Datasheet

c
L O
1
f =
2 R Cp
O
L c
1
C =
2 R fp
OPAMP
Co
Mute Circuit
Output
Enable
+
+
+
–
Conventional Solution
Mute Circuit
Output
Enable
3.3 V
+
–
TPA6138A2 Solution
VDD
VSS
GND
VDD
VDD/2
GND
DirectPath™
9 V–12 V
TPA6138A2
SLOS704A –JANUARY 2011– REVISED MAY 2011 www.ti.com
APPLICATION INFORMATION
Single-supply line-driver amplifiers typically require dc-blocking capacitors. The top drawing in Figure 7 illustrates
the conventional line-driver-amplifier connection to the load and output signal. DC blocking capacitors are often
large in value. The headphone load (typical resistive values of 16 Ω to 32 Ω) combine with the dc blocking
capacitors to form a high-pass filter. Equation 1 shows the relationship between the load impedance (R
L
), the
capacitor (C
O
), and the cutoff frequency (f
C
).
(1)
C
O
can be determined using Equation 2, where the load impedance and the cutoff frequency are known.
(2)
If f
C
is low, the capacitor must then have a large value because the load resistance is small. Large capacitance
values require large package sizes. Large package sizes consume PCB area, stand high above the PCB,
increase cost of assembly, and can reduce the fidelity of the audio output signal.
Figure 7. Conventional and DirectPath Line Driver
The DirectPath amplifier architecture operates from a single supply but makes use of an internal charge pump to
provide a negative voltage rail. Combining the user-provided positive rail and the negative rail generated by the
IC, the device operates in what is effectively a split-supply mode. The output voltages are now centered at zero
volts with the capability to swing to the positive rail or negative rail. The DirectPath amplifier requires no output
dc-blocking capacitors. The bottom block diagram and waveform of Figure 7 illustrate the ground-referenced
line-driver architecture. This is the architecture of the TPA6138A2.
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Product Folder Link(s): TPA6138A2