LTC3703
applicaTions inForMaTion
approach its ESR and the rolloff due to the capacitor will
stop,leaving6dB/octaveand90°ofphaseshift(Figure11).
C2
C1
IN
R2
–6dB/OCT
GAIN
R1
FB
–6dB/OCT
–
R
OUT
0
FREQ
–90
B
V
+
REF
GAIN
–180
–270
–360
A
V
PHASE
–12dB/OCT
0
FREQ
–90
PHASE
–180
–270
–360
3703 F13
–6dB/OCT
Figure 13. Type 2 Schematic and 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. LTC3703 circuits using
conventional switching grade electrolytic output capaci-
tors can often get acceptable phase margin with Type 2
compensation.
3703 F11
Figure 11. Transfer Function of Buck Modulator
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
requirements 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
something 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.
“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
circuitsusinglowESRtantalumorOS-CONoutputcapaci-
torsneedType3compensationtoobtainacceptablephase
margin with a high bandwidth feedback loop.
C1
IN
GAIN
R1
FB
–
–6dB/OCT
R
B
OUT
0
FREQ
–90
V
+
REF
–180
–270
–360
IN
C2
PHASE
C1
C3
R3
R2
R1
–6dB/OCT
GAIN
3703 F12
FB
–
+6dB/OCT
–6dB/OCT
Figure 12. Type 1 Schematic and Transfer Function
R
B
OUT
0
FREQ
–90
V
+
REF
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.”
–180
–270
–360
PHASE
3703 F14
Figure 14. Type 3 Schematic and Transfer Function
3703fc
19
Linear Systems [ Linear Systems ]
