LM4842 datasheet(21/31 Pages) NSC

21 / 31 page

Application Information (Continued)

The BEEP IN pin is used only to detect the beep signal’s

magnitude: it does not pass the signal to the output amplifi-

ers. The LM4842’s shutdown mode must be deactivated

before a system alert signal is applied to the BEEP IN pin.

CAP-LESS HEADPHONE (SINGLE-ENDED) AMPLIFIER

OPERATION

An internal pull−up circuit is connected to the HP Sense (Pin

21 HP-IN on the MT/MH packages) headphone amplifier

control pin. When this pin is left unconnected, V

DD is applied

to the HP−IN. This turns off Amp B +OUT (not seen in Fig 5,

see Fig 2 Pin 15) and switches Amp A +OUT’s input signal

from an audio signal to the V

DD/2 voltage present on pin 28

(Amp A + OUT). The result is muted bridge-connected loads.

Quiescent current consumption is reduced when the IC is in

this single−ended mode.

Figure 5 above shows the implementation of the LM4842’s

headphone control function. An internal comparator with a

nominal 400mV offset monitors the signal present at the

−OUT B output. It compares this signal against the signal

applied to the HP−IN pin (Notice in Figure 5, Pin 21 is

shorted to Pin 17 without a headphone plugged in). When

these signals are equal, as in the case when a BTL is

connected to the amplifier, an internal comparator forces the

LM4842 to maintain bridged−amplifier operation. When the

HP−IN pin is externally floated, such as when headphones

are connected to the jack shown in Figure 5, an internal

pull−up forces V

DD on the internal comparator’s HP−IN in-

puts. This changes the comparator’s output state and en-

ables the headphone function: it turns off Amp B +OUT (not

seen in Fig 5, see Fig 2 Pin 15), switches the Amp A +OUT

input signal from an audio signal to the V

DD/2 DC voltage

present on pin 28, and mutes the bridge-connected loads.

Amp A -OUT and Amp B -OUT drive the headphones.

Figures 2 and 6 also show suggested headphone jack elec-

trical connections. The jack is designed to mate with a

three−wire plug. The plug’s tip and ring should each carry

one of the two stereo output signals, whereas the sleeve

provides the return to Amp A +OUT. A headphone jack with

one control pin contact is sufficient to drive the HP−IN pin

when connecting headphones.

A switch can replace the headphone jack contact pin. When

a switch shorts the HP−IN pin to V

DD (An open switch

contact will accomplish this because there is an internal

pull-up resistor), the bridge−connected speakers are muted

and Amp A -OUT and Amp B -OUT drive the stereo head-

phones. When a switch shorts the HP−IN pin to GND (pulling

down the internal pull-up resistor), the LM4842 operates in

bridge mode. If headphone drive is not needed, short the

HP−IN pin to the −OUTB pin.

Figure 6 shows an optional resistor connected between the

amplifier output that drives the headphone jack sleeve and

ground. This resistor provides a ground path that supresses

power supply hum. This hum may occur in applications such

as notebook computers in a shutdown condition and con-

nected to an external powered speaker. The resistor’s 100

Ω

value is a suggested starting point. Its final value must be

determined based on the tradeoff between the amount of

noise suppression that may be needed and minimizing the

additional current drawn by the resistor (25mA for a 100

Ω

resistor and a 5V supply).

MUTE FUNCTION

The LM4842 mutes the amplifier and DOCK outputs when

DD is applied to the MUTE pin. Even while muted, the

LM4842 will amplify a system alert (beep) signal whose

magnitude satisfies the BEEP DETECT circuitry. Applying

0V to the MUTE pin returns the LM4842 to normal, unmuted

operation. Prevent unanticipated mute behavior by connect-

ing the MUTE pin to V

DD or ground. Do not let the mute pin

float.

MICRO-POWER SHUTDOWN

The voltage applied to the SHUTDOWN pin controls the

LM4842’s shutdown function. Activate micro-power shut-

down by applying ground (logic low) to the SHUTDOWN pin.

When activated, the LM4842’s micro-power shutdown fea-

ture turns off the amplifier’s bias circuitry, reducing the sup-

ply current. On the demo board, the micro-power shutdown

feature is controlled by a single pole switch that connects the

shutdown pin to either V

DD, for normal operation, or directly

20028196

FIGURE 5. Headphone Sensing Circuit

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