PWM
Over Current Protection
CH1:(5V/Div)
CH2:(5V/Div)
VSS
Time (25ms/Div)
Figure 7. The Over Current Protection in the soft start
interval
Over Current Protection
CH1:(5V/Div)
CH2:(5V/Div)
PWM
VSS
Time (25ms/Div)
Figure 8. Over Current Protection at steady state
Current Ratio Setting
IL
L
DCR
+VC-
R1
C
R2
Figure 9. Application circuit for current ratio setting
DS9245A-05 March 2007
RT9245A
For some case with preferable current ratio instead of
current balance, the corresponding technique is provided.
Due to different physical environment of each channel, it
is necessary to slightly adjust current loading between
channels. Figure 9 shows the application circuit of GM for
current ratio requirement. Applying KVL along L+DCR
branch and R1+C//R2 branch :
L
dIL
dt
+ DCR ×IL
=
R1(
VC
R2
+ C dVC
dt
) + VC
=
R1C
dVC
dt
+
R1 + R2
R2
VC
For
VC
=
R2
R1 + R2
DCR ×IL
Look for its corresponding conditions :
L
dIL
dt
+ DCR×IL
= (R1//R2)×C×DCR× dIL
dt
+ DCR×IL
Let L = (R1//R2)×C
DCR
Thus if L = (R1//R2) × C
DCR
Then
VC
=
R2
R1+ R2
× DCR
× IL
With internal current balance function, this phase would
share (R1+R2)/R2 times current than other phases.
Figure 10 & 11 show different settings for the power stages.
Figure 12 shows the performance of current ratio compared
with conventional current balance function in Figure 13.
IL
0.3uH
0.6m
470
1uF
470
Figure 10. GM4 Setting for current ratio function
IL
0.3uH
0.6m
470
1uF
Figure 11. GM1~3 Setting for current ratio function
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