Datasheet

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MIDRAIL BYPASS CAPACITOR, C
(B)
1

C
(B)
 230 kΩ


1

C
i
R
i

(6)
1

C
(B)
 230 kΩ


1

C
i
R
i


1
R
L
C
(C)
(8)
USING LOW-ESR CAPACITORS
OUTPUT COUPLING CAPACITOR, C
(C)
5-V VERSUS 3.3-V OPERATION
f
c

1
2 R
L
C
(C)
(7)
TPA6112A2
SLOS342A – DECEMBER 2000 – REVISED SEPTEMBER 2004
Table 1. Common Load Impedances vs Low-
Frequency Output Characteristics in SE Mode
The midrail bypass capacitor, C
(B)
, serves several
R
L
C
(C)
LOWEST FREQUENCY
important functions. During start up, C
(B)
determines
32 Ω 68 µF 73 Hz
the rate at which the amplifier starts up. This helps to
push the start-up pop noise into the subaudible range
10,000 Ω 68 µF 0.23 Hz
(so low it can not be heard). The second function is to
47,000 Ω 68 µF 0.05 Hz
reduce noise produced by the power supply caused
by coupling into the output drive signal. This noise is
As Table 1 indicates, headphone response is ad-
from the midrail generation circuit internal to the
equate, and drive into line level inputs (a home stereo
amplifier. The capacitor is fed from a 230-k Ω source
for example) is very good.
inside the amplifier. To keep the start-up pop as low
as possible, maintain the relationship shown in
The output coupling capacitor required in
Equation 6 .
single-supply SE mode also places additional con-
straints on the selection of other components in the
amplifier circuit. With the rules described earlier still
valid, add the following relationship:
Consider an example circuit where C
(B)
is 1 µF, C
i
is
1 µF, and R
i
is 20 k Ω . Subsitituting these values into
the equation 9 results in: 6.25 ≤ 50 which satisfies the
rule. Bypass capacitor, C
(B)
, values of 0.1 µF to 1 µF
ceramic or tantalum low-ESR capacitors are rec-
ommended for the best THD and noise performance.
Low-ESR capacitors are recommended throughout
this application. A real capacitor can be modeled
simply as a resistor in series with an ideal capacitor.
The voltage drop across this resistor minimizes the
In a typical single-supply, single-ended (SE) configur-
beneficial effects of the capacitor in the circuit. The
ation, an output coupling capacitor (C
(C)
) is required
lower the equivalent value of this resistance, the
to block the dc bias at the output of the amplifier, thus
more the real capacitor behaves like an ideal capaci-
preventing dc currents in the load. As with the input
tor.
coupling capacitor, the output coupling capacitor and
impedance of the load form a high-pass filter
governed by Equation 7 .
The TPA6112A2 was designed for operation over a
supply range of 2.5 V to 5.5 V. This data sheet
provides full specifications for 5-V and 3.3-V oper-
ation, since these are considered to be the two most
The main disadvantage, from a performance stand-
common supply voltages. There are no special con-
point, is that the typically-small load impedance drives
siderations for 3.3-V versus 5-V operation as far as
the low-frequency corner higher. Large values of C
(C)
supply bypassing, gain setting, or stability. The most
are required to pass low frequencies into the load.
important consideration is that of output power. Each
Consider the example where a C
(C)
of 68 µF is
amplifier in theTPA6112A2 can produce a maximum
chosen and loads vary from 32 Ω to 47 k Ω . Table 1
voltage swing of V
DD
– 1 V. This means, for 3.3-V
summarizes the frequency response characteristics
operation, clipping starts to occur when V
O(PP)
= 2.3 V
of each configuration.
as opposed when V
O(PP)
= 4 V while operating at 5 V.
The reduced voltage swing subsequently reduces
maximum output power into the load before distortion
becomes significant.
15