Table 1. Design Requirements

10.2.1.2.1 Component Selection

Figure 40 shows the TPA2005D1 typical schematic with differential inputs and Figure 42 shows the TPA2005D1 with differential inputs and input capacitors, and Figure 43 shows the TPA2005D1 with single-ended inputs. Differential inputs should be used whenever possible because the single-ended inputs are much more susceptible to noise.

10.2.1.2.2 Input Resistors (R_I)

The input resistors (R_I) set the gain of the amplifier according to equation Equation 20.

Equation 20.

Resistor matching is important in fully differential amplifiers. The balance of the output on the reference voltage depends on matched ratios of the resistors. CMRR, PSRR, and cancellation of the second harmonic distortion diminish if resistor mismatch occurs. Therefore, it is recommended to use 1% tolerance resistors or better to keep the performance optimized. Matching is more important than overall tolerance. Resistor arrays with 1% matching can be used with a tolerance greater than 1%.

Place the input resistors close to the TPA2005D1 to limit noise injection on the high-impedance nodes.

For optimal performance the gain should be set to 2 V/V or lower. Lower gain allows the TPA2005D1 to operate at its best, and keeps a high voltage at the input making the inputs less susceptible to noise.

10.2.1.2.3 Decoupling Capacitor (C_S)

The TPA2005D1 is a high-performance class-D audio amplifier that requires adequate power supply decoupling to ensure the efficiency is high and total harmonic distortion (THD) is low. For higher frequency transients, spikes, or digital hash on the line, a good low equivalent-series-resistance (ESR) ceramic capacitor, typically 1 μF, placed as close as possible to the device V_DD lead works best. Placing this decoupling capacitor close to the TPA2005D1 is important for the efficiency of the class-D amplifier, because any resistance or inductance in the trace between the device and the capacitor can cause a loss in efficiency. For filtering lower-frequency noise signals, a 10 μF or greater capacitor placed near the audio power amplifier would also help, but it is not required in most applications because of the high PSRR of this device.

10.2.2.2.1 Input Capacitors (C_I)

The TPA2005D1 does not require input coupling capacitors if the design uses a differential source that is biased from 0.5 V to V_DD - 0.8 V (shown in Figure 40). If the input signal is not biased within the recommended common-mode input range, if needing to use the input as a high pass filter (shown in Figure 42), or if using a single-ended source (shown in Figure 43), input coupling capacitors are required.

The input capacitors and input resistors form a high-pass filter with the corner frequency, f_c, determined in equation Equation 21.

Equation 21.

The value of the input capacitor is important to consider as it directly affects the bass (low frequency) performance of the circuit. Speakers in wireless phones cannot usually respond well to low frequencies, so the corner frequency can be set to block low frequencies in this application.

Equation Equation 22 is reconfigured to solve for the input coupling capacitance.

Equation 22.

If the corner frequency is within the audio band, the capacitors should have a tolerance of ±10% or better, because any mismatch in capacitance causes an impedance mismatch at the corner frequency and below, and causes pop. Any capacitor in the audio path should have a rating of X7R or better.

For a flat low-frequency response, use large input coupling capacitors (1 μF). However, in a GSM phone the ground signal is fluctuating at 217 Hz, but the signal from the codec does not have the same 217 Hz fluctuation. The difference between the two signals is amplified, sent to the speaker, and heard as a 217 Hz hum.