LTC1704 Linear Technology, LTC1704 Datasheet - Page 19

• Manufacturer Part Number: LTC1704
• Description: 550kHz Synchronous Switching Regulator Controller Plus Linear Regulator Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
pensation network. All of these components affect loop
behavior and need to be accounted for in the loop compen-
sation. The modulator consists of the internal PWM gen-
erator, the output MOSFET drivers and the external
MOSFETs themselves. From a feedback loop point of view,
it looks like a linear voltage transfer function from COMP
to SW and has a gain roughly equal to the input voltage. It
has fairly benign AC behavior at typical loop compensation
frequencies with significant phase shift appearing at half
the switching frequency.
The external inductor/output capacitor combination makes
a more significant contribution to loop behavior. These
components cause a second order LC roll-off at the
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 7).
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 LTC1704 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
something less than 360 phase shift at the point that the
loop gain falls to 0dB. The simplest strategy is to set up the
Figure 7. Transfer Function of Buck Modulator
A
0
V
PHASE
GAIN
U
U
–12dB/OCT
–6dB/OCT
W
1704 F05
FREQ
–90
–180
–270
–360
U
feedback amplifier as an inverting integrator, with the 0dB
frequency lower than the LC pole (Figure 8). This “Type 1”
configuration is stable but transient response will be less
than exceptional if the LC pole is at a low frequency.
Figure 9 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 hap-
pens close to the LC pole, limiting the total phase shift due
to the LC. The additional phase compensation in the feed-
back 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 compen-
sate for LC combinations where low capacitor ESR keeps
the phase shift near 180 for an extended frequency range.
LTC1704 circuits using conventional switching grade elec-
trolytic output capacitors can often get acceptable phase
margin with Type 2 compensation.
“Type 3” loops (Figure 10), 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 LTC1704
circuits use 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
R1
R2
V
REF
FB
Figure 8. Type 1 Schematic and Transfer Function
–
+
C1
LTC1704/LTC1704B
COMP
0
PHASE
GAIN
–6dB/OCT
19
1704bfa
FREQ
–90
–180
–270
–360
1704 F06