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LTC1702

APPLICATIONS INFORMATION

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”

OUT

1702 F10a

VREF

–

GAIN

(dB)

PHASE

(DEG)

1702 F10b

–90

–180

–270

PHASE

GAIN

–6dB/OCT

Figure 10a. Type 2 Amplifier Schematic Diagram

Figure 10b. Type 2 Amplifier Transfer Function

configuration is stable but transient response will be less

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

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

the LC roll-off happens close to the LC pole, limiting the

total phase shift due to the LC. The additional phase

compensation in the feedback amplifier allows the 0dB

point to be at or above the LC pole frequency, improving

loop bandwidth substantially over a simple type 1 loop. It

has limited ability to compensate for LC combinations

where low capacitor ESR keeps the phase shift near 180

for an extended frequency range. LTC1702 circuits using

conventional switching grade electrolytic output capaci-

tors can often get acceptable phase margin with type 2

compensation.

GAIN

(dB)

PHASE

(DEG)

1702 F08

–90

–180

–6dB/OCT

PHASE

GAIN

–12dB/OCT

Figure 8. Transfer Function of Buck Modulator

OUT

1702 F09a

VREF

–

GAIN

(dB)

PHASE

(DEG)

1702 F09b

–90

–180

–270

GAIN

PHASE

–6dB/OCT

Figure 9a. Type 1 Amplifier Schematic Diagram

Figure 9b. Type 1 Amplifier Transfer Function

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