Data Sheet

LT8350S

analog.com

Rev. A

26 of 36

Spread Spectrum Frequency Modulation

Switching regulators can be particularly troublesome for applications where electromagnetic interference (EMI) is a

concern. The LT8350S implements a triangle spread spectrum frequency modulation scheme to improve EMI

performance. With the SYNC/MODE pin connected to INTVCC, the LT8350S spreads its switching frequency 23%

(typical) above the internal oscillator frequency. Figure 36 and Figure 37 show the noise spectrum of the typical

application (See Figure 41) when the part operates at 12VOUT, 2.5A, and 350kHz with ferrite bead EMI filter and spread

spectrum enabled.

Figure 36. Conducted Average EMI (CISPR25)

Figure 37. Conducted Peak EMI (CISPR25)

Frequency Synchronization and Operation Mode Selection

The LT8350S switching frequency can be synchronized to an external clock using the SYNC/MODE pin. It is

recommended to drive the SYNC/MODE with a 10% to 90% duty cycle waveform. Due to the use of a phase-locked

loop (PLL) inside, there is no restriction between the synchronization 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.

SYNC/MODE pin also has four selectable modes for optimization of performance:

1) Connect to INTVCC: For spread spectrum frequency modulation and force continuous mode at light load.

2) Connect to VREF: For spread spectrum frequency modulation and pulse-skipping mode at light load.

3) Float: For internal oscillator frequency and force continuous mode at light load.

4) Connect to GND: For internal oscillator frequency and pulse-skipping mode at light load.

80

70

60

50

40

30

20

10

0

150k

1M

10M

FREQUENCY (Hz)

108M

CLASS 5 AVERAGE LIMIT

NOISE FLOOR

SSFM ON WITH FILTER