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

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

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-

may occur. (See Turn On Characteristics).

Figure 3 shows the LM4841’s lack of transients in the differ-

ential signal (Trace B) across a BTL 8

Ω 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

32

Ω single-ended (SE) load. Again, no trace of output tran-

sients on Trace B can be observed.

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 8

Ω BTL

load.

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