LM3478MA National Semiconductor Corporation, LM3478MA Datasheet - Page 19

• Manufacturer Part Number: LM3478MA
• Description: High Efficiency Low-side N-channel Controller For Switching Regulator
• Manufacturer: National Semiconductor Corporation

POWER MOSFET SELECTION
As in a boost converter, parameters governing the selection
of the MOSFET are the minimum threshold voltage, V
(MIN)
reverse transfer capacitance, C
to source voltage, V
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
Similar to the boost converter, the average diode current is
equal to the output current. Schottky diodes are recommend-
ed.
SELECTION OF INDUCTORS L1 AND L2
Proper selection of inductors L1 and L2 to maintain continu-
ous current conduction mode requires calculations of the
following parameters.
Average current in the inductors:
, the on-resistance, R
V
SW(PEAK)
V
DS(MAX)
DS(MAX)
= V
DS(ON)
IN
. The peak switch voltage in a
> V
+ V
, the total gate charge, Q
RSS
SW(PEAK)
OUT
, and the maximum drain
+ V
DIODE
FIGURE 13. Typical SEPIC Converter
IN
+ V
g
, the
OUT
TH
.
19
In the above equation, V
FET, Q, and V
Peak to peak ripple current, to calculate core loss if neces-
sary:
Maintaining the condition I
rent conduction yields:
Peak current in the inductor, to ensure the inductor does not
saturate:
DIODE
is the forward voltage drop of the diode.
Q
I
L2AVE
is the on-state voltage of the MOS-
L
> Δi
= I
L
/2 to ensure continuous cur-
OUT
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