Datasheet

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APPLICATION INFORMATION
DEMODULATION FILTER DESIGN
Output A
C1
TAS5121I
L
Output B
L
C2
R
(Load)
S0016−01
TAS5121I
SLES122 – SEPTEMBER 2004
The TAS5121I amplifier outputs are driven by high-current DMOS transistors in an H-bridge configuration. These
transistors are either off or fully on.
The result is a square-wave output signal with a duty cycle that is proportional to the amplitude of the audio
signal. It is recommended that a second-order LC filter be used to recover the audio signal.
Figure 9. Demodulation Filter
The main purpose of the demodulation filter is to attenuate the high-frequency components of the output signals
that are out of the audio band.
Design of the demodulation filter significantly affects the audio performance of the power amplifier. Therefore, to
ensure proper operation of the OC protection circuit and meet the device THD+N specification, the selection of
the inductors used in the output filter should be carefully considered. The rule is that the inductance should
remain stable within the range of peak current seen at maximum output power and deliver approximately 5 µ H of
inductance at 15 A.
If this rule is observed, the TAS5121I should not have distortion issues due to the output inductors. This prevents
device damage due to overcurrent conditions because of inductor saturation in the output filter.
Another parameter to be considered is the idle current loss in the inductor. This can be measured or specified as
inductor dissipation (D). The target specification for dissipation is less than 0.05. If this specification is not met,
idle current increases.
In general, 10- µ H inductors suffice for most applications. The frequency response of the amplifier is slightly
altered by the change in output load resistance; however, unless tight control of frequency response is necessary
(better than 0.5 dB), it is not necessary to deviate from 10 µ H.
The graphs in Figure 10 display the inductance-versus-current characteristics of two inductors that are suggested
for use with the TAS5121I.
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