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LTC1703

1703fa

FEEDBACK LOOP/COMPENSATION1

Feedback Loop Types

In a typical LTC1703 circuit, the feedback loop consists of

the modulator, the external inductor and output capacitor,

and the feedback amplifier and its compensation network.

All of these components affect loop behavior and need to

be accounted for in the loop compensation. The modulator

consists of the internal PWM generator, the output MOSFET

drivers and the external MOSFETs themselves. From a

feedback loop point of view, it looks like a linear voltage

transfer function from COMP to SW and has a gain roughly

equal to the input voltage. It has fairly benign AC behavior

at typical loop compensation frequencies with significant

phase shift appearing at half the switching frequency.

The external inductor/output capacitor combination makes

a more significant contribution to loop behavior. These

components cause a second order LC roll-off at the output,

with the attendant 180

° phase shift. This roll-off is what

filters the PWM waveform, resulting in the desired DC

output voltage, but the phase shift complicates the loop

compensation if the gain is still higher than unity at the pole

frequency. Eventually (usually well above the LC pole

frequency), the reactance of the output capacitor will

approach its ESR, and the roll-off due to the capacitor will

stop, leaving 6dB/octave and 90

° of phase shift (Figure 8).

So far, the AC response of the loop is pretty well out of the

user’s control. The modulator is a fundamental piece of the

LTC1703 design, and the external L and C are usually

chosen based on the regulation and load current require-

ments without considering the AC loop response. The

feedback amplifier, on the other hand, gives us a handle

with which to adjust the AC response. The goal is to have

180

° phase shift at DC (so the loop regulates) and some-

thing less than 360

° phase shift at the point that the loop

gain falls to 0dB. The simplest strategy is to set up the

feedback amplifier as an inverting integrator, with the 0dB

frequency lower than the LC pole (Figure 9). This “type 1”

configuration is stable but transient response will be less

than exceptional if the LC pole is at a low frequency.

GAIN

(dB)

PHASE

(DEG)

1703 F08

–90

–180

–6dB/OCT

PHASE

GAIN

–12dB/OCT

Figure 8. Transfer Function of Buck Modulator

OUT

1703 F09a

VREF

–

GAIN

(dB)

PHASE

(DEG)

1703 F09b

–90

–180

–270

GAIN

PHASE

–6dB/OCT

Figure 9a. Type 1 Amplifier Schematic Diagram

Figure 9b. Type 1 Amplifier Transfer Function

Figure 10 shows an improved “type 2” circuit that uses an

additional pole-zero pair to temporarily remove 90

° of

phase shift. This allows the loop to remain stable with 90

more phase shift in the LC section, provided the loop

reaches 0dB gain near the center of the phase “bump.”

Type 2 loops work well in systems where the ESR zero in

APPLICATIO S I FOR ATIO

1The information in this section is based on the paper “The K Factor: A New Mathematical Tool for

Stability Analysis and Synthesis” by H. Dean Venable, Venable Industries, Inc. For complete paper,

see “Reference Reading #4” at www.linear-tech.com.

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