Datasheet

I
SWPEAK
V
SENSE
- D x (V
SL
+'V
SL
)
R
SEN
=
2
'V
OUT
=
I
OUT
1-D
+
'I
L2
( )
ESR
L2 >
(V
IN
- V
Q
)D
2I
OUT
f
S
L1 >
(V
IN
- V
Q
)(1-D)
2I
OUT
f
S
LM3481
SNVS346E –NOVEMBER 2007–REVISED APRIL 2012 www.ti.com
Peak to peak ripple current, to calculate core loss if necessary:
Maintaining the condition I
L
> ΔI
L
/2 to ensure continuous conduction mode yields the following minimum values
for L1 and L2:
Peak current in the inductor, to ensure the inductor does not saturate:
I
L1PK
must be lower than the maximum current rating set by the current sense resistor.
The value of L1 can be increased above the minimum recommended value to reduce input ripple and output
ripple. However, once ΔI
L1
is less than 20% of I
L1AVE
, the benefit to output ripple is minimal.
By increasing the value of L2 above the minimum recommendation, ΔI
L2
can be reduced, which in turn will
reduce the output ripple voltage:
where ESR is the effective series resistance of the output capacitor.
If L1 and L2 are wound on the same core, then L1 = L2 = L. All the equations above will hold true if the
inductance is replaced by 2L. A good choice for transformer with equal turns is Coiltronics CTX series Octopack.
Sense Resistor Selection
The peak current through the switch, I
SWPEAK
, can be adjusted using the current sense resistor, R
SEN
, to provide
a certain output current. Resistor R
SEN
can be selected using the formula:
SEPIC CAPACITOR SELECTION
The selection of SEPIC capacitor, C
S
, depends on the rms current. The rms current of the SEPIC capacitor is
given by:
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