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RT9245BPC Datasheet(PDF) 14 Page - Richtek Technology Corporation

Part # RT9245BPC
Description  Multi-Phase PWM Controller for K8 CPU Core Power Supply
PDF  23 Pages
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Manufacturer  RICHTEK [Richtek Technology Corporation]
Direct Link  http://www.richtek.com
Logo RICHTEK - Richtek Technology Corporation

RT9245BPC Datasheet(HTML) 14 Page - Richtek Technology Corporation

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RT9245B
14
DS9245B-02
March 2007
www.richtek.com
Figure 13
Current Balance Function
0
5
10
15
20
25
30
0
20
40
60
80
100
IOUT (A)
IL1
IL3
IL2
IL4
Figure 12
Current Ratio Function
0
5
10
15
20
25
30
35
0
15
30
45
60
75
90
IOUT (A)
IL3
IL4
IL2
IL1
Dead Zone Elimination
RT9245B samples and holds inductor current at 50%
period by time-sharing sourcing a current IX to RCSN. At
light load condition when inductor current is not balance,
voltage VX across the sensing capacitor would be negative.
It needs a negative IX to sense the voltage. However,
RT9245B CANNOT provide a negative IX and consequently
cannot sense negative inductor current. This results in
dead zone of load line performance as shown in Figure
14. Therefore a technique as shown in Figure 15 is required
to eliminate the dead zone of load line at light load
condition.
Figure 14 shows that dead zone of load line at light load
is eliminated by applying this technique.
Figure 14
Load Line without dead zone at light loads
1.23
1.24
1.25
1.26
1.27
1.28
1.29
1.3
1.31
0
5
10
15
20
25
IOUT (A)
RCSN open
RCSN = 82k
w/o Dead Zone Compensation
w/i Dead Zone Compensation
Referring to Figure 15, IX is expressed as :
where ILX_50% is the of inductor current at 50% period. To
make sure RT9245B could sense the inductor current,
right hand side of Equation (1) should always be positive:
Since RCSN2 >> RCSN in practical application, Equation (2)
could be simplified as :
(2)
(1)
CSN
LX
LX_50%
CSN2
LX
LX_50%
CSN2
OUT
X
R
R
x
I
R
R
x
I
R
V
I
+
+
=
0
R
R
x
I
R
R
x
I
R
V
CSN
LX
LX_50%
CSN2
LX
LX_50%
CSN2
OUT
≥
+
+
CSN
LX
LX_50%
CSN2
OUT
R
R
x
I
R
V
≥
Figure 15. Application circuit of GM
RCSN
GMx
Ix
+
-
RCSN2
I
LX
LX
R
LX
R
X
C
X
V
X
+-
V
OUT



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