ir3500a International Rectifier Corp., ir3500a Datasheet - Page 33

• Manufacturer Part Number: ir3500a
• Description: Xphase3 Vr11.0 & Amd Pvid Control Ic
• Manufacturer: International Rectifier Corp.

Select the series resistor R
operational temperature range. Then calculate R
TMAX from (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
the converter. Pre-select feedback resistor R
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
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
C
respectively.
E
are the equivalent inductance of output inductors and the equivalent capacitance of output capacitors
VO+
VDRP
Page 33 of 47
RFB
RDRP
(a) Type II compensation
R
DRP
R
R
TMAX
HOTSET
VDAC
HOTSET2
=
FB
FB
R
RCP
=
FB
1
R
Figure 22 - Voltage loop compensation network
∗
=
THERM
+
-
CCP1
R
n
(
L
to linearize the NTC thermistor, which has non-linear characteristics in the
R
∗
_
EAOUT
TMAX
MAX
R
*
O
CCP
EXP
*
+
G
FB,
[
CS
R
B
and calculate the droop resistor RDRP,
THERM
HOTSET
EAOUT
HOTSET1
1
6 .
( *
2
)
T
( *
L
_
VCCL
corresponding to the allowed maximum temperature
1
VO+
VDRP
MAX
O
RFB1
*I
−
O,
(b) Type III compensation
T
−
CP
where R
_
RFB
RDRP
. 1
CFB
ROOM
CDRP
1
) 6
and C
)]
O
DRP
CP
VDAC
is the required output impedance of
FB
from (26) and (27), where L
and C
RCP
+
-
CCP1
DRP
(25)
EAOUT
(22)
removed.
CCP
July 3, 2008
IR3500A
(23)
EAOUT
E
and