LM3481 Datasheet, PDF (19/22 Pages) National Semiconductor (TI) – High Efficiency Low-Side N-Channel Controller for Switching Regulators

SELECTION OF INDUCTORS L1 AND L2
Proper selection of the inductors L1 and L2 to maintain con-
stant current mode requires calculations of the following pa-
rameters.
Average current in the inductors:
I_L2AVE = I_OUT
Peak to peak ripple current, to calculate core loss if neces-
sary:
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. A good choice for transformer with equal turns
is Coiltronics CTX series Octopack.
SENSE RESISTOR SELECTION
The peak current through the switch, I_SWPEAK, can be adjusted
using the current sense resistor, R_SEN, to provide a certain
output current. Resistor R_SEN can be selected using the for-
mula:
Sepic Capacitor Selection
The selection of SEPIC capacitor, C_S, 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 con-
duction mode yields the following minimum values for L1 and
L2:
Peak current in the inductor, to ensure the inductor does not
saturate:
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 small (relative to capacitor
technology). The voltage rating of the SEPIC capacitor must
be greater than the maximum input voltage. Tantalum capac-
itors are the best choice for SMT, having high rms current
ratings relative to size. Ceramic capacitors could be used, but
the low C values will tend to cause larger changes in voltage
across the capacitor due to the large currents, and high C
value ceramics are expensive. Electrolytics work well for
through hole applications where the size required to meet the
rms current rating can be accommodated. There is an energy
balance between C_S and L1, which can be used to determine
the value of the capacitor. The basic energy balance equation
is:
Where
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 value to reduce input ripple and output ripple.
However, once ΔI_L1 is less than 20% of I_L1AVE, the benefit to
output ripple is minimal.
By increasing the value of L2 above the minimum recommen-
dation, ΔI_L2 can be reduced, which in turn will reduce the
output ripple voltage:
is the ripple voltage across the SEPIC capacitor, and
is the ripple current through the inductor L1. The energy bal-
ance equation can be solved to provide a minimum value for
C_S:
where ESR is the effective series resistance of the output ca-
pacitor.
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