LTC1873 Datasheet, PDF (21/32 Pages) Linear Technology – Dual 550kHz Synchronous 2-Phase Switching Regulator Controller with 5-Bit VID

APPLICATIO S I FOR ATIO
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
LTC1873 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.
C1
R1
IN
–
R_B
+
V_REF
OUT
1873 F09a
Figure 9a. Type 1 Amplifier Schematic Diagram
LTC1873
C2
C1
R2
R1
IN
–
R_B
+
V_REF
OUT
1873 F10a
Figure 10a. Type 2 Amplifier Schematic Diagram
GAIN
(dB)
–6dB/OCT
GAIN
0
PHASE
–6dB/OCT
PHASE
(DEG)
0
–90
–180
–270
1873 F10b
GAIN
(dB)
GAIN
PHASE
(DEG)
0
0
–6dB/OCT
–90
–180
PHASE
–270
1873 F09b
Figure 9b. Type 1 Amplifier Transfer Function
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.”
Figure 10b. Type 2 Amplifier Transfer Function
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. LTC1873 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
21