Spread spectrum is a factory option included in part number TPSM63603S. The purpose of spread spectrum is to eliminate peak emissions at specific frequencies by spreading these peaks across a wider range of frequencies than a part with fixed frequency operation. In most systems with the TPSM63603S, 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 emissions at higher harmonics that fall in the FM band. These harmonics often couple to the environment through electric fields around the switch node and inductor. The TPSM63603S uses a ±2% spread of frequencies that can spread energy smoothly across the FM and TV bands but is small enough to limit sub-harmonic emissions below the device switching frequency. Peak emissions at the module switching frequency are only reduced slightly, by less than 1 dB, while peaks in the FM band are typically reduced by more than 6 dB.

The TPSM63603S 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 1.5 Hz, which is below the audio band.

The spread spectrum is only available while the clock of TPSM63603S device are free running at their natural frequency. Any of the following conditions overrides spread spectrum, turning it off:

• The clock is slowed during dropout.

• Spread spectrum is active even if there is no load.

• At a high input voltage/low output voltage ratio when the device operates at minimum on-time, the internal clock is slowed, disabling spread spectrum.

• The clock is synchronized with an external clock.