Datasheet
SUMMING INPUT SIGNALS WITH THE TPA2005D1
Summing Two Differential Input Signals
Gain 1 +
V
O
V
I1
+ 2
150 kW
R
I1
ǒ
V
V
Ǔ
(4)
Gain 2 +
V
O
V
I2
+ 2
150 kW
R
I2
ǒ
V
V
Ǔ
(5)
_
+
IN–
IN+
PWM H–
Bridge
V
O+
V
O–
Internal
Oscillator
C
S
To Battery
V
DD
GND
Bias
Circuitry
R
I2
R
I2
+
–
Differential
Input 1
SHUTDOWN
R
I1
R
I1
+
–
Differential
Input 2
Filter-Free Class D
Summing a Differential Input Signal and a Single-Ended Input Signal
Gain 1 +
V
O
V
I1
+ 2
150 kW
R
I1
ǒ
V
V
Ǔ
(6)
Gain 2 +
V
O
V
I2
+ 2
150 kW
R
I2
ǒ
V
V
Ǔ
(7)
C
I2
+
1
ǒ
2p R
I2
f
c2
Ǔ
(8)
TPA2005D1
www.ti.com
............................................................................................................................................................... SLOS369F – JULY 2002 – REVISED JULY 2008
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 TPA2005D1 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.
Two extra resistors are needed for summing differential signals (a total of 5 components). The gain for each input
source can be set independently (see equations Equation 4 and Equation 5 , and Figure 35 ).
If summing left and right inputs with a gain of 1 V/V, use R
I1
= R
I2
= 300 k Ω .
If summing a ring tone and a phone signal, set the ring-tone gain to Gain 2 = 2 V/V, and the phone gain to gain
1 = 0.1 V/V. The resistor values would be. . .
• R
I1
= 3 M Ω , and = R
I2
= 150 k Ω .
Figure 35. Application Schematic With TPA2005D1 Summing Two Differential Inputs
Figure 36 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. The corner frequency of the single-ended
input is set by C
I2
, shown in equation Equation 8 . 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.
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