lm3478-mdc National Semiconductor Corporation, lm3478-mdc Datasheet - Page 18

• Manufacturer Part Number: lm3478-mdc
• Description: High Efficiency Low-side N-channel Controller For Switching Regulator
• Manufacturer: National Semiconductor Corporation

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Designing SEPIC Using LM3478
The duty cycle of a SEPIC is given by:
In the above equation, V
MOSFET, Q, and V
diode.
POWER MOSFET SELECTION
As in boost converter, the parameters governing the selec-
tion of the MOSFET are the minimum threshold voltage,
V
Q
mum drain to source voltage, V
voltage in a SEPIC is given by:
The selected MOSFET should satisfy the condition:
The peak switch current is given by:
The rms current through the switch is given by:
POWER DIODE SELECTION
The Power diode must be selected to handle the peak
current and the peak reverse voltage. In a SEPIC, the diode
peak current is the same as the switch peak current. The
off-state voltage or peak reverse voltage of the diode is V
+ V
current is equal to the output current. Schottky diodes are
recommended.
(Continued)
TH(MIN)
g
, the reverse transfer capacitance, C
OUT
. Similar to the boost converter, the average diode
, the on-resistance, R
V
SW(PEAK)
V
DIODE
DS(MAX)
= V
is the forward voltage drop of the
Q
IN
>
is the on-state voltage of the
+ V
DS(ON)
V
SW(PEAK)
DS(MAX)
OUT
, the total gate charge,
+ V
RSS
. The peak switch
DIODE
FIGURE 12. Typical SEPIC Converter
, and the maxi-
IN
18
SELECTION OF INDUCTORS L1 AND L2
Proper selection of the inductors L1 and L2 to maintain
constant current mode requires calculations of the following
parameters.
Average current in the inductors:
Peak to peak ripple current, to calculate core loss if neces-
sary:
Maintaining the condition I
rent mode yields:
I
L2AVE
L
>
= I
∆i
L
OUT
/2 to ensure constant cur-
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