LM2698 datasheet(14/17 Pages) NSC

14 / 17 page

Application Information (Continued)

3.3V SEPIC

The LM2698 can be used to implement a SEPIC technology.

The advantages of the SEPIC topology are that it can step

up or step down an input voltage, and it has low input current

ripple.

The conversion ratio for the SEPIC is :

where

D’ = 1−D

Solving for D yeilds:

To avoid subharmonic oscillations, it is recommended that

inductors L1 and L2 be the same inductance. Currents con-

ducted by the inductors are:

1 =IOUT(VOUT/VIN)

∆i

1 =VIND/(2*L1*fs)

2 =IOUT

∆i

1 =VIND/(2*L2*fs)

The switch sees a maximum current of I

1 +I2 + ∆i1 + ∆i2.If

L1 = L2 = L, the maximum switch current is given by:

OUT(1+VOUT/VIN)+VIND/(L*fs)

The maximum load current is limited by this relationship to

the switch current.

The polarity of C

SEPIC will change between each cycle, so a

ceramic capacitor should be used here. A high quality, low

ESR capacitor will directly improve efficiency because all the

load current passes through C

SEPIC.

IN should be chosen using the same relationship as in the

boost converter (see the C

IN section). CIN must be able to

provide the necessary RMS current.

20012631

FIGURE 7. 3.3V SEPIC Converter

LM2698 Datasheet(PDF) 14 Page - National Semiconductor (TI)