Datasheet

LTC3634
16
3634fb
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applicaTions inForMaTion
Choosing Compensation Components
Loop compensation is a complicated subject and Applica-
tion Note 76 is recommended reading for a full discussion
on maximizing loop bandwidth in a current mode switch-
ing regulator. This section will provide a quick method on
choosing proper components to compensate the L
TC3634
regulators.
Figure 4 shows the recommended components to be con
-
nected to the ITH pin, and Figure 5 shows an approximate
bode plot of the buck regulator loop using these compo-
nents. It is assumed that the major poles in the system
(the output capacitor pole and the error amplifier output
pole) are located at a frequency lower than the crossover
frequency
.
Figure 4. Compensation and Filtering Components
Figure 5. Bode Plot of Regulator Loop
ƒ
P
LOG (ƒ)
ƒ
C
ƒ
Z
3634 F05
–1
0dB
|H(s)|
–2
The first step is to choose the crossover frequency f
C
.
Higher crossover frequencies will result in a faster loop
transient response; however, in order to avoid higher or
-
der loop dynamics from the switching power stage, it is
recommended that f
C
not exceed one-tenth the switching
frequency (f
SW
).
Once f
C
is chosen, the value of R
COMP
that sets this cross-
over frequency can be calculated by the following equation:
R
COMP
=
2π • f
C
•C
OUT
g
m(EA)
•g
m(MOD)






V
OUT
V
FBREG






where g
m(EA)
is the error amplifier transconductance
(see the Electrical Characteristics section), and g
m(MOD)
is the modulator transconductance (the transfer function
from ITH voltage to current comparator threshold). For
the LTC3634, this transconductance is nominally 7Ω
–1
.
Once R
COMP
is determined, C
COMP
can be chosen to set
the zero frequency (f
Z
):
f
Z
=
1
2π •C
COMP
•R
COMP
For 90° of phase margin, f
Z
should be chosen to be less
than one-tenth of f
C
.
Since the ITH node is sensitive to noise coupling, a small
bypass capacitor (C
BYP
) may be used to filter out board
noise. However, this cap contributes a pole at f
P
and may
introduce some phase loss at the crossover frequency:
f
P
=
1
2π •C
BYP
•R
COMP
For best results, f
P
should be set high enough such that
phase margin is not significantly affected.
If necessary, a capacitor C
F
(as shown in Figure 3) may
be used to add some phase lead.
LTC3634
C
BYP
R
COMP
C
COMP
3634 F04
ITH
SGND