LT8350RVPBF datasheet(16/24 Pages) AD

16 / 24 page

LT8350

Rev. 0

For more information www.analog.com

The front page shows a typical LT8350 application circuit.

This Applications Information section serves as a guide-

line of selecting external components for typical applica-

tions. The examples and equations in this section assume

continuous conduction mode unless otherwise specified.

Switching Frequency Selection

The LT8350 uses a constant frequency control scheme

between 200kHz and 2MHz. Selection of the switching

frequency is a trade-off between efficiency and compo-

nent 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. In addition, the specific

application also plays an important role in switching fre-

quency selection. In a noise-sensitive system, the switch-

ing frequency is usually selected to keep the switching

noise out of a sensitive frequency band.

Switching Frequency Setting

The switching frequency of the LT8350 can be set by

the internal oscillator. With the SYNC/MODE 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 (kΩ)

200

249

400

124

600

78.7

800

56.2

1000

43.2

1200

33.2

1400

26.1

1600

21.5

1800

17.4

2000

14.3

APPLICATIONS INFORMATION

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 LT8350

implements a triangle spread spectrum frequency modu-

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

the LT8350 starts to spread its switching frequency

23% (typical) above the internal oscillator frequency.

Figure 9 and Figure 10 show the noise spectrum of the

typical application (Figure 14) when the part operates at

12VOUT, 2.5A and 350kHz with ferrite bead EMI filter and

spread spectrum enabled.

Figure 9. Conducted Average EMI (CISPR25)

Figure 10. Conducted Peak EMI (CISPR25)

CLASS 5 AVERAGE LIMIT

NOISE FLOOR

SSFM ON WITH FILTER

FREQUENCY (MHz)

0.1

100

–20

–10

8350 F09

CLASS 5 PEAK LIMIT

NOISE FLOOR

SSFM ON WITH FILTER

FREQUENCY (MHz)

0.1

100

–20

–10

8350 F10

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