rt9245c Richtek Technology Corporation, rt9245c Datasheet - Page 16

• Manufacturer Part Number: rt9245c
• Description: Multi-phase Controller Core Power Supply
• Manufacturer: Richtek Technology Corporation

RT9245C
Dead Zone Elimination
RT9245C samples and holds inductor current at 50%
period by time-sharing sourcing a current I
light load condition when inductor current is not balance,
voltage V
It needs a negative I
RT9245C CANNOT provide a negative I
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.
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16
35
30
25
20
15
10
5
0
30
25
20
15
10
5
0
0
0
X
across the sensing capacitor would be negative.
15
20
Current Balance Function
Current Ratio Function
X
30
Figure 12
to sense the voltage. However,
Figure 13
40
I
OUT
I
OUT
45
(A)
(A)
60
60
X
and consequently
I
L4
X
80
75
I
L2
to R
I
L3
I
I
I
I
L4
L3
L2
L1
CSN
I
L1
90
100
. At
Referring to Figure 15, I
where I
make sure RT9245C could sense the inductor current,
right hand side of Equation (1) should always be positive:
Since R
could be simplified as :
Figure 14 shows that dead zone of load line at light load
is eliminated by applying this technique.
I
R
R
X
V
V
CSN2
CSN2
=
OUT
OUT
1.31
1.29
1.28
1.27
1.26
1.25
1.24
1.23
R
1.3
V
CSN2
Load Line without dead zone at light loads
OUT
+
≥
LX_50%
CSN2
0
I
I
LX_50%
LX_50%
Figure 15. Application circuit
+
w/i Dead Zone Compensation
R
GMx
>> R
I
R
LX_50%
Ix
is the of inductor current at 50% period. To
CSN2
CSN
R
x
x
+
-
5
CSN
CSN2
R
R
I
LX
LX
LX
x
in practical application, Equation (2)
R
+
R
R
Figure 14
LX
CSN
CSN2
X
I
LX_50%
10
is expressed as :
R
R
+
w/o Dead Zone Compensation
LX
= 82k
CSN
X
I
R
I
OUT
LX_50%
R
CSN
DS9245C-02 March 2007
CSN
x
R
(A)
+
R
CSN
open
15
R
LX
C
x
V
LX
X
X
R
≥
LX
-
0
of GM
20
V
OUT
25
(1)
(2)