Datasheet

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APPLICATION SCHEMATICS
Audio
Input
Bias
Control
V
DD
700 mW
6
5
7
V
O
+
V
DD
1
2
4
BYPASS
IN–
V
DD
/2
C
I
C
S
1
µ
F
C
B
2.2
µ
F
SHUTDOWN
V
O
8
GND
From System Control
3 IN+
R
I
10 k
R
F
50 k
+
+
COMPONENT SELECTION
Gain-Setting Resistors, R
F
and R
I
BTL gain 2
R
F
R
I
(5)
Effective impedance
R
F
R
I
R
F
R
I
(6)
TPA721
SLOS231E NOVEMBER 1998 REVISED JUNE 2004
Figure 26 is a schematic diagram of a typical handheld audio application circuit, configured for a gain of –10 V/V.
Figure 26. TPA721 Application Circuit
The following sections discuss the selection of the components used in Figure 26 .
The gain for each audio input of the TPA721 is set by resistors R
F
and R
I
according to Equation 5 for BTL mode.
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 TPA721 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 is required for proper startup 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 k and 20 k. The effective impedance is calculated in Equation 6 .
As an example, consider an input resistance of 10 k and a feedback resistor of 50 k. The BTL gain of the
amplifier would be –10 V/V, and the effective impedance at the inverting terminal would be 8.3 k, 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 k, 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 of approximately 5 pF should be placed in parallel with R
F
when R
F
is greater than
50 k. This, in effect, creates a low-pass filter network with the cutoff frequency defined in Equation 7 .
15