LTC1704EGN#TRPBF【PDF 19/28 页】IC REG DL BUCK/LINEAR 16SSOP

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LTC1704/LTC1704B

1704bfa

APPLICATIO S I FOR ATIO

pensation network. All of these components affect loop

behavior and need to be accounted for in the loop compen-

sation. The modulator consists of the internal PWM gen-

erator, 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 7).

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

users control. The modulator is a fundamental piece of

the LTC1704 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

something 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 8). This Type 1

configuration is stable but transient response will be less

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

Figure 9 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 hap-

pens close to the LC pole, limiting the total phase shift due

to the LC. The additional phase compensation in the feed-

back 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 compen-

sate for LC combinations where low capacitor ESR keeps

the phase shift near 180?for an extended frequency range.

LTC1704 circuits using conventional switching grade elec-

trolytic output capacitors can often get acceptable phase

margin with Type 2 compensation.

Type 3 loops (Figure 10), 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 LTC1704

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

tors need Type 3 compensation to obtain acceptable phase

margin with a high bandwidth feedback loop.

Figure 7. Transfer Function of Buck Modulator

1704 F05

PHASE

6dB/OCT

12dB/OCT

GAIN

FREQ

180

270

360

Figure 8. Type 1 Schematic and Transfer Function

1704 F06

PHASE

6dB/OCT

GAIN

FREQ

180

270

360

COMP

REF

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