Testing
Simulation and Design Verification
All the blocks are simulated and verified individually at block level and at the top level. Simulations include AC, DC and transient analysis depending upon the functionality of individual blocks. Initially all the blocks are verified at schematic level and then RC parasitic extracted simulations are run.
Simulated Top Level typical waveforms
Post Silicon Testing
Fig: PCB Setup
Fig: Setup Image
Fig: Ramp generator waveform
Ramp generator frequency was lower by 25%, because of the additional parasitic capacitors coming from the PCB trace capacitances, which was underestimated during DV. The Iref of the Ramp generator was then adjusted to get the ramp frequency back to 500kHz.
Fig: Class D Amplifier Input output sinusoidal waveforms
Fig: Oscilloscope waveforms of Input and output Audio
Signals - Input Audio, Output PWM, Differential Audio outs, Speaker Input respectively
Efficiency Measurement Setup
Fig: Efficiency Measurement Setup
This a single tone in single tone out efficiency measurement setup based on the total input power and total output power delivered to the load. Two multimeters measure the DC currents of the Vdd-Analog and Vdd-Switching Paths respectively. The generated single tone sinusoidal signal is fed across a Load of 8.2 Ohms. We took the path trace resistances into account for the effective Rload.
* Rload also includes PCB trace losses
Table: Summary of Efficiency calculation
Efficiency across different input frequency and for output Vpp levels is calculated as tabulated above
Fig: Summary of Efficiency calculation
The above figure shows how the frequency Varies vs different Output Vpp levels across different frequencies. Efficiency depends heavily on output Vpp level, but is almost independent of input frequency. This is expected because the losses consist of 3 major components → Output Conduction Loss + Switching Loss + Quiescent Current losses (due the fixed Iq taken by the analog blocks). Of these Switching losses and Iq losses are independent of the output Vpp (Iq losses are always constant and switching loss depend on the ramp frequency). So as the total output power increases, the relative effect of these losses keep on decreasing. Losses are independent of the input tone frequency to the first order.