LTC1553LCSW#TRMPBF Linear Technology, LTC1553LCSW#TRMPBF Datasheet - Page 16

• Manufacturer Part Number: LTC1553LCSW#TRMPBF
• Manufacturer: Linear Technology

Specifications of LTC1553LCSW#TRMPBF

• Lead Free Status / RoHS Status: Compliant

LTC1553L
APPLICATIONS
Feedback Loop Compensation
The LTC1553L voltage feedback loop is compensated at
the COMP pin, attached to the output node of the internal
g
compensated properly with an RC + C network from COMP
to GND as shown in Figure 8a.
Loop stability is affected by the values of the inductor,
output capacitor, output capacitor ESR, error amplifier
transconductance and error amplifier compensation net-
work. The inductor and the output capacitor create a
double pole at the frequency:
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 shows the Bode plot of the overall transfer
function.
The compensation value used in this design is based on
the following criteria: f
the closed-loop frequency f
filter and the input resistor divider is compensated by the
gain of the PWM modulator and the gain of the error
amplifier (g
to frequency compensation can be used, the added
complication of input and/or output filters, unknown ca-
pacitor ESR, and gross operating point changes with input
voltage, load current variations, all suggest a more prac-
tical 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.
16
m
f
f
f
LC
ESR
Z
error amplifier. The feedback loop can generally be
=
=
2 (R
=
2
2 (ESR)(C
mERR
(L
1
C
)(C
O
1
)(R
)(C
1
C
)
U
C
OUT
). Although a mathematical approach
OUT
and f
SW
)
INFORMATION
)
= 12f
U
P
CO
=
, the attenuation due the LC
CO
2 (R
, f
Z
W
= f
1
C
)(C1)
LC
and f
respectively.
P
U
= 5f
CO
. At
Table 6. Suggested Compensation Network for 5V Input
Application Using Multiple Paralleled 330 F AVX TPS Output
Capacitors
Figure 8b. Bode Plot of the LTC1553L Overall Transfer Function
L
O
2.7
2.7
2.7
5.6
5.6
5.6
( H)
1
1
1
R
C
C
C
Figure 8a. Compensation Pin Hook-Up
C
1980
4950
1980
4950
1980
4950
O
COMP
990
990
990
f
10
Z
( F)
C1
f
LC
f
LTC1553L
ESR
R
f
f
C
SW
CO
1.8
3.6
9.1
5.1
10
24
10
20
51
(k )
= CLOSED-LOOP CROSSOVER
= LTC1553L SWITCHING
f
CO
FREQUENCY
ERR
FREQUENCY
– 20dB/DECADE
–
+
f
C
0.0047
0.0047
0.0036
P
0.022
C
0.01
0.01
0.01
0.01
0.01
FREQUENCY
( F)
DAC
1553L F08b
6
C1 (pF)
1553L F08a
SENSE
680
330
120
220
120
120
47
56
22