DRV632
SLOS681 –JANUARY 2011
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APPLICATION INFORMATION
LINE DRIVER AMPLIFIERS
Single-supply line-driver amplifiers typically require dc-blocking capacitors. The top drawing in Figure 6 illustrates
the conventional line-driver amplifier connection to the load and output signal. DC blocking capacitors are often
large in value. The line load (typical resistive values of 600 Ω to 10 kΩ) combines with the dc blocking capacitors
to form a high-pass filter. Equation 1 shows the relationship between the load impedance (RL), the capacitor
(CO), and the cutoff frequency (fC).
1
fc =
2pRLCO
(1)
CO can be determined using Equation 2, where the load impedance and the cutoff frequency are known.
1
CO
=
2pRLfc
(2)
If fC 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.
9 V–12 V
Conventional Solution
VDD
VDD/2
+
Mute Circuit
Co
+
+
–
Output
OPAMP
GND
Enable
3.3 V
DirectPath
VDD
DRV632 Solution
Mute Circuit
+
–
Output
GND
DRV632
VSS
Enable
Figure 6. Conventional and DirectPath Line Drivers
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. Combining this with the built-in click and pop
reduction circuit, the DirectPath amplifier requires no output dc blocking capacitors. The bottom block diagram
and waveform of Figure 6 illustrate the ground-referenced line-driver architecture. This is the architecture of the
DRV632.
8
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