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LTC3703

3703f

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. LTC3703 circuits using

conventional switching grade electrolytic output capaci-

tors can often get acceptable phase margin with Type 2

compensation.

“Type 3” loops (Figure 14) use two poles and two zeros to

obtain a 180

° phase boost in the middle of the frequency

band. A properly designed Type 3 circuit can maintain

acceptable loop stability even when low output capacitor

ESR causes the LC section to approach 180

° phase shift

well above the initial LC roll-off. As with a Type 2 circuit,

the loop should cross through 0dB in the middle of the

phase bump to maximize phase margin. Many LTC3703

circuits using low ESR tantalum or OS-CON output capaci-

tors need Type 3 compensation to obtain acceptable phase

margin with a high bandwidth feedback loop.

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

LTC3703 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 12). This “Type

1” configuration is stable but transient response is less

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

APPLICATIO S I FOR ATIO

3703 F11

PHASE

–6dB/OCT

–12dB/OCT

GAIN

FREQ

–90

–180

–270

–360

Figure 11. Transfer Function of Buck Modulator

3703 F12

PHASE

–6dB/OCT

GAIN

FREQ

–90

–180

–270

–360

OUT

–

VREF

Figure 12. Type 1 Schematic and Transfer Function

3703 F13

PHASE

–6dB/OCT

GAIN

FREQ

–90

–180

–270

–360

VREF

OUT

–

Figure 13. Type 2 Schematic and Transfer Function

3703 F14

PHASE

–6dB/OCT

+6dB/OCT

–6dB/OCT

GAIN

FREQ

–90

–180

–270

–360

VREF

OUT

–

Figure 14. Type 3 Schematic and Transfer Function

Figure 13 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

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