LTC3703_15 Datasheet, PDF (19/34 Pages) Linear Technology – 100V Synchronous Switching Regulator Controller

LTC3703
Applications Information
approach its ESR and the rolloff due to the capacitor will
stop, leaving 6dB/octave and 90° of phase shift (Figure 11).
GAIN
A_V
0
PHASE
–12dB/OCT
–6dB/OCT
FREQ
–90
–180
–270
–360
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.
IN
R1
FB
R_B
V_REF
C1
–
+
OUT 0
GAIN
–6dB/OCT
PHASE
FREQ
–90
–180
–270
–360
3703 F12
Figure 12. Type 1 Schematic and Transfer Function
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.”
C2
IN
R2
C1
R1
FB _–
R_B
V_REF +
–6dB/OCT
GAIN
OUT 0
PHASE
–6dB/OCT
FREQ
–90
–180
–270
–360
3703 F13
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.
“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
C2
C3 _R2
C1
R1 R3
FB _–
R_B
V_REF +
OUT 0
–6dB/OCT
GAIN
+6dB/OCT
–6dB/OCT
PHASE
FREQ
–90
–180
–270
–360
3703 F14
Figure 14. Type 3 Schematic and Transfer Function
3703fc
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