Table 2. Design Parameters

10.2.1.2.1 Surface Mount Inductor

Working inductance decreases as inductor current increases. if the drop in working inductance is severe enough, it may cause the boost converter to become unstable, or cause the TPA2080D1 to reach its current limit at a lower output power than expected. Inductor vendors specify currents at while inductor values decrease by a specific percentage. This can vary from 10% to 35%. Inductance is also affected by DC current and temperature.

10.2.1.2.2 Inductor Selection

Inductor current rating is determined by the requirements of the load. The inductance is determined by two factors: the minimum value required for stability and the maximum ripple current permitted in the application. Use Equation 1 to determine the required current rating. Equation 1 shows the approximate relationship between the average inductor current, I_L, to the load current, load voltage, and input voltage (I_PVDD, PVDD, and VBAT, respectively). Insert I_PVDD, PVDD, and VBAT into Equation 1 and solve for I_L. The inductor must maintain at least 90% of its initial inductance value at this current.

Equation 1.

Ripple current, ΔI_L, is peak-to-peak variation in inductor current. Smaller ripple current reduces core losses in the inductor and reduces the potential for EMI. Use Equation 2 to determine the value of the inductor, L. Equation 2 shows the relationship between inductance L, VBAT, PVDD, the switching frequency, f_BOOST, and ΔI_L. Insert the maximum acceptable ripple current into Equation 2 and solve for L.

Equation 2.

ΔI_L is inversely proportional to L. Minimize ΔI_L as much as is necessary for a specific application. Increase the inductance to reduce the ripple current. Do not use greater than 4.7 μH, as this prevents the boost converter from responding to fast output current changes properly. If using above 3.3 µH, then use at least 10-µF capacitance on PVDD to ensure boost converter stability.

The typical inductor value range for the TPA2080D1 is 2.2 μH to 3.3 µH. Select an inductor with less than 0.5-Ω DC resistance, DCR. Higher DCR reduces total efficiency due to an increase in voltage drop across the inductor.

10.2.1.2.3 Surface Mount Capacitors

Temperature and applied DC voltage influence the actual capacitance of high-K materials. Table 4 shows the relationship between the different types of high-K materials and their associated tolerances, temperature coefficients, and temperature ranges. Notice that a capacitor made with X5R material can lose up to 15% of its capacitance within its working temperature range.

In an application, the working capacitance of components made with high-K materials is generally much lower than nominal capacitance. A worst-case result with a typical X5R material might be –10% tolerance, –15% temperature effect, and –45% DC voltage effect at 50% of the rated voltage. This particular case would result in a working capacitance of 42% (0.9 × 0.85 × 0.55) of the nominal value.

Select high-K ceramic capacitors according to the following rules:

1. Use capacitors made of materials with temperature coefficients of X5R, X7R, or better.
2. Use capacitors with DC voltage ratings of at least twice the application voltage. Use minimum 10-V capacitors for the TPA2080D1.
3. Choose a capacitance value at least twice the nominal value calculated for the application. Multiply the nominal value by a factor of 2 for safety. If a 10-μF capacitor is required, use 20 µF.

The preceding rules and recommendations apply to capacitors used in connection with the TPA2080D1. The TPA2080D1 cannot meet its performance specifications if the rules and recommendations are not followed.

10.2.1.2.4 Boost Converter Capacitor Selection

The value of the boost capacitor is determined by the minimum value of working capacitance required for stability and the maximum voltage ripple allowed on PVDD in the application. Working capacitance refers to the available capacitance after derating the capacitor value for DC bias, temperature, and aging. Do not use any component with a working capacitance less than 6.8 µF. This corresponds to a 10-μF, 16-V capacitor or a 10-μF, 10-V capacitor.

Do not use above 22-μF capacitance as it will reduce the boost converter response time to large output current transients.

Equation 3 shows the relationship between the boost capacitance, C, to load current, load voltage, ripple voltage, input voltage, and switching frequency (I_PVDD, PVDD, ΔV, VBAT, and f_BOOST respectively).

Insert the maximum allowed ripple voltage into Equation 3 and solve for C. The 1.5 multiplier accounts for capacitance loss due to applied DC voltage and temperature for X5R and X7R ceramic capacitors.

Equation 3.

10.2.1.2.5 Decoupling Capacitors

The TPA2080D1 is a high-performance Class-D audio amplifier that requires adequate power supply decoupling. Adequate power supply decoupling to ensures that the efficiency is high and total harmonic distortion (THD) is low.

Place a low equivalent-series-resistance (ESR) ceramic capacitor, typically 0.1 µF, within 2 mm of the VBAT ball. Use X5R and X7R ceramic capacitors. This choice of capacitor and placement helps with higher frequency transients, spikes, or digital hash on the line. Additionally, placing this decoupling capacitor close to the TPA2080D1 is important, as any parasitic resistance or inductance between the device and the capacitor causes efficiency loss. In addition to the 0.1-μF ceramic capacitor, place a 2.2-µF to 10-µF capacitor on the VBAT supply trace. This larger capacitor acts as a charge reservoir, providing energy faster than the board supply, thus helping to prevent any droop in the supply voltage.

10.2.1.2.6 Input Capacitors

Input audio DC decoupling capacitors are recommended. The input capacitors and TPA2080D1 input impedance form a high-pass filter with the corner frequency, f_C, determined in Equation 4.

Any mismatch in capacitance between the two inputs will cause a mismatch in the corner frequencies. Severe mismatch may also cause turnon pop noise. Choose capacitors with a tolerance of ±10% or better. Use X5R and X7R ceramic capacitors.

Equation 4.

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. Not using input capacitors can increase output offset.

10.2.1.2.7 Boost Converter Component Section

The critical external components are summarized in Table 5.

Table 6. Table of Graphs