LTC1871 datasheet(28/36 Pages) LINER

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LTC1871

choice is AVX TPS series of surface mount tantalum. Also,

ceramic capacitors are now available with extremely low

ESR, ESL and high ripple current ratings.

SEPIC Converter: Input Capacitor Selection

The input capacitor of a SEPIC converter is less critical

than the output capacitor due to the fact that an inductor

is in series with the input and the input current waveform

is triangular in shape. The input voltage source impedance

determines the size of the input capacitor which is typically

in the range of 10

µF to 100µF. A low ESR capacitor is

recommended, although it is not as critical as for the

output capacitor.

The RMS input capacitor ripple current for a SEPIC con-

verter is:

RMS CIN

()

•

=∆

Please note that the input capacitor can see a very high

surge current when a battery is suddenly connected to the

input of the converter and solid tantalum capacitors can

fail catastrophically under these conditions. Be sure to

specify surge-tested capacitors!

SEPIC Converter: Selecting the DC Coupling Capacitor

The coupling capacitor C1 in Figure 16 sees nearly a

rectangular current waveform as shown in Figure 17.

During the switch off-time the current through C1 is IO(VO/

VIN) while approximately – IO flows during the on-time.

This current waveform creates a triangular ripple voltage

on C1:

∆=

−

CP P

OMAX

()

•

The maximum voltage on C1 is then:

C MAX

CP P

()

∆

−

which is typically close to VIN(MAX). The ripple current

through C1 is:

RMS C

O MAX

IN MIN

()

(

)

()

•

1 =

The value chosen for the DC coupling capacitor normally

starts with the minimum value that will satisfy 1) the RMS

current requirement and 2) the peak voltage requirement

(typically close to VIN). Low ESR ceramic and tantalum

capacitors work well here.

SEPIC Converter Design Example

The design example given here will be for the circuit shown

in Figure 18. The input voltage is 5V to 15V and the output

is 12V at a maximum load current of 1.5A (2A peak).

1. The duty cycle range is:









= 45 5

71 4

2. The operating mode chosen is pulse skipping, so the

MODE/SYNC pin is shorted to INTVCC.

3. The operating frequency is chosen to be 300kHz to

reduce the size of the inductors; the resistor from the

FREQ pin to ground is 80k.

4. An inductor ripple current of 40% is chosen, so the peak

input current (which is also the minimum saturation

current) is:

L PEAK

O MAX

IN MIN

12 0 5

()

(

)

()

••

•. •

















The inductor ripple current is:

∆=

O MAX

MAX

χ ••

–

.• .•

–.

()

04 15

0 714

1 0 714

APPLICATIO S I FOR ATIO

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