LM4867 datasheet(12/28 Pages) NSC

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Application Information (Continued)

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

PK, the amplifier is shutdown, pro-

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 (>60mV

RMS, typ) 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

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 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 1k

Ω resistor be-

tween each headphone connection and ground. Finally, re-

move any resistors connected to the HP-IN pin (typically two

100k

Ω resistors). Connect the HP-IN pin directly to the head-

phone jack control pin as shown in Figure 4.

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

DD or ground. To ensure

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

DD to the MUX CTRL pin selects stereo input 2

(−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.

20001356

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.

20001357

FIGURE 3. Single-ended 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 V

BYPASS output (Trace D) are applied

across a 32

Ω BTL load.

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