LM3481 Datasheet, PDF (18/22 Pages) National Semiconductor (TI) – High Efficiency Low-Side N-Channel Controller for Switching Regulators

Designing SEPIC Using LM3481
Since the LM3481 controls a low-side N-Channel MOSFET,
it can also be used in SEPIC (Single Ended Primary Induc-
tance Converter) applications. An example of SEPIC using
the LM3481 is shown in Figure 17. As shown in Figure 17, the
output voltage can be higher or lower than the input voltage.
The SEPIC uses two inductors to step-up or step-down the
input voltage. The inductors L1 and L2 can be two discrete
inductors or two windings of a coupled transformer since
equal voltages are applied across the inductor throughout the
switching cycle. Using two discrete inductors allows use of
catalog magnetics, as opposed to a custom transformer. The
input ripple can be reduced along with size by using the cou-
pled windings of transformer for L1 and L2.
Due to the presence of the inductor L1 at the input, the SEPIC
inherits all the benefits of a boost converter. One main ad-
vantage of SEPIC over a boost converter is the inherent input
to output isolation. The capacitor C_S isolates the input from
the output and provides protection against shorted or mal-
functioning load. Hence, the SEPIC is useful for replacing
boost circuits when true shutdown is required. This means
that the output voltage falls to 0V when the switch is turned
off. In a boost converter, the output can only fall to the input
voltage minus a diode drop.
The duty cycle of a SEPIC is given by:
In the above equation, V_Q is the on-state voltage of the MOS-
FET, Q1, and V_DIODE is the forward voltage drop of the diode.
POWER MOSFET SELECTION
As in a boost converter, the parameters governing the selec-
tion of the MOSFET are the minimum threshold voltage, V_TH
(MIN)^{, the on-resistance, R}DS(ON)^{, the total gate charge, Q}g^{, the}
reverse transfer capacitance, C_RSS, and the maximum drain
to source voltage, V_DS(MAX). The peak switch voltage in a
SEPIC is given by:
V_SW(PEAK) = V_IN + V_OUT + V_DIODE
The selected MOSFET should satisfy the condition:
V_DS(MAX) > V_SW(PEAK)
The peak switch current is given by:
Where ΔI_L1 and ΔI_L2 are the peak-to-peak inductor ripple cur-
rents of inductors L1 and L2 respectively.
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_IN + V_OUT.
Similar to the boost converter, the average diode current is
equal to the output current. Schottky diodes are recommend-
ed.
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FIGURE 17. Typical SEPIC Converter
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