LM4868 datasheet(12/28 Pages) NSC

12 / 28 page

Application Information (Continued)

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

LM4868’s one-half supply voltage on a plug’s sleeve con-

nection.

Driving

portable

notebook

computer

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 LM4868 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 LM4868 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 LM4868 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 (>25mV

RMS, typ) to mask any remaining transient that

may occur.

Figure 2 shows the LM4868’s lack of transients in the differ-

ential signal (Trace B) across a BTL 8

Ω load. The LM4868’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 LM4868 TO UPGRADE LM4863 AND LM4873

DESIGNS

The

LM4868’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

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.

The LM4868’s pin configuration simplifies the process of

upgrading systems that use the LM4863. Except for its four

MUX function pins, the LM4868’s pin configuration matches

the LM4863’s pin configuration. If the LM4868’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 should 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 LM4868’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.

20026756

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.

20026757

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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