16

LTC1553L

Feedback Loop Compensation

The LTC1553L voltage feedback loop is compensated at

the COMP pin, attached to the output node of the internal

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:

1

2

The ESR of the output capacitor forms a zero at the

frequency:

1

2

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:

1

2

1

2

Figure 8b shows the Bode plot of the overall transfer

function.

The compensation value used in this design is based on

filter and the input resistor divider is compensated by the

gain of the PWM modulator and the gain of the error

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.

Figure 8a. Compensation Pin Hook-Up

1553L F08a

DAC

LTC1553L

SENSE

COMP

C1

ERR

6

10

Figure 8b. Bode Plot of the LTC1553L Overall Transfer Function

– 20dB/DECADE

FREQUENCY

1553L F08b

FREQUENCY

FREQUENCY

Table 6. Suggested Compensation Network for 5V Input

Application Using Multiple Paralleled 330

µF AVX TPS Output

Capacitors

C1 (pF)

1

990

1.8

0.022

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