LM4842 National Semiconductor, LM4842 Datasheet - Page 21

• Manufacturer Part Number: LM4842
• Description: Stereo 2W Amplifiers with DC Volume Control Transient Free Outputs/ and Cap-less Headphone Drive
• Manufacturer: National Semiconductor

Application Information
The BEEP IN pin is used only to detect the beep signal’s
magnitude: it does not pass the signal to the output amplifi-
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
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
(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
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
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
on the internal comparator’s HP−IN in-
DD
/2 voltage present on pin 28
DD
(Continued)
(An open switch
FIGURE 5. Headphone Sensing Circuit
DD
/2 DC voltage
DD
is applied
21
ers. The LM4842’s shutdown mode must be deactivated
before a system alert signal is applied to the BEEP IN pin.
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 100Ω
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 100Ω
resistor and a 5V supply).
MUTE FUNCTION
The LM4842 mutes the amplifier and DOCK outputs when
V
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
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
is applied to the MUTE pin. Even while muted, the
DD
20028196
DD
or ground. Do not let the mute pin
, for normal operation, or directly
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