External Components Description
( Refer to Figure 1. )
Components
Functional Description
1.
Ri
Inverting input resistance which sets the closed-loop gain in conjunction with Rf. This resistor also forms a
high pass filter with C at fc = 1/(2πRiCi).
i
2.
Ci
Input coupling capacitor which blocks the DC voltage at the amplifier’s input terminals. Also creates a
highpass filter with Ri at fc = 1/(2πRiCi). Refer to the section, Proper Selection of External Components,
for an explanation of how to determine the value of Ci.
3.
4.
Rf
Feedback resistance which sets the closed-loop gain in conjunction with Ri.
Cs
Supply bypass capacitor which provides power supply filtering. Refer to the Power Supply Bypassing
section for information concerning proper placement and selection of the supply bypass capacitor.
5.
CB
Bypass pin capacitor which provides half-supply filtering. Refer to the section, Proper Selection of
External Components, for information concerning proper placement and selection of CB.
Application Information
ELIMINATING OUTPUT COUPLING CAPACITORS
Typical single-supply audio amplifiers that can switch be-
tween driving bridge-tied-load (BTL) speakers and
single-ended (SE) headphones use a coupling capacitor on
each SE output. This capacitor blocks the half-supply volt-
age to which the output amplifiers are typically biased and
couples the audio signal to the headphones. The signal
return to circuit ground is through the headphone jack’s
sleeve.
Figure 2 shows the LM4867’s lack of transients in the differ-
ential signal (Trace B) across a BTL 8Ω load. The LM4867’s
active-high SHUTDOWN pin is driven by the logic signal
shown in Trace A. Trace C is the VOUT- output signal and
trace D is the VOUT+ output signal. The shutdown signal
frequency is 1Hz with a 50% duty cycle. Figure 3 is gener-
ated with the same conditions except that the output drives a
32Ω single-ended (SE) load. Again, no trace of output tran-
sients is seen.
The LM4867 eliminates these coupling capacitors. Amp2A is
internally configured to apply VDD/2 to a stereo headphone
jack’s sleeve. This voltage matches the quiescent voltage
present on the Amp1A and Amp1B outputs that drive the
headphones. The headphones operate in a manner very
similar to a bridge-tied-load (BTL). The same DC voltage is
applied to both headphone speaker terminals. This results in
no net DC current flow through the speaker. AC current flows
through a headphone speaker as an audio signal’s output
amplitude increases on the speaker’s terminal.
USING THE LM4867 TO UPGRADE LM4863 AND LM4873
DESIGNS
The
LM4867’s
noise-free
operation
plus
coupling-capacitorless headphone operation and functional
compatibility with the LM4873 and the LM4863 simplifies
upgrading systems using these parts. Upgrading older de-
signs that use either the LM4863 or the LM4873 is easy.
Simply remove and short the coupling capacitors located
between the LM4873’s or LM4863’s Amp1A and Amp1B
outputs and the headphone connections. Also remove the
1kΩ resistor between each headphone connection and
ground. Finally, remove any resistors connected to the
HP-IN pin (typically two 100kΩ resistors). Connect the HP-IN
pin directly to the headphone jack control pin as shown in
Figure 4.
When operating as a headphone amplifier, the headphone
jack sleeve is not connected to circuit ground. Using the
headphone output jack as a line-level output will place the
LM4867’s one-half supply voltage on a plug’s sleeve con-
nection. Driving
a
portable notebook computer or
audio-visual display equipment is possible. This presents no
difficulty when the external equipment uses capacitively
coupled inputs. For the very small minority of equipment that
is DC-coupled, the LM4867 monitors the current supplied by
the amplifier that drives the headphone jack’s sleeve. If this
current exceeds 500mAPK, the amplifier is shutdown, pro-
tecting the LM4867 and the external equipment. For more
information, see the section titled ’Single-Ended Output
Power Performance and Measurement Considerations’.
OUTPUT TRANSIENT (’POPS AND CLICKS’)
ELIMINATED
The LM4867 contains advanced circuitry that eliminates out-
put transients (’pop and click’). This circuitry prevents all
traces of transients when the supply voltage is first applied,
when the part resumes operation after shutdown, or when
switching between BTL speakers and SE headphones. Two
circuits combine to eliminate pop and click. One circuit
mutes the output when switching between speaker loads.
Another circuit monitors the input signal. It maintains the
muted condition until there is sufficient input signal magni-
tude to mask any remaining transient that may occur.
DS200013-56
FIGURE 2. Differential output signal (Trace B) is devoid
of transients. The SHUTDOWN pin is driven by a
shutdown signal (Trace A). The inverting output (Trace
C) and the non-inverting output (Trace D) are applied
across an 8Ω BTL load.
9
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