6.2.6.6 Application Schematics For Use Case Examples

Figure 6-13. Use Case 1: Stereo Class D Amplifier With External Differential Power Stage

In Use Case 1, the external power stages are made of complementary low-cost MOSFETs. In addition to the R_DSON and drain breakdown voltage characteristics, the choice of these components is driven by a low gate threshold voltage, a low input capacitance, a low total gate charge and a fast turn-on time characteristics. Series resistance (10 Ω) added to the gates of the MOSFETs are optional and may be adjusted to optimize the gate drive. They help to limit the output current peaks driven by the I/Os into the MOSFET gates in some cases. The 10 kΩ resistors ensure an off condition when not driven and the capacitor/diode network (C1..C2/D1..D2) shifts the PMOS drive from the typical V_DDIO level (3.3V) to a higher supply voltage (e.g., a 5V power domain).

Figure 6-15. Use Case 2: Stereo Class D Amplifier With External Single-ended Power Stage

In the Use Case 2 application schematic, the drive network of the MOSFETs gates follows the principles described in Use Case 1.

A coupling capacitor (C_C) and an L-C low-pass filter (L_F, C_F) are added to the output of the power stage to remove both the DC and the high frequency components of the PWM signal. C_Cwith the resistive part of the speaker (R_SPK) forms a C-R high pass filter with a corner frequency of f_HP = 1 / (2 × PI × C_C × R_SPK).

L_F, C_F and R_SPK form a second-order low-pass filter of corner frequency f_C = 1 / (2 × PI × sqrt (L_F × C_F)) and of quality factor Q = R_SPK × sqrt (C_F / L_F). As a numerical example, consider the case f_HP = 200 Hz, f_C = 30 kHz, Q = 0.707 (maximum flat response) with R_SPK = 8 Ω. This leads to C_C = 100 µF, L_F = 60 µH, C_F = 470 nF.

Figure 6-17. Use Case 3A: Stereo Audio DAC With Active Differential-to-Single Low-Pass Filter

Figure 6-18. Use Case 3B: Stereo Audio DAC With Simple Passive Low-Pass Filter and Differential Outputs

Figure 6-19. Use Cases 3A and 3B: Waveforms

In Use Case 3A, the CLASSD is used as an audio DAC. In this case, the differential outputs of the CLASSD are used. The application schematic suggested in figure "Use Case 3A: Stereo Audio DAC With Active differential to Single Low-Pass Filter" above implements a third-order 10 kHz low-pass Butterworth filter and makes the differential to single-ended conversion. Note that in this schematic, the AVDD/2 point needs to be fed at low impedance (e.g., a buffered voltage). A simpler schematic (Use Case 3B) may also be possible, as shown in figure "Use Case 3B: Stereo Audio DAC With Simple Passive Low-Pass Filter and Differential Outputs" above, at the cost of higher out-of-band noise and differential outputs which may be acceptable in some applications.

Figure 6-20. Use Case 4A: Stereo Audio DAC With Active Low-Pass Filter and Single-ended Outputs

Figure 6-21. Use Case 4B: Stereo Audio DAC With Passive Low-Pass Filter and Single-ended Outputs

Figure 6-22. Use Cases 4A and 4B: Waveforms

In Use Case 4A, the CLASSD is used as an audio DAC with active low-pass filter. In this case, the single-ended outputs of the CLASSD are selected (PWMTYP = 0, trailing edge PWM) which leaves more I/Os to the application. A third-order 30 kHz low-pass Butterworth filter is shown in figure "Use Case 4A: Stereo Audio DAC With Active Low-Pass Filter and Single-ended Outputs". The AVDD/2 point can be fed at relatively high impedance as no current is drawn from this point (a simple resistive divider properly decoupled is acceptable). A reduced complexity schematic is presented in figure "Use Case 4B: Stereo Audio DAC With Passive Low-Pass Filter and Single-ended Outputs" above for less constrained applications.