4.1 Use Case 1 Output Filter Computation

In Use Case 1: Stereo Class D Amplifier with External Differential Power Stage, the positive terminal drives the positive alternates of the output signal (with the negative terminal shorted to ground), and the negative terminal drives the negative alternates (with the positive terminal shorted to ground). As a result, the output signal in this use case is predominantly made up of sharp spikes on either the P or the N side, rather than a modulated 50% PWM as in Single-Ended mode.

There is no major difference from the Single-Ended mode case regarding the component value computation (see Use Case 2 Output Filter Computation). However, the inductance is split into two devices, each having half of the overall value to preserve the circuit’s symmetry. Thus, the load should not be subjected to significant DC voltage, so the high-value serial capacitor can be omitted.

This can be simulated with Mindi™ using the 8Ohms 20Hz-20kHz maximally flat differential output filter.wxsch file, available from the MPLAB® Mindi Analog Simulator Software Library (see the Appendix).

Figure 4-3. Differential Mode Case Passive Smoothing Filter
Warning: With the single supply and chosen modulation scheme, the high-pass filtering capacitor(s) must be connected in parallel with the load and not divided into units connected between each side of the load and ground. Any common-mode filtering through capacitors connected to the ground would counteract the normal circuit operation and expose the capacitors to a high-frequency ripple current of potentially destructive magnitude.
Figure 4-4. Prohibited Smoothing Filter Configuration in Differential Mode

No smoothing filter operation in Differential mode:

For the lowest cost implementations and when driving inductive loads, such as a standard electrodynamic loudspeaker, acceptable sound quality may still be obtained without any smoothing filter. However, to prevent overcurrent conditions, observe the following:
  • Do not omit the specific smoothing filter when driving a capacitive load.
  • Do not add a capacitor in parallel with the load without implementing additional serial inductors.
Important: To preserve the sharpness of the short spikes generated by the Class D amplifier that convey most of the signal information in differential mode, do not add ferrite beads in series with the load. If EMC requirements are not met with the raw circuit, add a smoothing filter.