C1
1 µF
R1
499
+
R2
499
C2
1200 pF
R3
250
C4
4 µF
In_P 1, 2, 3, 4
In_M
Audio Source
co
1
¦
2
R
C
u S u
u
TAS54x4C Hardware Design Guidelines
1
TAS54x4C Hardware Design Guidelines
When designing an audio amplifier with the TAS5414C and TAS5424C class-D integrated devices, the
details of the full system design are not included in the data sheet. This design guide supplements the
data sheet with detailed hardware-related information on designing an audio amplifier product.
1.1
Circuit Design
1.1.1
Input
The input stage of the TAS5414C devices is single ended. The TAS5424C input stage is either differential
or balanced. The audio input pins have a bias voltage of 3.25 VDC with a tolerance of 2.8 VDC to 3.7
VDC. Therefore direct coupling to the input pins is not possible. An input coupling capacitor must be used.
The value of the capacitor is based on two criteria. The value of the capacitor must be large enough to
pass low frequencies, but not so large that the charge time on this capacitor is too long which can cause
pop and click noise at the output of the amplifier.
The input impedance of each input pin is 80 k
Ω
±20 k
Ω
. Use
to calculate the low-frequency
cutoff frequency (–3 dB).
where
•
ƒ
CO
= cutoff frequency in Hertz
•
R = 80 k
Ω
•
C = coupling capacitor
(1)
The recommended capacitor values for the TAS54x4C devices are 0.47 µF or 1 µF. For a 0.47 µF
capacitor with 20% tolerance the highest cutoff frequency can be calculated using 60 k
Ω
and 0.38 µF. The
worst-case cutoff frequency for this example is 7.1 Hz which allows for flat gain in the audio band.
The common mode rejection ratio, CMRR, is also affected by the input circuit. The balanced inputs of the
TAS5424C must have equal-value capacitors so that the CMRR is maximized at lower frequencies.
The single-ended input of the TAS5414C must also have the proper input capacitors for good low-
frequency common-mode performance. Because all four IN_P channels in the TAS5414C device share
the IN_M pin, the capacitor from IN_M to ground should be close to the sum of the capacitors on the four
IN_P pins. Additionally, a resistor in series with the IN_M capacitor should be placed between the IN_M
pin and ground that is equal to the parallel equivalent of the input series resistance on all IN_P channels
combined, including the output resistance of the DAC. For example, if 1-k
Ω
of series input resistance is on
each channel of the TAS5414C, a 250-
Ω
resistor should be placed between the IN_M pin and GND in
series with the IN_M capacitor. Following these guidelines will optimize noise performance.
shows an example input circuit that follows this practice.
Figure 1. TAS5414C Input Circuit
3
SLOA196 – June 2014
TAS54x4C Design Guide
Copyright © 2014, Texas Instruments Incorporated
