Datasheet

22
LTC1703
1703fa
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. LTC1703 circuits using
conventional switching grade electrolytic output capaci-
tors can often get acceptable phase margin with type 2
compensation.
“Type 3” loops (Figure 11) 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 LTC1703 circuits
using low ESR tantalum or OS-CON output capacitors
OUT
IN
R1
C2
C1
R2
R
B
1703 F10a
V
REF
+
–
Figure 10a. Type 2 Amplifier Schematic Diagram
GAIN
(dB)
PHASE
(DEG)
1703 F10b
00
–90
–180
–270
PHASE
GAIN
–6dB/OCT
–6dB/OCT
Figure 10b. Type 2 Amplifier Transfer Function
OUT
IN
R1
R3
C2
C1
C3
R2
R
B
1703 F11a
V
REF
+
–
GAIN
(dB)
PHASE
(DEG)
1703 F11b
00
–90
–180
–270
+6dB/OCT
–6dB/OCT
PHASE
GAIN
–6dB/OCT
Figure 11a. Type 3 Amplifier Schematic Diagram
Figure 11b. Type 3 Amplifier Transfer Function
need type 3 compensation to obtain acceptable phase
margin with a high bandwidth feedback loop.
Feedback Component Selection
Selecting the R and C values for a typical type 2 or type 3
loop is a nontrivial task. The applications shown in this data
sheet show typical values, optimized for the power com-
ponents shown. They should give acceptable performance
with similar power components, but can be way off if even
one major power component is changed significantly.
Applications that require optimized transient response will
need to recalculate the compensation values specifically
for the circuit in question. The underlying mathematics are
complex, but the component values can be calculated in a
straightforward manner if we know the gain and phase of
the modulator at the crossover frequency.
Modulator gain and phase can be measured directly from
a breadboard, or can be simulated if the appropriate
parasitic values are known. Measurement will give more
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