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LT8494

8494fa

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APPLICATIONS INFORMATION

For the coupled inductor SEPIC:

IL(PEAK) =IOUT • 1+

VOUT

VIN • η









+ VIN •DC

2•L • f

IL(AVG) =IOUT • 1+

VOUT

VIN • η













Note: Inductor current can be higher during load tran-

sients. It can also be higher during short-circuit and

start-up if inadequate soft-start capacitance is used. Thus,

IL(PEAK) may be higher than the switch current limit of

2.95A, and the RMS inductor current is approximately

equal to IL(AVG). Choose an inductor having sufficient

saturation current and RMS current ratings.

Capacitor Selection

Low ESR (equivalent series resistance) capacitors should

be used at the output to minimize the output ripple volt-

age. Multilayer ceramic capacitors are an excellent choice,

as they have an extremely low ESR and are available in

very small packages. X5R or X7R dielectrics are preferred,

as these materials retain their capacitance over wider volt-

age and temperature ranges. Always use a capacitor with

a sufficient voltage rating. Many capacitors rated at 2.2µF

to 20µF, particularly 0805 or 0603 case sizes, have greatly

reduced capacitance at the desired output voltage. Solid

tantalum or OS-CON capacitors can be used, but they will

occupy more board area than a ceramic and will have a

higher ESR with greater output ripple.

Ceramic capacitors also make a good choice for the input

decoupling capacitor, which should be placed as closely as

possible to the VIN and BIAS pins of the LT8494. A 2.2µF

to 4.7µF input capacitor is sufficient for most applications.

Audible Noise

Ceramic capacitors are small, robust and have very low

ESR. However, due to their piezoelectric nature, ceramic

capacitors can sometimes create audible noise when

used with the LT8494. During Burst Mode operation, the

LT8494 regulator’s switching frequency depends on the

load current, and at very light loads the regulator can

excite the ceramic capacitor at audio frequencies, gen-

erating audible noise. Since LT8494 operates at a lower

current limit during Burst Mode operation, the noise is

typically very quiet. If this is unacceptable, use a high per-

formance tantalum or electrolytic capacitor at the output.

Diode Selection

The diode used in boost or SEPIC topologies conducts

current only during the switch off-time. During the switch

on-time, the diode has reverse voltage across it. The peak

reverse voltage is equal to VOUT in the boost topology

and equal to (VOUT + VIN) in the SEPIC topology. Use a

diode with a reverse voltage rating greater than the peak

reverse voltage.

An additional consideration is the reverse leakage cur-

rent. The leakage current appears to the output as load

current and affects the efficiency, most noticeably, under

light load conditions. In Burst Mode operation, after the

inductor current vanishes, the reverse voltage across the

boost diode is approximately equal to VOUT – VIN in the

boost topology and VOUT in the SEPIC topology. The per-

centage of time that the diode is reverse biased increases

as load current decreases.

Schottky diodes that have larger forward voltages often

have less leakage, so a trade-off exists between light

load and high load efficiency. Also the Schottky diodes

with larger reverse bias ratings may have less leakage at

a given output voltage, therefore, superior leakage per-

formance can be achieved at the expense of diode size.

Finally, keep in mind that the leakage current of a power

Schottky diode goes up exponentially with junction tem-

perature. Therefore, the Schottky diode must be selected

with care to avoid excessive increase in light load supply

current at high temperatures.

Soft-Start

The LT8494 contains a soft-start circuit to limit peak

switch currents during start-up. High start-up current is

inherent in switching regulators since the feedback loop

is saturated due to VOUT being far from its final value. The

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