LM4841 Datasheet, PDF (14/31 Pages) National Semiconductor (TI) – Stereo 2W Amplifiers with DC Volume Control, Transient Free Outputs, and Cap-less Headphone Drive

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.
The LM4841 eliminates these coupling capacitors. Amplifi-
erA+ (pin 28 on MT/MH) is internally configured to apply
V_DD/2 to a stereo headphone jack’s sleeve. This voltage
matches the quiescent voltage present on the AmpAout- and
AmpBout- outputs that drive the headphones. The head-
phones 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
LM4841’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 LM4841 monitors the current supplied by
the amplifier that drives the headphone jack’s sleeve. If this
current exceeds 500mA_PK, the amplifier is shutdown, pro-
tecting the LM4841 and the external equipment.
OUTPUT TRANSIENT (’POPS AND CLICKS’)
ELIMINATED
The LM4841 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 (>22mV_RMS, typ) to mask any remaining transient that
may occur. (See Turn On Characteristics).
20028095
FIGURE 3. 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.
Figure 3 shows the LM4841’s lack of transients in the differ-
ential signal (Trace B) across a BTL 8Ω load. The LM4841’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 4 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 on Trace B can be observed.
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