External Components Description

( Refer to Figure 1. )

Components

Functional Description

1.

R

i

Inverting input resistance which sets the closed-loop gain in conjunction with R

high pass filter with C

2.

C

i

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

3.

R

f

Feedback resistance which sets the closed-loop gain in conjunction with R

4.

C

s

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.

5.

C

B

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

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 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.

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 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.

Figure 2 shows the LM4867’s lack of transients in the differ-

ential signal (Trace B) across a BTL 8

Ω 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

32

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

sients is seen.

USING THE LM4867 TO UPGRADE LM4863 AND LM4873

DESIGNS

The

LM4867’s

noise-free

operation

plus

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

Ω resistor between each headphone connection and

ground. Finally, remove any resistors connected to the

HP-IN pin (typically two 100k

Ω resistors). Connect the HP-IN

pin directly to the headphone jack control pin as shown in

Figure 4.

DS200013-56

FIGURE 2. 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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