Texas Instruments TPA005D02 - Manual (Page 21)

The TPA005D02 Audio Power Amplifier Evaluation Module
3-7 Details
3.2.3 EVM Design Considerations
Circuit design and layout plays a large role in the creation or reduction of
distortion in class D amplifiers, and the high frequency switching
characteristics of class D audio power amplifier output stages offer some
interesting design challenges over conventional class AB amplifiers.
The main goal of the design of this EVM is to offer the best performance with
the smallest components, without sacrificing performance. For this reason
surface mount technology (SMT) parts are used whenever possible. The
major design considerations are discussed below and refer to Figures 3–2 and
3–3 unless otherwise noted. The actual parts used in the EVM are listed in
Table 3–3.
The audio signal path is the most critical, so the discussion begins there.
3.2.3.1 Input Filter
The first consideration is the desired frequency bandwidth (BW). High-fidelity
audio requires a flat 20Hz to 20kHz bandwidth. The low frequency –3-dB point
is set using an ac coupling capacitor at the amplifier inputs (IC pins 4, 5, 44,
and 45) which creates a high-pass filter (HPF). The –3-dB point for a first-order
HPF is found using the equation
f
LO
+ 1
(2
p
RC)
where R = 10 k
Ω
is the input resistance of the amplifier and C = 1
µ
F for the
value of capacitors C2, C3, C6, and C7. These values give a –3-dB point of
15.9 Hz—close to the desired –3-dB point of 20 Hz. Ceramic capacitors are
preferred over electrolytic for their small size, low equivalent series resistance
(ESR), low noise, and long life. The smallest ceramic SMT package currently
available is 0603, yet availability necessitates using an 0805 package.
Other considerations are stability over temperature, voltage rating, and cost.
A tradeoff exists between the size, temperature characteristics, rated voltage,
and capacitance value. For a given package size, for example, an increase in
the voltage rating means a decrease in capacitance. The same applies to
improved temperature stability.
Temperature stability has little impact on the input capacitors since they
primarily couple the ac input signal and are not expected to dissipate large
amounts of heat. The input voltage is low for the class D EVM (less than
0.5 Vrms), so the rated voltage can also be low. An 0805 SMT package with
a rated voltage of 5 V and Y5V temperature characteristics would be ideal for
this application. Availability and cost constraints, however, dictated the use of
the power supply bypass capacitors that were finally selected, which meet all
requirements and are rated at 16V. If better matching of the left and right
channels is needed or a harsh environment will be encountered, then a
capacitor with X7R temperature characteristics should be considered.