TPA6205A1
www.ti.com
SLOS490A–JULY 2006–REVISED AUGUST 2006
Summing
a
Differential Input Signal and a
This leakage current creates a dc offset voltage at
the input to the amplifier that reduces useful
headroom, especially in high gain applications. For
this reason, a ceramic capacitor is the best choice.
When polarized capacitors are used, the positive
side of the capacitor should face the amplifier input
in most applications, as the dc level there is held at
VDD/2, which is likely higher than the source dc level.
It is important to confirm the capacitor polarity in the
application.
Single-Ended Input Signal
Figure 31 shows how to sum a differential input
signal and a single-ended input signal. Ground noise
can couple in through IN+ with this method. It is
better to use differential inputs. To assure that each
input is balanced, the single-ended input must be
driven by a low-impedance source even if the input is
not in use. Both input nodes must see the same
impedance for optimum performance, thus the use of
RP and CP.
Decoupling Capacitor (CS)
V
R
R
O
I1
F
V
V
ǒ Ǔ
Gain 1 +
+ *
The TPA6205A1 is a high-performance CMOS audio
amplifier that requires adequate power supply
decoupling to ensure the output total harmonic
distortion (THD) is as low as possible. Power supply
decoupling also prevents oscillations for long lead
lengths between the amplifier and the speaker. For
higher frequency transients, spikes, or digital hash
on the line, a good low equivalent-series- resistance
(ESR) ceramic capacitor, typically 0.1 µF to 1 µF,
placed as close as possible to the device VDD lead
works best. For filtering lower frequency noise
signals, a 10-µF or greater capacitor placed near the
audio power amplifier also helps, but is not required
in most applications because of the high PSRR of
this device.
V
I1
(6)
(7)
V
V
R
O
I2
F
V
V
ǒ Ǔ
Gain 2 +
+ *
R
I2
Where
CP = CI1 // CI2
RP = RI1 // RI2
USING LOW-ESR CAPACITORS
Low-ESR capacitors are recommended throughout
this applications section. A real (as opposed to ideal)
capacitor can be modeled simply as a resistor in
series with an ideal capacitor. The voltage drop
across this resistor minimizes the beneficial effects of
the capacitor in the circuit. The lower the equivalent
value of this resistance the more the real capacitor
behaves like an ideal capacitor.
SUMMING INPUT SIGNALS WITH THE
TPA6205A1
Most wireless phones or PDAs need to sum signals
at the audio power amplifier or just have two signal
sources that need separate gain. The TPA6205A1
makes it easy to sum signals or use separate signal
sources with different gains. Many phones now use
the same speaker for the earpiece and ringer, where
the wireless phone would require a much lower gain
for the phone earpiece than for the ringer. PDAs and
phones that have stereo headphones require
summing of the right and left channels to output the
stereo signal to the mono speaker.
DIFFERENTIAL OUTPUT VERSUS
SINGLE-ENDED OUTPUT
Figure 32 shows a Class-AB audio power amplifier
(APA) in
a fully differential configuration. The
TPA6205A1 amplifier has differential outputs driving
both ends of the load. There are several potential
benefits to this differential drive configuration, but
initially consider power to the load. The differential
drive to the speaker means that as one side is
slewing up, the other side is slewing down, and vice
versa. This in effect doubles the voltage swing on the
load as compared to a ground referenced load.
Plugging 2 × VO(PP) into the power equation, where
voltage is squared, yields 4× the output power from
the same supply rail and load impedance (see
Equation 8).
Summing Two Differential Input Signals
Two extra resistors are needed for summing
differential signals (a total of 10 components). The
gain for each input source can be set independently
(see Equation 4 and Equation 5, and Figure 30).
V
R
R
O
I1
F
V
V
ǒ Ǔ
Gain 1 +
+ *
V
I1
(4)
(5)
V
V
R
O
I2
F
V
V
ǒ Ǔ
Gain 2 +
+ *
R
I2
12
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