Datasheet

15
LTC1530
1530fa
f
L
C
LC
O
OUT
=
()
1
2π
The ESR of the output capacitor and the output capacitor
value form a zero at the frequency:
f
ESR C
ESR
OUT
=
()( )( )
1
2π
The compensation network used with the error amplifier
must provide enough phase margin at the 0dB crossover
frequency for the overall open-loop transfer function. The
zero and pole from the compensation network are:
f
RC
and f
RC
Z
CC
P
C
=
()()()
=
()()()
1
2
1
21ππ
respectively. Figure 8b shows the Bode plot of the overall
transfer function.
The compensation values used in this design are based on
the following criteria, f
SW
= 12f
CO
, f
Z
= f
LC
, f
P
= 5f
CO
. At the
closed-loop frequency f
CO
, the attenuation due to the LC
filter and the input resistor divider is compensated by the
gain of the PWM modulator and the gain of the error
amplifier (g
mERR
)(R
C
).
Although a mathematical approach to frequency compen-
sation can be used, the added complication of input and/
or output filters, unknown capacitor ESR, and gross
operating point changes with input voltage, load current
variations and frequency of operation all suggest a more
practical empirical method. This can be done by injecting
a transient current at the load and using an RC network box
to iterate toward the final compensation values or by
obtaining the optimum loop response using a network
analyzer to find the actual loop poles and zeros.
Table 2 shows the suggested compensation components
for 5V input applications based on the inductor and output
capacitor values. The values were calculated using mul-
tiple paralleled 330µF AVX TPS series surface mount
tantalum capacitors for the output capacitor. The opti-
mum component values might deviate from the suggested
values slightly because of board layout and operating
condition differences.
APPLICATIO S I FOR ATIO
WUUU
C1
R
C
C
C
LTC1530
V
OUT
COMP
1530 F08a
+
–
ERR
BG
3
4
Figure 8a. Compensation Pin Hook-Up
LOOP GAIN
FREQUENCY
1530 F08b
–20dB/DECADE
f
SW
= LTC1530 SWITCHING FREQUENCY
f
CO
= CLOSED-LOOP CROSSOVER FREQUENCY
f
Z
f
LC
f
ESR
f
CO
f
P
Figure 8b. Bode Plot of the LTC1530 Overall
Transfer Function
Table 2. Suggested Compensation Network for a 5V Input
Application Using Multiple Paralleled 330µF AVX TPS Output
Capacitors for 2.5V Output
L
O
(µH) C
O
(µF) R
C
(kΩ)C
C
(µF) C1 (pF)
1 990 1.3 0.022 1000
1 1980 2.7 0.022 470
1 4950 6.8 0.01 220
2.7 990 3.6 0.022 330
2.7 1980 7.5 0.01 220
2.7 4950 18 0.01 68
5.6 990 7.5 0.01 220
5.6 1980 15 0.01 100
5.6 4950 36 0.0047 47
An alternate output capacitor is the Sanyo MV-GX series.
Using multiple paralleled 1500µF Sanyo MV-GX capaci-
tors for the output capacitor, Table 3 shows the suggested
compensation components for 5V input applications based
on the inductor and output capacitor values.