LM4867MT National Semiconductor, LM4867MT Datasheet - Page 9

• Manufacturer Part Number: LM4867MT
• Description: Output-Transient-Free Dual 2.1W Audio Amplifier Plus No Coupling Capacitor Stereo Headphone Function
• Manufacturer: National Semiconductor

External Components Description
Application Information
ELIMINATING OUTPUT COUPLING CAPACITORS
Typical single-supply audio amplifiers that can switch be-
tween
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.
The LM4867 eliminates these coupling capacitors. Amp2A is
internally configured to apply V
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.
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.
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 500mA
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.
Components
( Refer to Figure 1. )
1.
2.
3.
4.
5.
driving
Driving
C
C
R
R
C
B
s
i
i
f
bridge-tied-load
a
Inverting input resistance which sets the closed-loop gain in conjunction with R
high pass filter with C
Input coupling capacitor which blocks the DC voltage at the amplifier’s input terminals. Also creates a
highpass filter with R
for an explanation of how to determine the value of C
Feedback resistance which sets the closed-loop gain in conjunction with R
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.
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 C
portable
PK
, the amplifier is shutdown, pro-
DD
/2 to a stereo headphone
notebook
(BTL)
i
at f
i
at f
speakers
c
computer
c
= 1/(2 R
= 1/(2 R
and
i
C
i
or
C
i
). Refer to the section, Proper Selection of External Components,
i
).
Functional Description
9
Figure 2 shows the LM4867’s lack of transients in the differ-
ential signal (Trace B) across a BTL 8
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
32
sients is seen.
USING THE LM4867 TO UPGRADE LM4863 AND LM4873
DESIGNS
The
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
1k
ground. Finally, remove any resistors connected to the
HP-IN pin (typically two 100k resistors). Connect the HP-IN
pin directly to the headphone jack control pin as shown in
Figure 4 .
FIGURE 2. Differential output signal (Trace B) is devoid
shutdown signal (Trace A). The inverting output (Trace
C) and the non-inverting output (Trace D) are applied
of transients. The SHUTDOWN pin is driven by a
single-ended (SE) load. Again, no trace of output tran-
resistor between each headphone connection and
i
.
LM4867’s
across an 8
noise-free
i
.
f
BTL load.
. This resistor also forms a
load. The LM4867’s
operation
B
.
DS200013-56
www.national.com
plus