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LTC1703

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. LTC1703 circuits using

conventional switching grade electrolytic output capaci-

tors can often get acceptable phase margin with type 2

compensation.

“Type 3” loops (Figure 11) 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 LTC1703 circuits

using low ESR tantalum or OS-CON output capacitors

OUT

1703 F10a

VREF

–

Figure 10a. Type 2 Amplifier Schematic Diagram

GAIN

(dB)

PHASE

(DEG)

1703 F10b

–90

–180

–270

PHASE

GAIN

–6dB/OCT

Figure 10b. Type 2 Amplifier Transfer Function

OUT

1703 F11a

VREF

–

GAIN

(dB)

PHASE

(DEG)

1703 F11b

–90

–180

–270

+6dB/OCT

–6dB/OCT

PHASE

GAIN

–6dB/OCT

Figure 11a. Type 3 Amplifier Schematic Diagram

Figure 11b. Type 3 Amplifier Transfer Function

need type 3 compensation to obtain acceptable phase

margin with a high bandwidth feedback loop.

Feedback Component Selection

Selecting the R and C values for a typical type 2 or type 3

loop is a nontrivial task. The applications shown in this data

sheet show typical values, optimized for the power com-

ponents shown. They should give acceptable performance

with similar power components, but can be way off if even

one major power component is changed significantly.

Applications that require optimized transient response will

need to recalculate the compensation values specifically

for the circuit in question. The underlying mathematics are

complex, but the component values can be calculated in a

straightforward manner if we know the gain and phase of

the modulator at the crossover frequency.

Modulator gain and phase can be measured directly from

a breadboard, or can be simulated if the appropriate

parasitic values are known. Measurement will give more

APPLICATIO S I FOR ATIO

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