lm4867 National Semiconductor Corporation, lm4867 Datasheet - Page 12

• Manufacturer Part Number: lm4867
• Description: Output-transient-free Dual 2.1w Audio Amplifier Plus No Coupling Capacitor Stereo Headphone Function
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
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. Driving a portable notebook computer or audio-
visual display equipment is possible. This presents no diffi-
culty 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 (
may occur.
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.
USING THE LM4867 TO UPGRADE LM4863 AND LM4873
DESIGNS
The
capacitorless headphone operation and functional compat-
ibility with the LM4873 and the LM4863 simplifies upgrading
systems using these parts. Upgrading older designs 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 be-
tween each headphone connection and ground. Finally, re-
move any resistors connected to the HP-IN pin (typically two
100kΩ resistors). Connect the HP-IN pin directly to the head-
phone jack control pin as shown in Figure 4.
>
LM4867’s
60mV
RMS
, typ) to mask any remaining transient that
noise-free
PK
, the amplifier is shutdown, pro-
operation
(Continued)
plus
coupling-
12
FIGURE 2. Differential output signal (Trace B) is devoid
The LM4867’s pin configuration simplifies the process of
upgrading systems that use the LM4863. Except for its four
MUX function pins, the LM4867’s pin configuration matches
the LM4863’s pin configuration. If the LM4867’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 must 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 LM4867’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.
shutdown signal (Trace A). The inverting output (Trace
(Trace C) and the V
devoid of transients. The SHUTDOWN pin is driven by
C) and the non-inverting output (Trace D) are applied
FIGURE 3. Single-ended output signal (Trace B) is
a shutdown signal (Trace A). The inverting output
of transients. The SHUTDOWN pin is driven by a
DD
to the MUX CTRL pin selects stereo input 2
across an 8Ω BTL load.
across a 32Ω BTL load.
BYPASS
output (Trace D) are applied
DD
or ground. To ensure
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