In the LMQ664x0 family of devices, spread spectrum is a factory option. To find which parts have spread spectrum enabled, see Section 5.

Spread spectrum reduces peak emissions at specific frequencies by spreading these peaks across a wider range of frequencies than a part with fixed-frequency operation. The LMQ664x0 implements a modulation pattern designed to reduce low frequency-conducted emissions from the first few harmonics of the switching frequency. 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 LMQ664x0 uses a spread of frequencies, which can spread energy smoothly across the FM and TV bands. The device implements dual random spread spectrum (DRSS). DRSS is a combination of a triangular frequency spreading pattern and pseudorandom frequency hopping. The combination allows the spread spectrum to be very effective at spreading the energy at the following:

• Fundamental switching harmonic with slow triangular pattern
• High frequency harmonics with additional pseudo-random jumps at the switching frequency

The advantage of DRSS is its equivalent harmonic attenuation in the upper frequencies with a smaller fundamental frequency deviation. This reduces the amount of input current and output voltage ripple that is introduced at the modulating frequency. Additionally, the LMQ664x0 also allows the user to further reduce the output voltage ripple caused by the spread spectrum modulating pattern.

The spread spectrum is only available while the clock of the device is free running at its 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 Electrical Characteristics.
• The clock is synchronized with an external clock.