LM3478MASLASHNOPB datasheet(25/43 Pages) TI2

25 / 43 page

L2 >

(VIN - VQ)D

2IOUTfS

L1 >

(VIN - VQ)(1-D)

2IOUTfS

LM3478

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SNVS085X – JULY 2000 – REVISED DECEMBER 2017

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Typical Applications (continued)

8.2.2.2 Detailed Design Procedure

8.2.2.2.1

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 using Equation 35.

VSW(PEAK) = VIN + VOUT + VDIODE

(35)

The selected MOSFET should satisfy the condition:

VDS(MAX) > VSW(PEAK)

(36)

The peak switch current is given using Equation 37.

(37)

The RMS current through the switch is given using Equation 38.

(38)

8.2.2.2.2

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.

8.2.2.2.3

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 can be calculated using Equation 39.

(39)

IL2AVE = IOUT

(40)

Peak to peak ripple current, to calculate core loss if necessary using Equation 41 and Equation 42.

(41)

(42)

Maintaining the condition IL > ΔiL/2 to ensure continuous current conduction yields Equation 43 and Equation 44.

(43)

(44)

Peak current in the inductor, use Equation 45 and Equation 46 to ensure the inductor does not saturate.

(45)

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