LM3478MM National Semiconductor, LM3478MM Datasheet - Page 18
LM3478MM
Manufacturer Part Number
LM3478MM
Description
Pulse Width Modulation (PWM) Controller IC
Manufacturer
National Semiconductor
Datasheet
1.LM3478MM.pdf
(21 pages)
Specifications of LM3478MM
Peak Reflow Compatible (260 C)
No
Leaded Process Compatible
No
Mounting Type
Surface Mount
Features
High Efficiency
Package / Case
8-MSOP
Lead Free Status / RoHS Status
Contains lead / RoHS non-compliant
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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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