LT3460(2003) Ver la hoja de datos (PDF) - Linear Technology
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LT3460 Datasheet PDF : 12 Pages
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OPERATIO
Feedback Loop Compensation
The LT3460 has an internal feedback compensation net-
work as shown in Figure 1 (RC and CC). However, because
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 VOUT to FB (CF in
Figure 2) will correct the loop compensation for most
applications.
L1
22µH
D1
VIN
5V
5
1
C1
4.7µF
VIN
SW
R2
130k
LT3460
4
OFF ON SHDN
3
FB
R1
GND
15k
2
C1: TAIYO YUDEN X5R JMK212BJ475KG
C2: TAIYO YUDEN X5R EMK316BJ105
D1: CENTRAL SEMICONDUCTOR CMDSH2-3
L1: MURATA LQH32CN-220 OR EQUIVALENT
CF
22pF
VOUT
12V
70mA
C2
1µF
3460 F02
Figure 2. 5V to 12V Step-Up Converter
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:
GP(s)
=
K1
s • C2
,
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 RC, CC can be approxi-
mated as follows in medium frequency range:
GC(s)
=
K2
•
s
•
RC • CC
s • CC
+
1
The zero
fZ
=
2
•
π
1
• RC
•
CC
LT3460
is about 70kHz.
The feedback loop gain T(s) = K3 • GP(s) • GC(s). If it
crosses over 0dB far before fZ, the phase margin will be
small. Figure 3 is the Bode plot of the feedback loop gain
measured from the converter shown in Figure 2 without
the feedforward capacitor CF. The result agrees with
the previous discussion: Phase margin of about 20° is
insufficient.
60
50
GAIN
40
30
20
10 PHASE
0
–10
–20
–30
–40
1
10
100
FREQUENCY (kHz)
Figure 3
90
45
0
–45
–90
–135
–180
–225
–270
–315
–360
1000
3460 F03
In order to improve the phase margin, a feed-forward
capacitor CF in Figure 2 can be used.
Without the feed-forward capacitor, the transfer function
from VOUT to FB is:
FB = R1
VOUT R1+ R2
With the feed-forward capacitor CF, the transfer function
becomes:
FB
VOUT
=
R1
R1+ R2
•
s
s • R2 • CF + 1
•
R1• R2
R1+ R2
•
CF
+
1
The feed-forward capacitor CF generates a zero and a pole.
The zero always appears before the pole. The frequency
distance between the zero and the pole is determined only
by the ratio between VOUT and FB. To give maximum phase
3460f
5
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