LM4842MH datasheet(17/31 Pages) NSC

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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 LM4842 eliminates these coupling capacitors. Amplifi-

erA+ (pin 28 on MT/MH) is internally configured to apply

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

manner

very

similar

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

LM4842’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 LM4842 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 LM4842 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 LM4842 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).

Figure 3 shows the LM4842’s lack of transients in the differ-

ential signal (Trace B) across a BTL 8

Ω load. The LM4842’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

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

sients on Trace B can be observed.

EXPOSED-DAP PACKAGE PCB MOUNTING

CONSIDERATIONS

The LM4842’s exposed-DAP (die attach paddle) package

(MH,LQ) provides a low thermal resistance between the die

and the PCB to which the part is mounted and soldered. This

allows rapid heat transfer from the die to the surrounding

PCB copper traces, ground plane and, finally, surrounding

air. The result is a low voltage audio power amplifier that

produces 2.1W at

≤ 1% THD with a 4Ω load. This high power

is achieved through careful consideration of necessary ther-

20028198

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.

20028199

FIGURE 4. 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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