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IR3500
Page 33 of 47
June 12, 2007
)]
1
1
(
*
[
*
_
_
ROOM
MAX
L
THERM
B
THERM
R
TMAX
R
T
T
EXP
=
(22)
Select the series resistor R
HOTSET2
to linearize the NTC thermistor, which has non-linear characteristics in the
operational temperature range. Then calculate R
HOTSET1
corresponding to the allowed maximum temperature
TMAX from (23).
*
)
(
2
1
=
R
HOTSET
6
)
(
+
VCCL
R
R
HOTSET
TMAX
(23)
VOLTAGE LOOP COMPENSATION
The adaptive voltage positioning (AVP) is usually adopted in the computer applications to improve the transient
response and reduce the power loss at heavy load. Like current mode control, the adaptive voltage positioning
loop introduces an extra zero to the voltage loop and splits the double poles of the power stage, which makes the
voltage loop compensation much easier.
Adaptive voltage positioning lowers the converter voltage by R
O
*I
O,
where R
O
is the
required
output impedance of
the converter. Pre-select feedback resistor R
FB,
and calculate the droop resistor RDRP,
G
R
R
R
O
CS
MAX
R
L
FB
DRP
n
=
*
_
(25)
The selection of compensation types depends on the output capacitors used in the converter. For applications
using Electrolytic, Polymer or AL-Polymer capacitors and running at lower frequency, type II compensation shown
in Figure 22(a) is usually enough. While for the applications using only ceramic capacitors and running at higher
frequency, type III compensation shown in Figure 22(b) is preferred.
For applications where AVP is not required, the compensation is the same as for the regular voltage mode
control. For converters using Polymer, AL-Polymer, and ceramic capacitors, which have much higher ESR zero
frequency, type III compensation is required as shown in Figure 22(b) with R
DRP
and C
DRP
removed.
RCP
CCP1
EAOUT
CCP
RFB
RDRP
VO+
VDRP
VDAC
+
-
EAOUT
FB
CFB
CDRP
RCP
EAOUT
CCP1
CCP
RFB
RDRP
VO+
VDRP
VDAC
FB
+
-
EAOUT
RFB1
(a) Type II compensation
(b) Type III compensation
Figure 22 - Voltage loop compensation network
Type II Compensation for AVP Applications
Determine the compensation at no load, the worst case condition. Choose the crossover frequency fc between
1/10 and 1/5 of the switching frequency per phase. Assume the time constant of the resistor and capacitor across
the output inductors matches that of the inductor, and determine R
CP
and C
CP
from (26) and (27), where L
E
and
C
E
are the equivalent inductance of output inductors and the equivalent capacitance of output capacitors
respectively.