Spread spectrum is configurable using the SPSP 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 LM6x4xx-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 LM6x4xx-Q1 uses a ±4% (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 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 LM6x4xx-Q1
also allows you to further reduce the output voltage ripple caused by the spread spectrum modulating pattern. With the SPSP pin grounded, the spread spectrum is disabled. With the SPSP pin tied to VCC, the spread spectrum is on. With the SPSP pin tied through a resistor to ground, the spread spectrum is on. Also, a modulating tone correction is applied to the switcher to reduce the output voltage ripple caused by the frequency modulation. The resistor is usually around 20 kΩ, and can be more precisely calculated using Equation 4
Equation 4. Figure 8-11 Output Ripple Without
Ripple Cancellation Showing VSW (top), FSW (middle),
VOUT (bottom) Figure 8-12 Output Ripple with Ripple
Cancellation Showing VSW (top), FSW (middle),
The spread spectrum is only available while the clock of the LM6x4xx-Q1 are free running at their 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. This is normally not seen above 750-mA load. 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 Timing Characteristics.
- The clock is synchronized with an external clock.