LM3478_09 Datasheet, PDF (18/22 Pages) National Semiconductor (TI) – High Efficiency Low-Side N-Channel Controller for Switching Regulator

The peak switch current is given by:
The rms current through the switch is given by:
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 rec-
ommended to reduce input ripple and output ripple. However,
once D_IL1 is less than 20% of I_L1AVE, the benefit to output ripple
is minimal.
By increasing the value of L2 above the minimum recom-
mended, Δ_IL2 can be reduced, which in turn will reduce the
output ripple voltage:
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_IN + V_OUT.
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:
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_SW(PEAK) can be adjust-
ed using the current sense resistor, R_SEN, to provide a certain
output current. Resistor R_SEN can be selected using the for-
mula:
I_L2AVE = I_OUT
Peak to peak ripple current, to calculate core loss if neces-
sary:
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:
Maintaining the condition I_L > Δi_L/2 to ensure continuous cur-
rent conduction yields:
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:
Peak current in the inductor, to ensure the inductor does not
saturate:
where
is the ripple voltage across the SEPIC capacitor, and
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