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LT8390

8390fa

For more information www.linear.com/LT8390

APPLICATIONS INFORMATION

The front page shows a typical LT8390 application circuit.

ThisApplicationsInformationsectionservesasaguideline

of selecting external components for typical applications.

The examples and equations in this section assume

continuous conduction mode unless otherwise specified.

Switching Frequency Selection

The LT8390 uses a constant frequency control scheme

between 150kHz and 650kHz. Selection of the switching

frequency is a tradeoff between efficiency and component

size. Low frequency operation improves efficiency by

reducing MOSFET switching losses, but requires larger

inductor and capacitor values. For high power applica-

tions, consider operating at lower frequencies to minimize

MOSFET heating from switching losses. For low power

applications, consider operating at higher frequencies to

minimize the total solution size.

Inaddition,thespecificapplicationalsoplaysanimportant

role in switching frequency selection. In a noise-sensitive

system,theswitchingfrequencyisusuallyselectedtokeep

the switching noise out of a sensitive frequency band.

Switching Frequency Setting

The switching frequency of the LT8390 can be set by the

internal oscillator. With the SYNC/SPRD pin pulled to

ground, the switching frequency is set by a resistor from

the RT pin to ground. Table 1 shows RT resistor values

for common switching frequencies.

Table 1. Switching Frequency vs RT Value (1% Resistor)

fOSC (kHz)

RT (k)

150

309

200

226

300

140

400

100

500

600

650

51.1

Spread Spectrum Frequency Modulation

Switching regulators can be particularly troublesome for

applications where electromagnetic interference (EMI) is

a concern. To improve the EMI performance, the LT8390

implements a triangle spread spectrum frequency modu-

lation scheme. With the SYNC/SPRD pin tied to INTVCC,

the LT8390 starts to spread its switching frequency ±15%

aroundtheinternaloscillatorfrequency.Figure9andFigure

10 show the noise spectrum comparison of the front page

applicationbetweenspreadspectrumenabledanddisabled.

Figure 9. Average Conducted EMI Comparison

Figure 10. Peak Conducted EMI Comparison

FREQUENCY (kHz)

150

2000

8390 F09

SPREAD ON

SPREAD OFF

CISPR25

CONDUCTED EMI

AVERAGE LIMIT

8390 F10

CISPR25

CONDUCTED EMI

PEAK LIMIT

SPREAD ON

SPREAD OFF

FREQUENCY (kHz)

150

2000

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