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SS8028

increase the available output current, but limit it to about

twice the calculated value. When too large an inductor is

used, the output voltage ripple will increase without

providing much additional output current. In conditions of

value can be used to give smaller physical size, but

overshoot of the inductor current will occur (see Current

Limit Overshoot section).

Inductor Selection – SEPIC Regulator

For a SEPIC regulator using the SS8028, the

approximate inductance value can be calculated using

the formula below. As for the boost inductor selection, a

larger or smaller value can be used.

Current Limit Overshoot

The SS8028 uses a constant off-time control scheme;

the MOSFET is turned off after the 350mA current limit is

reached. When the current limit is reached and the

MOSFET actually turns off, there is a 100ns delay time.

During this time, the inductor current exceeds the current

limit by a small amount. The formula below can calculate

the peak inductor current.

L

x 100ns

systems with high input voltages and smaller

inductance values, the current overshoot will be most

apparent. This overshoot can be useful as it helps

increase the amount of available output current. By

using a small inductance value, the current limit

overshoot can be quite high. Even though it is

internally current limited to 350mA, the internal

MOSFET of the SS8028 can handle larger currents

without any problem, but the total efficiency will suffer.

500mA.

Capacitor Selection

Low ESR (Equivalent Series Resistance) capacitors

should be used at the output to minimize the output

ripple voltage and the peak-to-peak transient voltage.

Multilayer ceramic capacitors (MLCC) are the best

choice, as they have a very low ESR and are

available in very small packages. Their small size

makes them a good match with the SS8028’s

SOT-23 package. If solid tantalum capacitors (like the

AVX TPS, Sprague 593D families) or OS-CON

capacitors are used, they will occupy more volume

than ceramic ones and the higher ESR increases the

output ripple voltage. It is important to use a capacitor

with a sufficient voltage rating.

A low ESR surface-mount ceramic capacitor also

makes a good selection for the input bypass capacitor,

which should be placed as close as possible to the

SS8028. A 4.7µF input capacitor is sufficient for

most applications.

Diode Selection

For most SS8028 applications, the high switching

frequency requires high-speed Schottky diodes, such

as the Motorola MBR0530 (0.5A, 30V) with their low

forward voltage drop and fast switching speed. Many

different manufacturers make equivalent parts, but

make sure that the component is rated for at least

0.35A. To achieve high efficiency, the average

current rating of the Schottky diodes should be

greater than the peak switching current. Choose a

reverse breakdown voltage greater than the output

voltage.

Lowering Output Ripple Voltage

The SS8028 supplies energy to the load in bursts by

ramping up the inductor current, then delivering that

current to the load. Using low ESR capacitors will help

L = 2

Rev.2.02 12/06/2003