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PSPKR = PSUP – PSUP THEORETICAL (at max output power)
(1)
PSPKR = PSUP / POUT – PSUP THEORETICAL / POUT (at max output power)
(2)
PSPKR = 1/ηMEASURED – 1/ηTHEORETICAL (at max output power)
(3)
WHEN TO USE AN OUTPUT FILTER
GAIN SETTING VIA GAIN0 AND GAIN1 INPUTS
SLOS291F – MARCH 2000 – REVISED MARCH 2007
APPLICATION INFORMATION (continued)
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, PSUP THEORETICAL, from the actual supply power, PSUP, at maximum output power,
POUT. The switching power dissipated in the speaker is the inverse of the measured efficiency, ηMEASURED, minus
the theoretical efficiency,
η
THEORETICAL.
The maximum efficiency of the TPA2000D2 with an 8-
load is 85%. Using
Equation 3 with the efficiency at
maximum power from
Figure 2 (78%), we see that 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.
Design the TPA2000D2 without the filter if the traces from amplifier to speaker are short. The TPA2000D2
passed FCC and CE radiated emissions with no shielding with speaker wires 8 inches (20,32 cm) 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.
The gain of the TPA2000D2 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, ZI, 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 k
, which is the absolute minimum input impedance of the TPA2000D2. At the lower gain
settings, the input impedance could increase to as high as 115 k
.
Table 2. Gain Settings
AMPLIFIER GAIN
INPUT IMPEDANCE
(dB)
(k
)
GAIN1
GAIN0
TYP
0
8
104
0
1
12
74
1
0
17.5
44
1
23.5
24
11