TPA3124D2_1 Datasheet, PDF (14/27 Pages) Texas Instruments – 15-W STEREO CLASS-D AUDIO POWER AMPLIFIER

TPA3124D2
SLOS578 – MAY 2008........................................................................................................................................................................................................ www.ti.com
INPUT CAPACITOR, C_I
In the typical application, input capacitor C_I is required to allow the amplifier to bias the input signal to the proper
dc level for optimum operation. In this case C_I and the input impedance of the amplifier (Z_I) form a high-pass filter
with the corner frequency determined in Equation 2.
–3 dB
1
f_c = _{2p Zi Ci}
f_c
(2)
The value of C_I is important, as it directly affects the bass (low-frequency) performance of the circuit. Consider
the example where Z_I is 20 kΩ and the specification calls for a flat bass response down to 20 Hz. Equation 2 is
reconfigured as Equation 3.
1
C_i = _{2p Zi fc}
(3)
In this example, C_I is 0.4 µF; so, one would likely choose a value of 0.47 µF as this value is commonly used. If
the gain is known and is constant, use Z_I from Table 2 to calculate C_I. A further consideration for this capacitor is
the leakage path from the input source through the input network, C_I, and the feedback network to the load. This
leakage current creates a dc offset voltage at the input to the amplifier that reduces useful headroom, especially
in high-gain applications. For this reason, a low-leakage tantalum or ceramic capacitor is the best choice. When
polarized capacitors are used, the positive side of the capacitor should face the amplifier input in most
applications as the dc level there is held at 2 V, which is likely higher than the source dc level. Note that it is
important to confirm the capacitor polarity in the application. Additionally, lead-free solder can create dc offset
voltages, and it is important to ensure that boards are cleaned properly.
Single-Ended Output Capacitor, C_O
In single-ended (SE) applications, the dc blocking capacitor forms a high-pass filter with the speaker impedance.
The frequency response rolls off with decreasing frequency at a rate of 20 dB/decade. The cutoff frequency is
determined by
f_c = πC_OZ_L
Table 3 shows some common component values and the associated cutoff frequencies:
Speaker Impedance (Ω)
4
6
8
Table 3. Common Filter Responses
f_c = 60 Hz (–3 dB)
680
470
330
C_SE - DC Blocking Capacitor (µF)
f_c = 40 Hz (–3 dB)
1000
680
470
f_c = 20 Hz (–3 dB)
2200
1500
1000
Output Filter and Frequency Response
For the best frequency response, a flat-passband output filter (second-order Butterworth) may be used. The
output filter components consist of the series inductor and capacitor to ground at the LOUT and ROUT pins.
There are several possible configurations, depending on the speaker impedance and whether the output
configuration is single-ended (SE) or bridge-tied load (BTL). Table 4 lists the recommended values for the filter
components. It is important to use a high-quality capacitor in this application. A rating of at least X7R is required.
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