Active EMI filter provides a higher level of EMI attenuation and a smaller solution size than a standard passive π-filter. Passive π-filters use large inductors and capacitors to attenuate the ripple and noise on the input DC bus. Passive filters are typically most effective at reducing the switching frequency harmonics to comply with EMI in the low-frequency range, less than 30 MHz.

Active EMI filter has a high gain, wide bandwidth amplifier, and a low output impedance that can source and sink current. It senses (at the SEN pin) any disturbance on the DC input bus and injects (at the INJ pin) a cancellation signal out of phase with the noise source to reduce the conducted emissions.

To maintain low I_Q at light loads, the LM25149 automatically enables active EMI filter when the load current is greater than 40% of the current limit value, and disables the active EMI filter when the load current is less then 30% of the current limit value. The active EMI filter is disabled by pulling the CNFG pin below 0.8 V after the LM25149 has been configured.

Figure 8-2 Active EMI Filter