LM3478MM datasheet(21/32 Pages) TI1

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LM3478

LM3478-Q1

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SNVS085V – JULY 2000 – REVISED FEBRUARY 2013

Figure 36. Typical SEPIC Converter

POWER MOSFET SELECTION

As in a boost converter, parameters governing the selection of the MOSFET are the minimum threshold voltage,

VTH(MIN), the on-resistance, RDS(ON), the total gate charge, Qg, the reverse transfer capacitance, CRSS, and the

maximum drain to source voltage, VDS(MAX). The peak switch voltage in a SEPIC is given by:

VSW(PEAK) = VIN + VOUT + VDIODE

(35)

The selected MOSFET should satisfy the condition:

VDS(MAX) > VSW(PEAK)

(36)

The peak switch current is given by:

(37)

The RMS current through the switch is given by:

(38)

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 VIN + VOUT. Similar to the boost converter, the average diode current is equal to the output current.

Schottky diodes are recommended.

SELECTION OF INDUCTORS L1 AND L2

Proper selection of inductors L1 and L2 to maintain continuous current conduction mode requires calculations of

the following parameters.

Average current in the inductors:

(39)

IL2AVE = IOUT

(40)

Peak to peak ripple current, to calculate core loss if necessary:

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