TPA2001D2
1-W FILTERLESS STEREO CLASS-D AUDIO POWER AMPLIFIER
SLOS292A – MARCH 2000 – REVISED APRIL 2000
APPLICATION INFORMATION
effects of applying a square wave into a speaker
Audio specialists have said for years not to apply a square wave to speakers. If the amplitude of the waveform
is high enough and the frequency of the square wave is within the bandwidth of the speaker, the square wave
could cause the voice coil to jump out of the air gap and/or scar the voice coil. A 250-kHz switching frequency,
however, is not significant because the speaker cone movement is proportional to 1/f
2
for frequencies beyond
the audio band. Therefore, the amount of cone movement at the switching frequency is very small. However,
damage could occur to the speaker if the voice coil is not designed to handle the additional power. To size the
speaker for added power, the ripple current dissipated in the load needs to be calculated by subtracting the
theoretical supplied power, P
SUP THEORETICAL
, from the actual supply power, P
SUP
, at maximum output power,
P
OUT
. The switching power dissipated in the speaker is the inverse of the measured efficiency,
η
MEASURED
,
minus the theoretical efficiency,
η
THEORETICAL
.
P
SPKR
= P
SUP
– P
SUP THEORETICAL
(at max output power)
P
SPKR
= P
SUP
/ P
OUT
– P
SUP THEORETICAL
/ P
OUT
(at max output power)
P
SPKR
= 1/η
MEASURED
– 1/η
THEORETICAL
(at max output power)
(1)
(2)
(3)
The maximum efficiency of the TPA2001D2 with an 8-Ω load is 85%. Using equation 3 with the efficiency at
maximum power (78%) there is an additional 106 mW dissipated in the speaker. The added power dissipated
in the speaker is not an issue as long as it is taken into account when choosing the speaker.
when to use an output filter
Design the TPA2001D2 without the filter if the traces from amplifier to speaker are short. The TPA2001D2
passed FCC and CE radiated emissions with no shielding with speaker wires 8 inches long or less. Notebook
PCs and powered speakers where the speaker is in the same enclosure as the amplifier are good applications
for class-D without a filter.
A ferrite bead filter can often be used if the design is failing radiated emissions without a filter, and the frequency
sensitive circuit is greater than 1 MHz. This is good for circuits that just have to pass FCC and CE because FCC
and CE only test radiated emissions greater than 30 MHz. If choosing a ferrite bead, choose one with high
impedance at high frequencies, but very low impedance at low frequencies.
Use an output filter if there are low frequency (< 1 MHz) EMI sensitive circuits and/or there are long leads from
amplifier to speaker.
gain setting via GAIN0 and GAIN1 inputs
The gain of the TPA2001D2 is set by two input terminals, GAIN0 and GAIN1.
The gains listed in Table 2 are realized by changing the taps on the input resistors inside the amplifier. This
causes the input impedance, Z
I
, to be dependent on the gain setting. The actual gain settings are controlled
by ratios of resistors, so the actual gain distribution from part-to-part is quite good. However, the input
impedance may shift by 30% due to shifts in the actual resistance of the input resistors.
For design purposes, the input network (discussed in the next section) should be designed assuming an input
impedance of 20 kΩ, which is the absolute minimum input impedance of the TPA2001D2. At the higher gain
settings, the input impedance could increase as high as 115 kΩ.
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•
DALLAS, TEXAS 75265
7
TI [ TEXAS INSTRUMENTS ]
