Datasheet
LTC3775
11
3775fa
APPLICATIONS INFORMATION
Figure 1 shows a Type 3 amplifi er. The transfer function of
this amplifi er is given by the following equation:
V
COMP
V
OUT
=
–1+ sR2C1
()
1+ s(R
A
+ R3)C3
sR
A
C1+ C2
()
1+ s(C1|| C2) R2
()
1+ sC3R3
( )
The RC network across the error amplifi er and the feed-
forward components R3 and C3 introduce two pole-zero
pairs to obtain a phase boost at the system unity-gain
frequency, f
C
. In theory, the zeros and poles are placed
symmetrically around f
C
, and the spread between the zeros
and the poles is adjusted to give the desired phase boost
at f
C
. However, in practice, if the crossover frequency
is much higher than the LC double-pole frequency, this
method of frequency compensation normally generates
a phase dip within the unity bandwidth and creates some
concern regarding conditional stability.
If conditional stability is a concern, move the error ampli-
fi er’s zero to a lower frequency to avoid excessive phase
dip. The following equations can be used to compute the
feedback compensation component values:
f switching frequency
f
LC
f
R
SW
LC
OUT
ESR
=
=
π
=
π
1
2
1
2
EESR OUT
C
choose:
f crossover frequency
f
ff
C
SW
ZERR LC
==
==
π
10
1
2
1( )
RRC
f
f
RRC
ff
ZRES
C
A
PERR ESR
21
5
1
233
2
1
()
()
==
π +
()
= ==
π
==
π
1
2212
5
1
233
2
RC C
ff
RC
PRES C
(||)
()
Required error amplifi er gain at frequency f
C
:
A
V(CROSSOVER)
40log 1+
f
C
f
LC
2
–20log 1+
f
C
f
ESR
2
–20log A
MOD
()
20log
R2
R
A
•
1+
f
LC
f
C
1+
f
P2(RES)
f
C
+
f
P2(RES)
–f
Z2(RES)
f
Z2(RES)
1+
f
C
f
ESR
+
f
LC
f
ESR
–f
LC
1+
f
P2(RES)
f
C
where A
MOD
is the modulator and line feedforward gain
and is equal to:
A
VDC
V
V
V
VV
MOD
IN MAX MAX
SAW
≈ = ≈
()
•
•.
.
/
40 0 95
125
30
Once the value of resistor R
A
and the pole and zero loca-
tions have been decided, the values of C1, R2, C2, R3 and
C3 can be obtained from the above equations.
Compensating a switching power supply feedback loop is
a complex task. The applications shown in this data sheet
show typical values, optimized for the power components
shown. Though similar power components should suffi ce,
substantially changing even one major power component
may degrade performance signifi cantly. Stability also may
depend on circuit board layout. To verify the calculated
component values, all new circuit designs should be
prototyped and tested for stability.
–
+
V
OUT
V
REF
R
A
R3
C3
R2
C1
GAIN (dB)
C2
FB
R
B
COMP
FREQ
–1
–1+1
GAIN
PHASE
BOOST
0
PHASE (DEG)
–90
–180
–270
–380
3775 F01
Figure 1. Type 3 Amplifi er Compensation