RT9245C 데이터 시트보기 (PDF) - Richtek Technology
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RT9245C Datasheet PDF : 25 Pages
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RT9245C
35
30
25
20
15
10
5
0
0
Current Ratio Function
IL4
IL3
IL2
IL1
15
30
45
60
75
90
IOUT (A)
Figure 12
Current Balance Function
30
IL3
25
IL4
20
IL1
IL2
15
10
5
0
0
20
40
60
80
100
IOUT (A)
Figure 13
Dead Zone Elimination
RT9245C 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,
RT9245C 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.
Load Line without dead zone at light loads
1.31
1.3
w/o Dead Zone Compensation
1.29
RCSN open
1.28
1.27
1.26
RCSN = 82k
1.25 w/i Dead Zone Compensation
1.24
1.23
0
5
10
15
20
25
IOUT (A)
Figure 14
LX
ILX
RLX
RX
CX
+
-
GMx
+ VX -
RCSN
Ix
RCSN2
VOUT
Figure 15. Application circuit of GM
Referring to Figure 15, IX is expressed as :
IX = VOUT + ILX_50% x RLX + ILX_50% x RLX
(1)
RCSN2
RCSN2
RCSN
where ILX_50% is the of inductor current at 50% period. To
make sure RT9245C could sense the inductor current,
right hand side of Equation (1) should always be positive:
VOUT + ILX_50% x RLX + ILX_50% x RLX ≥ 0
(2)
RCSN2
RCSN2
RCSN
Since RCSN2 >> RCSN in practical application, Equation (2)
could be simplified as :
VOUT ≥ ILX_50% x RLX
RCSN2
RCSN
Figure 14 shows that dead zone of load line at light load
is eliminated by applying this technique.
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16
DS9245C-02 March 2007
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