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IR3093
Page 26 of 39
07/15/04
SETBIAS
FD
BIASOUT
I
SET
V
V
R
(15)
Compensation of the Current Share Loop
The crossover frequency of the current share loop should be at least one decade lower than that of the voltage
loop in order to eliminate the interaction between the two loops. A 22nF capacitor from SCOMP to LGND is good
for most of the applications. If necessary
have a 1k resistor in series with the Csc to make the current loop a little
bit faster.
Compensation of Voltage Loop
The adaptive voltage positioning is used in the computer applications to meet the load line requirements. Like
current mode control, the adaptive voltage positioning loop introduces extra zero to the voltage loop and splits the
double poles of the power stage, which make the voltage loop compensation much easier.
Resistors R
FB
and R
DRP
are chosen according to Equations (12) and (13), and the selection of compensation
types depends on the capacitors used. For the applications using Electrolytic, Polymer or AL-Polymer capacitors,
type II compensation shown in Figure 11 (a) is usually enough. While for the applications with only low ESR
ceramic capacitors, type III compensation shown in Figure 11 (b) is preferred.
(a) Type II compensation
(b) Type III compensation
Figure 11 . Voltage loop compensation network
Type II Compensation
Determine the compensation at no load, the worst case condition. Assume the time constant of the resistor and
capacitor across the output inductors matches that of the inductor, the crossover frequency of the voltage loop can
be estimated by Equations (16), where C
E
and R
CE
are the equivalent capacitance and ESR of output capacitors
respectively and R
LE
is the equivalent resistance of inductor DCR.
R
f
S
R
COMP
and C
COMP
have limited effect on the crossover frequency, and are used only to fine tune the crossover
frequency and transient load response. Choose the desired crossover frequency fc1 around fc estimated by
Equation (16) and determine R
COMP and
C
COMP.
)
*
(
*
2
CE
LE
FB
CS
E
DRP
R
C
R
R
G
C
(16)
CFB
CDRP
RCOMP
EAOUT
CCP1
CCOMP
RFB
RDRP
VO+
VDRP
VDAC
FB
+
-
EAOUT
RFB1
RCOMP
CCP1
EAOUT
CCOMP
RFB
RDRP
VO+
VDRP
VDAC
+
-
EAOUT
FB