LM3478 MDC National Semiconductor, LM3478 MDC Datasheet - Page 18

• Manufacturer Part Number: LM3478 MDC
• Description: LM3478/LM3478Q High Efficiency Low-Side N-Channel Controller for Switching Regulator; ; Container: Tray
• Manufacturer: National Semiconductor

www.national.com
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:
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:
I
L2AVE
L
> Δi
= I
L
/2 to ensure continuous cur-
OUT
IN
+ V
OUT
.
18
I
the current sense resistor.
The value of L1 can be increased above the minimum rec-
ommended to reduce input ripple and output ripple. However,
once D
is minimal.
By increasing the value of L2 above the minimum recom-
mended, Δ
output ripple voltage:
where ESR is the effective series resistance of the output ca-
pacitor.
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 re-
placed by 2L.
SENSE RESISTOR SELECTION
The peak current through the switch, I
ed using the current sense resistor, R
output current. Resistor R
mula:
Sepic Capacitor Selection
The selection of the SEPIC capacitor, CS, depends on the
rms current. The rms current of the SEPIC capacitor is given
by:
The SEPIC capacitor must be rated for a large ACrms current
relative to the output power. This property makes the SEPIC
much better suited to lower power applications where the rms
current through the capacitor is relatively small (relative to
capacitor technology). The voltage rating of the SEPIC ca-
pacitor must be greater than the maximum input voltage.
There is an energy balance between CS and L1, which can
be used to determine the value of the capacitor. The basic
energy balance equation is:
where
is the ripple voltage across the SEPIC capacitor, and
L1PK
must be lower than the maximum current rating set by
IL1
is less than 20% of I
IL2
can be reduced, which in turn will reduce the
SEN
L1AVE
can be selected using the for-
, the benefit to output ripple
SEN
SW(PEAK)
, to provide a certain
can be adjust-