Class-D amplifier outputs are switching outputs, having carrier frequency along with the audio signal content. These switching frequencies lead to concerns with EMI radiation due to long traces running to speaker loads. To meet the EMI radiation specs like FCC/CE, Class-D amplifier outputs typically have EMI filters. In this application note we briefly describe the different techniques used to select and realize EMI filters . The Post filter Feedback feature in TAS27XX family of devices is also explained and filter selection care abouts regarding the same are also listed. The audio amplifier performance is also measured in the presence of the bead. Further an excel sheet based calculator is provided to the user to help assess the feasibility of EMI filter to be used for post filter feedback with some examples.
All trademarks are the property of their respective owners.
EMI filters at the Class-D output can be realized using the following techniques.
A large capacitance >1nF can be connected to the amplifier output directly. This connection is not recommended as the output impedance of the Class-D is of a very low order approximately 0.5Ω which does not offer significant filtering (Filter cutoff> 300MHz). Further, in TAS27xx generation of devices increasing the capacitance at the Class-D output has negligible impact edge rate. Also, any capacitor directly connected to the Class-D output can significant degrade device efficiency and cause very high switching currents which can cause the device to shut down or reliability issues. For these reasons in TAS27XX family of devices the recommendation is to not connect maximum 200pF capacitance directly at the Class-D output.