A key parameter for mobile audio applications is the efficiency of the used amplifier. This is also a point where the biggest impact of the different load can be seen and new considerations have to be taken. The efficiency of audio amplifiers for electrodynamic speaker is often calculated based on the average power at the output [
7].
$$ P_{\text{Average}}=\frac{1}{T} \int_{0}^{T} u_{\text{(t)}}\cdot i _{\text{(t)}} \, \mathrm{d}t = V_{\text{RMS}} \cdot I_{\text{RMS}} \cdot \cos(\varPhi), $$
(3)
where
\(P_{\text{Average}}\) is the average active power,
\(T\) the period of the signal,
\(u_{\text{(t)}}\) the current value of the voltage,
\(i_{\text{(t)}}\) the current value of the current,
\(V_{\text{RMS}}\) the root mean square (RMS) value of the voltage and
\(I_{\text{RMS}}\) the RMS value of the current. As shown above, the power factor for MEMS speakers is approaching zero which would result in a very low efficiency. For this reason the apparent power will be used to calculate the efficiency of amplifiers with capacitive loads [
7].
$$ \eta= \frac{P_{\text{out}}}{P_{\text{out}}+P_{\text{diss}}}, $$
(4)
where
\(P_{\text{out}}\) is the apparent output power and
\(P_{\text{diss}}\) the amplifier dissipation. The losses of the amplifier can be separated into following parts [
7]:
$$ P_{\text{diss}}=P_{\text{Q}}+P_{\text{CL}}+P_{\text{SW}}+P_{\text{BD}}+P_{\text{Filt}}, $$
(5)
where
\(P_{\text{Q}}\) is the quiescent power loss of the amplifier,
\(P_{\text{CL}}\) the conduction losses of the output drivers,
\(P_{\text{SW}}\) the switching losses,
\(P_{\text{BD}}\) the bulk diode of the driving transistors and
\(P_{\text{Filt}}\) the losses inside the output filter and load. Considering the four parts of the power losses [
7] calculated in (
6) to (
10)
$$\begin{aligned} P_{\text{CL}} =&I_{\text{out,rms}}^{2} \cdot R_{\text{ds,on}} \end{aligned}$$
(6)
$$\begin{aligned} P_{\text{SW}} =&\sum_{i} F_{\text{SW}} \cdot V_{\text{CP}}^{2} \cdot C_{\text{P,i}} \end{aligned}$$
(7)
$$\begin{aligned} P_{\text{BD}} =& V_{\text{SD}} \cdot F_{\text{SW}} \cdot ( I_{\text{pk}} \cdot t_{\text{dt}} + I_{\text{rrm}} \cdot t_{\text{rr}}) \end{aligned}$$
(8)
$$\begin{aligned} P_{\text{FILT}} =&I_{\text{out,rms}}^{2} \cdot \vert Z_{\text{L}} \vert \cdot\cos ( \varPhi ) \end{aligned}$$
(9)
$$\begin{aligned} &{}+ C_{\text{load}} \cdot V_{\text{out,rms}}^{2} \cdot2 \cdot\pi \cdot f \cdot df , \end{aligned}$$
(10)
where
\(I_{\text{out,rms}}\) is the output RMS current,
\(R_{ \text{ds,on}}\) is the drain source on resistance of the output transistor,
\(F_{\text{SW}}\) the switching frequency,
\(V_{\text{CP}} ^{2}\) the voltage at each parasitic capacitance at the output,
\(C_{\text{P,i}}\) the parasitic capacitances at the output,
\(V_{ \text{SD}}\) the body diode voltage drop,
\(I_{\text{pk}}\) the peak output current,
\(t_{\text{dt}}\) the dead time,
\(I_{\text{rrm}}\) the body diode reverse recovery current,
\(t_{\text{rr}}\) the reverse recovery time,
\(|Z_{\text{L}}| \cdot\cos ( \varPhi ) \) the resistive part of the output filter,
\(C_{\text{load}}\) the load capacitance and
\(f\) the applied frequency at the output, two major drawbacks of a Class D amplifier together with capacitive loads for ultra low power applications can be seen. First the conduction losses and the body diode losses are related to the output current (either rms or peak). Especially at high frequencies this current will be considerable high which causes high losses inside the output switches.
Second, the size of the inductor can be a critical parameter, especially for a small load capacitance as it is the case for MEMS speakers. Considering a load of 100 nF and a cutoff frequency of 29 kHz would results in an inductor value of approximately 300 μH for the output filter.