LM4868MT National Semiconductor, LM4868MT Datasheet - Page 12

• Manufacturer Part Number: LM4868MT
• Description: Output-Transient-Free Dual 2.1W Audio Amplifier Plus No Coupling Capacitor Stereo Headphone Function
• Manufacturer: National Semiconductor

www.national.com
Application Information
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
LM4868’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 LM4868 monitors the current supplied by
the amplifier that drives the headphone jack’s sleeve. If this
current exceeds 500mA
tecting the LM4868 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 LM4868 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 (
may occur.
FIGURE 2. Differential output signal (Trace B) is devoid
Figure 2 shows the LM4868’s lack of transients in the differ-
ential signal (Trace B) across a BTL 8Ω load. The LM4868’s
active-high SHUTDOWN pin is driven by the logic signal
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
>
25mV
Driving
RMS
across an 8Ω BTL load.
, typ) to mask any remaining transient that
a
portable
PK
, the amplifier is shutdown, pro-
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(Continued)
computer
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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.
USING THE LM4868 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
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.
The LM4868’s pin configuration simplifies the process of
upgrading systems that use the LM4863. Except for its four
MUX function pins, the LM4868’s pin configuration matches
the LM4863’s pin configuration. If the LM4868’s MUX func-
tionality is not needed when replacing an LM4863, connect
the MUX CTRL pin to either V
correct amplifier operation, unused MUX inputs should be
tied to GND. As shown in Table 1, grounding the MUX CTRL
pin selects stereo input 1 (−IN A1 and −IN B1), whereas
applying V
(−IN A2 and −IN B2).
The LM4868’s unique headphone sense circuit requires a
dual switch headphone jack. Replace the four-terminal head-
phone jack used with the LM4863 and LM4873 with the
five-terminal headphone jack, such as the Switchcraft
35RAPC4BH3, shown in Figure 4. Connect the +OUT A
(Amp2A) pin to the five-terminal headphone jack’s sleeve
pin.
(Trace C) and the V
devoid of transients. The SHUTDOWN pin is driven by
FIGURE 3. Single-ended output signal (Trace B) is
a shutdown signal (Trace A). The inverting output
LM4868’s
DD
to the MUX CTRL pin selects stereo input 2
across a 32Ω BTL load.
BYPASS
noise-free
output (Trace D) are applied
DD
or ground. To ensure
operation
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