LM3481MM datasheet(21/30 Pages) TI1

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LM3481

VIN

ISEN

COMP

AGND

UVLO

VCC

PGND

FA/SYNC/SD

VIN = 3.0V to 24V

VOUT = 5V, 1A

CIN

15 PF, 35V x2

COUT

100 PF, 10V

1 PF, ceramic

RSEN

0.05:

10 PH

0.47 µF

MBRS130LT3

RFA

40 k:

CSEN

1 nF

IRF7807

4.7 k:

0.1 PF

RF1

60 k:

RF2

20 k:

10 k:

5.1V

ISWPEAK = IL1(AVG) + IOUT +

'IL1 + 'IL2

LM3481

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SNVS346E – NOVEMBER 2007 – REVISED APRIL 2012

Power MOSFET Selection

As in a boost converter, the 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

The selected MOSFET should satisfy the condition:

VDS(MAX) > VSW(PEAK)

The peak switch current is given by:

Where

ΔIL1 and ΔIL2 are the peak-to-peak inductor ripple currents 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 VIN + VOUT. Similar to the boost converter, the average diode current is equal to the output current.

Schottky diodes are recommended.

Figure 34. Typical SEPIC Converter

Selection of Inductors L1 and L2

Proper selection of the inductors L1 and L2 to maintain constant current mode requires calculations of the

following parameters.

Average current in the inductors:

IL2AVE = IOUT

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