LT3790 datasheet(17/30 Pages) LINER

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LT8390A

8390afa

For more information www.linear.com/LT8390A

APPLICATIONS INFORMATION

ThefrontpageshowsatypicalLT8390Aapplicationcircuit.

ThisApplicationsInformationsectionservesasaguideline

of selecting external components for typical applications.

The examples and equations in this section assume con-

tinuous conduction mode unless otherwise specified.

Switching Frequency Selection

The LT8390A uses a constant frequency control scheme

between 600kHz and 2MHz. 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 LT8390A 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 (MHz)

RT (k)

0.6

267

0.8

191

1.0

147

1.2

118

1.4

97.6

1.6

82.5

1.8

66.5

2.0

59.0

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 LT8390A

implements a triangle spread spectrum frequency modu-

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

the LT8390A starts to spread its switching frequency 25%

above the internal oscillator frequency. Figure 9 and Figure

10 show the noise spectrum of the front page application

when spread spectrum enabled.

Figure 9. Average Conducted EMI

Figure 10. Peak Conducted EMI

FREQUENCY (MHz)

–10

0.1

8390a F09

SSFM ON WITH EMI FILTER

NOISE FLOOR

CISPER 25 CLASS 5 PEAK LIMITS

FREQUENCY (MHz)

–10

0.1

8390a F10

SSFM ON WITH EMI FILTER

NOISE FLOOR

CISPER 25 CLASS 5 PEAK LIMITS

Frequency Synchronization

The LT8390A switching frequency can be synchronized to

an external clock using the SYNC/SPRD pin. Driving the

SYNC/SPRD with a 50% duty cycle waveform is always a

good choice, otherwise maintain the duty cycle between

10%and90%.Duetotheuseofaphase-lockedloop(PLL)

inside, there is no restriction between the synchronization

frequency and the internal oscillator frequency. The rising

edge of the synchronization clock represents the begin-

ning of a switching cycle, turning on switches A and C,

or switches A and D.

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