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PRODUCT SPECIFICATION
RC5053
13
P
The ESR of the output capacitor forms a zero at the
frequency:
The compensation network at the error amplifier output is to
provide enough phase margin at the 0dB crossover frequency
for the overall closed-loop transfer function. The zero and
pole from the compensation network are:
Figure 8b. Bode Plot of the RC5053
Overall Transfer Function
Figure 8b shows the Bode plot of the overall transfer func-
tion. The compensation value used in this design is based on
the following criteria: f
SW
= 12f
CO
, f
Z
= f
LC
and f
P
= 5f
CO
.
At the closed-loop frequency f
CO
, the attenuation due the LC
filter and the input resistor divider is compensated by the
gain of the PWM modulator and the gain of the error ampli-
fier (g
mERR
)(R
C
).
Although a mathematical approach to frequency compensa-
tion can be used, the added complication of input and/or out-
put filters, unknown capacitor ESR, and gross operating
point changes with input voltage, load current variations, 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.
f
ESR
)
C
OUT
2
ESR
(
)
--------------------1
=
f
Z
π
R
C
(
)
C
C
(
)
2
=
f
P
π
R
C
(
)
C1
(
)
2
=
f
Z
f
CO
f
P
f
LC
f
ESR
FREQUENCY
–20dB /DECADE
f
SW
= RC5053 SWITCHING
FREQUENCY
f
CO
= CLOSED-LOOP CROSSOVER
FREQUENCY
L
Table 4.
Suggested Compensation Network for 5V
Input Application Using Multiple Paralleled 330
μ
F AVX
TPS Output Capacitors
Table 4 shows the suggested compensation components for
5V input applications based on the inductor and output
capacitor values. The values were calculated using multiple
paralleled 330
μ
F AVX TPS series surface mount tantalum
capacitors as the output capacitor. The
optimum component values might deviate from the
suggested values slightly because of board layout and
operating condition differences.
An alternate output capacitor is the Sanyo MV-GX series.
Using multiple parallel 1500
μ
F Sanyo MV-GX capacitors
for the output capacitor, Table 5 shows the suggested com-
pensation component value for a 5V input application based
on the inductor and output capacitor values.
Table 5.
Suggested Compensation Network for 5V
Input Application Using Multiple Paralleled 1500
μ
F
SANYO MV-GX Output Capacitors
VID0 to VID4, PWRGD and FAULT
The digital inputs (VID0 to VID4) program the internal DAC
which in turn controls the output voltage. These digital input
controls are intended to be static and are not designed for
high speed switching. Forcing V
OUT
to step from a high to a
low voltage by changing the VID
n
pins quickly can cause
FAULT to trip.
L
O
(
μ
H)
1
1
1
2.7
2.7
2.7
5.6
5.6
5.6
C
O
(
μ
F)
990
1980
4950
990
1980
4950
990
1980
4950
R
C
(k
)
3.6
6.8
22
10
20
51
20
39
100
C
C
(pF)
10000
4700
2200
3300
2200
1000
2200
1000
470
C1 (pF)
470
220
100
150
68
47
68
47
33
L
O
(
μ
H)
1
1
1
2.7
2.7
2.7
5.6
5.6
5.6
C
O
(
μ
F)
4500
6000
9000
4500
6000
9000
4500
6000
9000
R
C
(k
)
9.1
10
18
22
30
47
47
62
91
C
C
(pF)
3300
3300
2200
1500
1000
680
680
470
330
C1 (pF)
150
100
68
47
47
33
33
33
22