The purpose of spread spectrum is to reduce peak emissions at specific frequencies by spreading these peaks across a wider range of frequencies than a part with fixed-frequency operation. The TPSM365R1x implements a modulation pattern designed to reduce low frequency-conducted emissions from the first few harmonics of the switching frequency can be easily filtered. The pattern can also help reduce the higher harmonics that are more difficult to filter, which can fall in the FM band. These harmonics often couple to the environment through electric fields around the switch node and inductor. The TPSM365R1x uses a spread of frequencies, which can spread energy smoothly across the FM and TV bands, but is small enough to limit subharmonic emissions below the switching frequency of the part. Pseudo random frequency hopping allows the spread spectrum to be very effective at spreading the energy.

The TPSM365R1x 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 the device is free running at the natural frequency. Any of the following conditions overrides spread spectrum, turning it off:

• The clock is slowed due to operation at low-input voltage – this is operation in dropout.
• The clock is slowed under light load in auto mode. Note that if you are operating in FPWM mode, spread spectrum can be active, even if there is no load.
• The clock is slowed due to high input to output voltage ratio. This mode of operation is expected if on time reaches minimum on time. See the Section 6.5.
• The clock is synchronized with an external clock.