LM3488MM datasheet(20/24 Pages) NSC

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Designing SEPIC Using LM3488

(Continued)

The duty cycle of a SEPIC is given by:

In the above equation, V

Q is the on-state voltage of the

MOSFET, Q, and V

DIODE is the forward voltage drop of the

diode.

POWER MOSFET SELECTION

As in boost converter, the parameters governing the selec-

tion of the MOSFET are the minimum threshold voltage,

TH(MIN), the on-resistance, RDS(ON), the total gate charge,

g, the reverse transfer capacitance, CRSS, and the maxi-

mum drain to source voltage, V

DS(MAX). The peak switch

voltage in a SEPIC is given by:

SW(PEAK) =VIN +VOUT +VDIODE

The selected MOSFET should satisfy the condition:

DS(MAX) > VSW(PEAK)

The peak switch current is given by:

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

OUT. 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 the inductors L1 and L2 to maintain

constant current mode requires calculations of the following

parameters.

Average current in the inductors:

L2AVE =IOUT

Peak to peak ripple current, to calculate core loss if neces-

sary:

maintains the condition I

L >

∆i

L to ensure constant current

mode.

10138844

FIGURE 14. Typical SEPIC Converter

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