TPA3100D2 Datasheet, PDF (28/31 Pages) Texas Instruments – 20-W STEREO CLASS-D AUDIO POWER AMPLIFIER

TPA3100D2
SLOS469B – OCTOBER 2005 – REVISED OCTOBER 2005
www.ti.com
Load
R_L
V_L= V_IN
To APA
GND
RC Low-Pass Filters
R_FILT
R_FILT
C_FILT
V_OUT
C_FILT
AP Analyzer Input
C_ANA
R_ANA
C_ANA
R_ANA
Figure 34. Measurement Low-Pass Filter Derivation Circuit-Class-D APAs
The transfer function for this circuit is shown in Equation 5 where ω_O = R_EQC_EQ, R_EQ = R_FILT || R_ANA and
C_EQ = (C_FILT + C_ANA). The filter frequency should be set above f_MAX, the highest frequency of the measurement
bandwidth, to avoid attenuating the audio signal. Equation 6 provides this cutoff frequency, f_C. The value of R_FILT
must be chosen large enough to minimize current that is shunted from the load, yet small enough to minimize the
attenuation of the analyzer-input voltage through the voltage divider formed by R_FILT and R_ANA. A general rule is
that R_FILT should be small (~100 Ω) for most measurements. This reduces the measurement error to less than
1% for R_ANA ≥ 10 kΩ.
( VOUT
V_IN
)⁼
( ) RANA
R_ANA + R_FILT
( )^w
1 + j _wO
(5)
f_c = Ö2 x f_max
(6)
An exception occurs with the efficiency measurements, where R_FILT must be increased by a factor of ten to
reduce the current shunted through the filter. C_FILT must be decreased by a factor of ten to maintain the same
cutoff frequency. See Table 3 for the recommended filter component values.
Once f_C is determined and R_FILT is selected, the filter capacitance is calculated using . When the calculated value
is not available, it is better to choose a smaller capacitance value to keep f_C above the minimum desired value
calculated in Equation 7.
1
C_FILT = _{2p x fc x RFILT}
(7)
Table 3 shows recommended values of R_FILT and C_FILT based on common component values. The value of f_C
was originally calculated to be 28 kHz for an f_MAX of 20 kHz. C_FILT, however, was calculated to be 57,000 pF, but
the nearest values of 56,000 pF and 51,000 pF were not available. A 47,000-pF capacitor was used instead, and
f_C is 34 kHz, which is above the desired value of 28 kHz.
Table 3. Typical RC Measurement Filter Values
MEASUREMENT
Efficiency
All other measurements
R_FILT
1000 Ω
100 Ω
C_FILT
5,600 pF
56,000 pF
28