LTC3703 Linear Technology, LTC3703 Datasheet - Page 19

• Manufacturer Part Number: LTC3703
• Description: 100V Synchronous Switching Regulator Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Figure 13 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
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
LTC3703 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 12). This “Type
1” configuration is stable but transient response is less
than exceptional if the LC pole is at a low frequency.
IN
R1
R
V
B
REF
FB
Figure 12. Type 1 Schematic and Transfer Function
Figure 11. Transfer Function of Buck Modulator
–
+
A
C1
V
0
PHASE
GAIN
U
OUT
0
U
–12dB/OCT
PHASE
–6dB/OCT
GAIN
W
–6dB/OCT
FREQ
–90
–180
–270
–360
3703 F11
U
3703 F12
FREQ
–90
–180
–270
–360
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. LTC3703 circuits using
conventional switching grade electrolytic output capaci-
tors can often get acceptable phase margin with Type 2
compensation.
“Type 3” loops (Figure 14) 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 LTC3703
circuits using low ESR tantalum or OS-CON output capaci-
tors need Type 3 compensation to obtain acceptable phase
margin with a high bandwidth feedback loop.
IN
IN
R1
R
B
R1
R
V
B
REF
FB
Figure 13. Type 2 Schematic and Transfer Function
R3
V
Figure 14. Type 3 Schematic and Transfer Function
REF
FB
C3
–
+
R2
–
+
R2
C2
C2
C1
C1
OUT
OUT
0
0
PHASE
GAIN
GAIN
–6dB/OCT
–6dB/OCT
+6dB/OCT
LTC3703
PHASE
–6dB/OCT
–6dB/OCT
19
3703 F13
FREQ
–90
–180
–270
–360
3703f
3703 F14
FREQ
–90
–180
–270
–360