Datasheet
V
O
=
D
¸
¹
·
¨
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§
(V
IN
x K)+
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V
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V
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=
1 - D
D
x K
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1
+ +
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R
R
L1
¸
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¹
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D
2
D
2
'
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R
R
ON
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¹
·
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¹
·
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D
D
2
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¹
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R
L2
V
D
V
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1
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R
R
L1
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·
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D
2
D
2
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R
ON
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D
D
2
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R
L2
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D
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x
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I
L1
and
D
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D
R
¸
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LMR62421
www.ti.com
SNVS734B –OCTOBER 2011–REVISED APRIL 2013
Using inductor volt-second balance & capacitor charge balance, the following equations are derived:
(24)
(25)
Therefore:
(26)
One can see that all variables are known except for the duty cycle (D). A quadratic equation is needed to solve
for D. A less accurate method of determining the duty cycle is to assume efficiency, and calculate the duty cycle.
(27)
(28)
Table 1. Efficiencies for Typical SEPIC Application
Vin 2.7V Vin 3.3V Vin 5V5V
Vo 3.1V Vo 3.1V Vo 3.1V
lin 770 mA lin 600mA lin 375 mA
lo 500 mA lo 500mA lo 500 mA
η 75% η 80% η 83%
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