The purpose of spread spectrum is to eliminate peak emissions at specific frequencies by spreading these emissions across a wider range of frequencies. In most systems containing the LM644x0-Q1, low-frequency conducted emissions from the first few harmonics of the switching frequency can be easily filtered. A more difficult design criterion is reduction of the emissions at higher harmonics that fall in the FM frequency band. These harmonics often couple to the environment through electric fields around the switch node and inductor. The LM644x0-Q1 uses a ±2% spread of frequencies, which can spread energy smoothly across the FM and TV bands but is small enough to limit subharmonic emissions below the converter switching frequency. Peak emissions at the switching frequency of the converter are only reduced slightly, by less than 1dB, while peaks in the FM band are typically reduced by more than 6dB.

Use the MODE/SYNC pin to enable or disable spread spectrum in the LM644x0-Q1. The LM644x0-Q1 uses a cycle-to-cycle frequency hopping method based on a linear feedback shift register (LFSR). This intelligent pseudo-random generator limits cycle-to-cycle frequency changes to limit output ripple. The pseudo-random pattern repeats at less than 1.5Hz, which is below the audio band.

Spread spectrum is only available while the clock of the LM644x0-Q1 is free running at the natural frequency. Any of the following conditions overrides spread spectrum, turning it off:

• The clock is slowed when operating in dropout.
• The clock is slowed at light load in AUTO mode. In FPWM mode, spread spectrum is active even if there is no load.
• At a high-input-voltage to low-output-voltage conversion ratio when the device operates at the minimum on time, the internal clock is slowed, disabling spread spectrum. Refer to the Timing Characteristics for more detail.
• The clock is synchronized to an external clock signal.