TPA0312_16 Datasheet, PDF (19/37 Pages) Texas Instruments – 2.6-W STEREO AUDIO POWER AMPLIFIER WITH FOUR SELECTABLE GAIN SETTINGS AND MUX CONTROL

TPA0312
www.ti.com
SLOS335A – DECEMBER 2000 – REVISED OCTOBER 2004
GAIN SETTING VIA GAIN0 AND GAIN1 INPUTS
The gain of the TPA0312 is set by two input terminals, GAIN0 and GAIN1.
GAIN0
0
0
1
1
X
Table 1. GAIN SETTINGS
GAIN1
0
1
0
1
X
SE/BTL
0
0
0
0
1
A_V
6 dB
10 dB
15.6 dB
21.6 dB
4.1 dB
The gains listed in Table 1 are realized by changing the taps on the input resistors inside the amplifier. This
causes the input impedance, Z_I, to be dependant 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
will shift by 30% due to shifts in the actual resistance of the input impedance.
For design purposes, the input network (discussed in the next section) should be designed assuming an input
impedance of 10 kΩ, which is the absolute minimum input impedance of the TPA0312. At the lower gain settings,
the input impedance could increase as high as 115 kΩ.
INPUT RESISTANCE
Each gain setting is achieved by varying the input resistance of the amplifier, which can range from its smallest
value to over 6 times that value. As a result, if a single capacitor is used in the input high pass filter, the –3-dB or
cutoff frequency also changes by over 6 times. If an additional resistor is connected from the input pin of the
amplifier to ground, as shown in the following figure, the variation of the cutoff frequency is much reduced.
Z_F
C
Input
Signal
IN
Z_I
R
The typical input impedance at each gain setting is given in the table below:
A_v
21.6 dB
15.6 dB
10 dB
6 dB
Z_I
25 kΩ
45 kΩ
70 kΩ
90 kΩ
The –3-dB frequency can be calculated using Equation 1:
ƒ_–3
dB
+
2p
1
C^ǒR
ø
RI^Ǔ
(1)
If the filter must be more accurate, the value of the capacitor should be increased while the value of the resistor
to ground should be decreased. In addition, the order of the filter could be increased.
INPUT CAPACITOR, C_I
In the typical application, an input capacitor, C_I, is required to allow the amplifier to bias the input signal to the
proper dc level for optimum operation. In this case, C_I and the input impedance of the amplifier, Z_I, form a
high-pass filter with the corner frequency determined in Equation 2.
19