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POWER MANAGEMENT

SC1408

COMPONENT SELECTION

SEPIC Converter

Again, with the SEPIC topology, the value of the sense

resistor is the primary determining factor for maximum

to add Vin to Vo and use this value as the output voltage

in the output current curves or in the equation for Boost

converter.

Output Voltage

Output voltage setting works exactly the same in SEPIC

topology as in Boost, including the ability to set to 5V by

connecting the FB pin to GND. Care must be taken to

ensure that the IC supply (pin2; BST) does not exceed its

16.5V rating. In the circuit of Fig.2: This requires maxi-

mum output voltage to be limited to 16.5V-Vin. Higher

output voltages are possible with different IC supply strat-

egies.

Inductor

The SEPIC topology requires a coupled inductor. Again A

good choice for most applications is 22µH. Smaller induc-

tor values result in higher peak currents and increase out-

put ripple, while larger values will result in slower loop re-

sponse.

Transistor selection

The choice of FET can be critical, especially in battery pow-

ered applications where the converter must be able to use

all of the available energy in the battery. This requires that

the converter be capable of starting up from very low

input voltages. For example a two cell alkaline system’s

Applications Information (Cont.)

terminal voltage will drop to 1.8V as it approaches full

discharge. For these demanding applications, a FET with

low V

should be at least 0.5V less than the minimum input volt-

age.

Diode

For most applications, a Schottky diode should be used

as the output rectifier. It will be subjected to reverse volt-

ages of at least V

what less than the Inductor peak current. Industry stan-

dard 1N5817 series or an equivalent surface mount part

would be suitable.

Output Capacitors

Output capacitors should be low ESR to minimize ripple

voltage and maximize efficiency. Low ESR tantalums,

OSCONs or the newer Polymer capacitors should be used.

Input Capacitors

Input capacitors on a SEPIC converter are less critical

than the output capacitors, since there are no fast cur-

rent pulses drawn from the input supply. A 100

µF tanta-

lum will be adequate for most applications.

Series Capacitors

The Series capacitor(s) must be capable of handling an

RMS current given by:-

IN

O

O

RMS

V

5

.

0

V

I

I

+

=