LTC1553L View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC1553L
Linear Technology
'LTC1553L' PDF : 20 Pages View PDF
LTC1553L
APPLICATIONS INFORMATION
Feedback Loop Compensation
The LTC1553L voltage feedback loop is compensated at
the COMP pin, attached to the output node of the internal
gm error amplifier. The feedback loop can generally be
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:
fLC
=
1
2π√(LO)(COUT)
The ESR of the output capacitor forms a zero at the
frequency:
fESR
=
1
2Ï€(ESR)(COUT)
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:
fZ
=
1
2Ï€(RC)(CC)
and
fP
=
1
2Ï€(RC)(C1)
respectively.
Figure 8b shows the Bode plot of the overall transfer
function.
The compensation value used in this design is based on
the following criteria: fSW = 12fCO, fZ = fLC and fP = 5fCO. At
the closed-loop frequency fCO, 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
amplifier (gmERR)(RC). Although a mathematical approach
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.
COMP
10
LTC1553L
ERR
6 SENSE
RC
C1
DAC
CC
1553L F08a
Figure 8a. Compensation Pin Hook-Up
fSW = LTC1553L SWITCHING
FREQUENCY
fCO = CLOSED-LOOP CROSSOVER
fZ
FREQUENCY
– 20dB/DECADE
fLC fESR fCO
fP
FREQUENCY
1553L F08b
Figure 8b. Bode Plot of the LTC1553L Overall Transfer Function
Table 6. Suggested Compensation Network for 5V Input
Application Using Multiple Paralleled 330µF AVX TPS Output
Capacitors
LO (µH)
1
CO (µF)
990
RC (kΩ)
1.8
CC (µF)
0.022
C1 (pF)
680
1
1980
3.6
0.01
330
1
4950
9.1
0.01
120
2.7
990
5.1
0.01
220
2.7
1980
10
0.01
120
2.7
4950
24
0.0047
47
5.6
990
10
0.01
120
5.6
1980
20
0.0047
56
5.6
4950
51
0.0036
22
16
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