R
2
2
•
OUT
s
10
FREQUENCY (kHz)
•
s R
Figure 3
R
R R
and FB. To give maximum phase
•
1
1
•
F
2
Z
F
R
generates a zero and a pole.
, the phase margin will be
. The result agrees with
•
2
2
C
100
•
F
F
C
, the transfer function
F
1
P
1
3460 F03
(s) • G
1000
LT3460
90
45
0
–45
–90
–135
–180
–225
–270
–315
–360
C
(s). If it
5
3460f