Datasheet

21
LTC1702
1702fa
APPLICATIONS INFORMATION
WUU
U
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”
OUT
IN
R1
C2
C1
R2
R
B
1702 F10a
V
REF
+
–
GAIN
(dB)
PHASE
(DEG)
1702 F10b
00
–90
–180
–270
PHASE
GAIN
–6dB/OCT
–6dB/OCT
Figure 10a. Type 2 Amplifier Schematic Diagram
Figure 10b. Type 2 Amplifier Transfer Function
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.”
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. LTC1702 circuits using
conventional switching grade electrolytic output capaci-
tors can often get acceptable phase margin with type 2
compensation.
GAIN
(dB)
PHASE
(DEG)
1702 F08
A
V
00
–90
–180
–6dB/OCT
PHASE
GAIN
–12dB/OCT
Figure 8. Transfer Function of Buck Modulator
OUT
IN
R1
C1
R
B
1702 F09a
V
REF
+
–
GAIN
(dB)
PHASE
(DEG)
1702 F09b
00
–90
–180
–270
GAIN
PHASE
–6dB/OCT
Figure 9a. Type 1 Amplifier Schematic Diagram
Figure 9b. Type 1 Amplifier Transfer Function