Spread spectrum is configurable using the CONFIG pin. Spread spectrum eliminates peak emissions at specific frequencies by spreading these peaks across a wider range of frequencies than a part with fixed-frequency operation. The LM644A2-Q1 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 LM644A2-Q1 uses a ±10% (typical) 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 psuedorandom jumps at the switching frequency

The spread spectrum is only available while the clocks of the The LM644A2-Q1 are free running at the natural frequency. Any of the following conditions overrides the clock and can interfere with spread spectrum:

• 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 the device is operating in FPWM mode, spread spectrum is 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.