Datasheet

21
LTC1702A
1702afa
APPLICATIONS INFORMATION
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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 8).
OUT
IN
R1
C2
C1
R2
R
B
1702A F10a
V
REF
+
–
GAIN
(dB)
PHASE
(DEG)
1702A 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
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
LTC1702A 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.
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.”
GAIN
(dB)
PHASE
(DEG)
1702A 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
1702A F09a
V
REF
+
–
GAIN
(dB)
PHASE
(DEG)
1702A F09b
00
–90
–180
–270
GAIN
PHASE
–6dB/OCT
Figure 9a. Type 1 Amplifier Schematic Diagram
Figure 9b. Type 1 Amplifier Transfer Function