Datasheet

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SELECTION OF COMPONENTS
Audio
Input
Bias
Control
V
DD
= 5 V
1-W
Internal
Speaker
6
5
8
7
V
O
1
V
O
2
V
DD
1
2
3
4
IN+
IN−
BYPASS
SHUTDOWN (see Note A)
V
DD
/2
C
I
R
I
R
F
C
F
50 kΩ 50 kΩ
46 kΩ
46 kΩ
C
B
C
S
NOTE A: SHUTDOWN must be held low for normal operation and asserted high for shutdown mode.
−
+
−
+
Gain Setting Resistors, R
F
and R
I
Gain   2

R
F
R
I

(5)
Effective Impedance 
R
F
R
I
R
F
 R
I
(6)
TPA4861
SLOS163C – SEPTEMBER 1996 – REVISED JUNE 2004
Figure 37 is a schematic diagram of a typical notebook computer application circuit.
Figure 37. TPA4861 Typical Notebook Computer Application Circuit
The gain for the TPA4861 is set by resistors R
F
and R
I
according to Equation 5 .
BTL mode operation brings about the factor of 2 in the gain equation due to the inverting amplifier mirroring the
voltage swing across the load. Given that the TPA4861 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 .
As an example, consider an input resistance of 10 kΩ and a feedback resistor of 50 kΩ. The 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 of approximately 5 pF should be placed in parallel with R
F
. This, in effect, creates a
low-pass filter network with the cutoff frequency defined in Equation 7 .
16