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LT8392

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

The front page shows a typical LT8392 application circuit.

This Applications Information section serves as a guideline

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 LT8392 uses a constant frequency control scheme

between 150kHz and 650kHz. 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 import-

ant role in switching frequency selection. In a noise-sensi-

tive system, the switching frequency is usually selected to

keep the switching noise out of a sensitive frequency band.

Switching Frequency Setting

The switching frequency of the LT8392 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 LT8392

implements a triangle spread spectrum frequency mod-

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

the LT8392 starts to spread its switching frequency

±15% around 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

8392 F09

SSFM ON WITH EMI FILTER

NOISE FLOOR

CISPER 25 CLASS 5 PEAK LIMITS

FREQUENCY (MHz)

–10

0.1

8392 F10

SSFM ON WITH EMI FILTER

NOISE FLOOR

CISPER 25 CLASS 5 PEAK LIMITS

Frequency Synchronization

The LT8392 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% and 90%. Due to the use of a phase-locked loop

(PLL) inside, there is no restriction between the synchro-

nization frequency and the internal oscillator frequency.

The rising edge of the synchronization clock represents

the beginning of a switching cycle, turning on switches A

and C, or switches A and D.

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