LTC1702AIGN datasheet(21/36 Pages) LINER

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LTC1702A

1702afa

APPLICATIONS INFORMATION

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).

OUT

1702A F10a

VREF

–

GAIN

(dB)

PHASE

(DEG)

1702A F10b

–90

–180

–270

PHASE

GAIN

–6dB/OCT

Figure 10a. Type 2 Amplifier Schematic Diagram

Figure 10b. Type 2 Amplifier Transfer Function

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

LTC1702A 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.

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.”

GAIN

(dB)

PHASE

(DEG)

1702A F08

–90

–180

–6dB/OCT

PHASE

GAIN

–12dB/OCT

Figure 8. Transfer Function of Buck Modulator

OUT

1702A F09a

VREF

–

GAIN

(dB)

PHASE

(DEG)

1702A 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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