Datasheet

LT3460/LT3460-1
6
3460fb
OPERATION
Figure 3
FREQUENCY (kHz)
PHASE
GAIN
1
GAIN (dB)
60
50
40
30
20
PHASE (DEG)
10
0
–10
–20
–30
–40
90
45
0
–45
–90
–135
–180
–225
–270
–315
–360
10 100 1000
3460 F03
Figure 4
60
50
40
30
20
10
0
–10
–20
–30
–40
PHASE (DEG)
90
45
0
–45
–90
–135
–180
–225
–270
–315
–360
FREQUENCY (kHz)
PHASE
GAIN
1
GAIN (dB)
10 100 1000
3460 F04
The feedback loop gain T(s) = K3 • G
P
(s) • G
C
(s). If it
crosses over 0dB far before f
Z
, 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 C
F
. The result agrees with
the previous discussion: Phase margin of about 20° is
insuffi cient.
In order to improve the phase margin, a feed-forward
capacitor C
F
in Figure 2 can be used.
Without the feed-forward capacitor, the transfer function
from V
OUT
to FB is:
FB
V
OUT
=
R1
R1+ R2
With the feed-forward capacitor C
F
, the transfer function
becomes:
FB
V
OUT
=
R1
R1+ R2
•
s•R2•C
F
+ 1
s•
R1• R2
R1+ R
2
•C
F
+ 1
The feed-forward capacitor C
F
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 V
OUT
and FB. To give maximum
phase margin, C
F
should be chosen so that the midpoint
frequency between the zero and the pole is at the cross
over frequency.
With C
F
= 20pF, the feedback loop Bode plot is reshaped
as shown in Figure 4. The phase margin is about 60°.
Figure 2. 5V to 12V Step-Up Converter
OFF ON
V
IN
5V
L1
22μH
D1
R2
130k
R1
15k
C2
1μF
C
F
22pF
C1
4.7μF
V
OUT
12V
70mA
51
3
2
4
V
IN
SW
FB
SHDN
GND
LT3460
C1: TAIYO YUDEN X5R JMK212BJ475KG
C2: TAIYO YUDEN X5R EMK316BJ105
D1: CENTRAL SEMICONDUCTOR CMDSH2-3
L1: MURATA LQH32CN-220 OR EQUIVALENT
3460 F02
The feed-forward capacitor increases the gain at high
frequency. The feedback loop therefore needs to have
enough attenuation at the switching frequency to reject
the switching noise. Additional internal compensation
components have taken this into consideration.
For most of the applications of LT3460/LT3460-1, the
output capacitor ESR zero is at very high frequency and
can be ignored. If a low frequency ESR zero exists, for
example, when a high-ESR Tantalum capacitor is used at
the output, the phase margin may be enough even without
a feed-forward capacitor. In these cases, the feed-forward
capacitor should not be added because it may cause
the feedback loop to not have enough attenuation at the
switching frequency.