TPA301_15 Datasheet, PDF (16/29 Pages) Texas Instruments – 350-mW MONO AUDIO POWER AMPLIFIER

TPA301
SLOS208E – JANUARY 1998 – REVISED JUNE 2004
www.ti.com
Audio
Input
C_F
5 pF
C_I
R_I
0.47 µF 10 kΩ
R_F
50 kΩ
4 ^{IN -}
3 IN+
C_B
2.2 µF
2 BYPASS
V_DD/2
-
+
-
+
From System Control
1 SHUTDOWN
Bias
Control
V_DD 6
V_O+ 5
V_DD
C_S
1 µF
V_O- 8
7
GND
350 mW
Figure 26. TPA301 Application Circuit
The following sections discuss the selection of the components used in Figure 26.
COMPONENT SELECTION
Gain Setting Resistors, R_F and R_I
The gain for each audio input of the TPA301 is set by resistors R_F and R_I according to Equation 5 for BTL mode.
ǒ Ǔ BTL Gain + A_V + * 2
RF
RI
(5)
BTL mode operation brings about the factor 2 in the gain equation due to the inverting amplifier mirroring the
voltage swing across the load. Given that the TPA301 is a MOS amplifier, the input impedance is high;
consequently, input leakage currents are not generally a concern, although noise in the circuit increases as the
value of R_F increases. In addition, a certain range of R_F values are required for proper start-up operation of the
amplifier. Taken together it is recommended that the effective impedance seen by the inverting node of the
amplifier be set between 5 kΩ and 20 kΩ. The effective impedance is calculated in Equation 6.
Effective Impedance
+
RFRI
RF ) RI
(6)
As an example, consider an input resistance of 10 kΩ and a feedback resistor of 50 kΩ. The BTL gain of the
amplifier would be -10 V/V, and the effective impedance at the inverting terminal would be 8.3 kΩ, which is well
within the recommended range.
For high-performance applications, metal film resistors are recommended because they tend to have lower noise
levels than carbon resistors. For values of R_F above 50 kΩ, the amplifier tends to become unstable due to a pole
formed from R_F and the inherent input capacitance of the MOS input structure. For this reason, a small
compensation capacitor, C_F, of approximately 5 pF should be placed in parallel with R_F when R_F is greater than
50 kΩ. This, in effect, creates a low-pass filter network with the cutoff frequency defined in Equation 7.
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