LTC1702 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1702' PDF : 36 Pages View PDF
LTC1702
APPLICATIONS INFORMATION
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”
GAIN
(dB)
AV
GAIN
0
–12dB/OCT
PHASE
(DEG)
0
PHASE
–90
–180
–6dB/OCT
1702 F08
Figure 8. Transfer Function of Buck Modulator
C1
R1
IN
–
OUT
RB
+
VREF
1702 F09a
Figure 9a. Type 1 Amplifier Schematic Diagram
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.
C2
C1
R2
R1
IN
–
RB
+
VREF
OUT
1702 F10a
Figure 10a. Type 2 Amplifier Schematic Diagram
GAIN
(dB)
GAIN
PHASE
(DEG)
0
0
–6dB/OCT
–90
–180
PHASE
–270
1702 F09b
Figure 9b. Type 1 Amplifier Transfer Function
GAIN
(dB)
–6dB/OCT
GAIN
0
–6dB/OCT
PHASE
(DEG)
0
–90
PHASE
–180
–270
1702 F10b
Figure 10b. Type 2 Amplifier Transfer Function
1702fa
21
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