lm4839mtx

Manufacturer Part Numberlm4839mtx
DescriptionStereo 2w Audio Power Amplifiers With Dc Volume Control, Bass Boost, And Input Mux
ManufacturerNational Semiconductor Corporation
lm4839mtx datasheet
 


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Application Information
Higher value capacitors need more time to reach a quiescent
DC voltage (usually V
/2) when charged with a fixed cur-
DD
rent. The amplifier’s output charges the input capacitor
through the feedback resistor, R
. Thus, pops can be mini-
f
mized by selecting an input capacitor value that is no higher
than necessary to meet the desired −3dB frequency.
As shown in Figure 1 , the input resistors (RIN = 20K) and the
input capacitosr (CIN = 0.33µF) produce a −6dB high pass
filter cutoff frequency that is found using Equation (7).
As an example when using a speaker with a low frequency
limit of 150Hz, the input coupling capacitor using Equation
(7), is 0.063µF. The 0.33µF input coupling capacitor shown
in Figure 1 allows the LM4839 to drive high efficiency, full
range speaker whose response extends below 30Hz.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4839 contains circuitry that minimizes turn-on and
shutdown transients or “clicks and pops”. For this discus-
sion, turn-on refers to either applying the power supply volt-
age or when the shutdown mode is deactivated. While the
power supply is ramping to its final value, the LM4839’s
internal amplifiers are configured as unity gain buffers. An
internal current source changes the voltage of the BYPASS
pin in a controlled, linear manner. Ideally, the input and
outputs track the voltage applied to the BYPASS pin. The
gain of the internal amplifiers remains unity until the voltage
on the bypass pin reaches 1/2 V
. As soon as the voltage
DD
on the bypass pin is stable, the device becomes fully opera-
tional. Although the BYPASS pin current cannot be modified,
changing the size of C
alters the device’s turn-on time and
B
the magnitude of “clicks and pops”. Increasing the value of
C
reduces the magnitude of turn-on pops. However, this
B
presents a tradeoff: as the size of C
time increases. There is a linear relationship between the
size of C
and the turn-on time. Here are some typical
B
turn-on times for various values of C
C
B
0.01µF
0.1µF
0.22µF
0.47µF
1.0µF
DOCKING STATION
Applications such as notebook computers can take advan-
tage of a docking station to connect to external devices such
as monitors or audio/visual equipment that sends or receives
line level signals. The LM4839 has two outputs, Right Dock
and Left Dock which connect to outputs of the internal input
amplifiers that drive the volume control inputs. These input
>
amplifiers can drive loads of
1k (such as powered speak-
ers) with a rail-to-rail signal. Since the output signal present
on the RIGHT DOCK and LEFT DOCK pins is biased to
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/2, coupling capacitors should be connected in series
V
(Continued)
DD
with the load. Typical values for the coupling capacitors are
0.33µF to 1.0µF. If polarized coupling capacitors are used,
connect their ’+’ terminals to the respective output pin.
Since the DOCK outputs precede the internal volume con-
trol, the signal amplitude will be equal to the input signal’s
magnitude and cannot be adjusted. However, the input am-
plifier’s closed-loop gain can be adjusted using external
resistors. These 20K resistors are shown in Figure 1 (RIN,
RF ) and they set each input amplifier’s gain to -1. Use
Equation 8 to determine the input and feedback resistor
values for a desired gain.
(7)
Adjusting the input amplifier’s gain sets the minimum gain for
that channel. Although the single ended outputs of the
Bridge Output Amplifiers can be used to drive line level
outputs, it is recommended that the R & L Dock Outputs
simpler signal path be used for better performance.
STEREO-INPUT MULTIPLEXER (STEREO MUX)
The LM4839 has two stereo inputs. The MUX CONTROL pin
controls which stereo input is active. Applying 0V to the MUX
CONTROL pin selects stereo input 1. Applying V
MUX CONTROL pin selects stereo input 2.
BEEP DETECT FUNCTION
Computers and notebooks produce a system ’beep’ signal
that drives a small speaker. The speaker’s auditory output
signifies that the system requires user attention or input. To
accommodate this system alert signal, the LM4839’s beep
input pin is a mono input that accepts the beep signal.
Internal level detection circuitry at this input monitors the
beep signal’s magnitude. When a signal level greater than
V
/2 is detected on the BEEP IN pin, the bridge output
DD
amplifiers are enabled. The beep signal is amplified and
applied to the load connected to the output amplifiers. A valid
beep signal will be applied to the load even when MUTE is
active. Use the input resistors connected between the BEEP
increases, the turn-on
IN pin and the stereo input pins to accommodate different
B
beep signal amplitudes. These resistors are shown as
200k
devices in Figure 1. Use higher value resistors to
:
reduce the gain applied to the beep signal. The resistors
B
must be used to pass the beep signal to the stereo inputs.
The BEEP IN pin is used only to detect the beep signal’s
T
magnitude: it does not pass the signal to the output amplifi-
ON
ers. The LM4839’s shutdown mode must be deactivated
2ms
before a system alert signal is applied to the BEEP IN pin.
20ms
44ms
MICRO-POWER SHUTDOWN
94ms
The voltage applied to the SHUTDOWN pin controls the
LM4839’s shutdown function. Activate micro-power shut-
200ms
down by applying V
the LM4839’s micro-power shutdown feature turns off the
amplifier’s bias circuitry, reducing the supply current. The
logic threshold is typically V
shutdown current is achieved by applying a voltage that is as
near as V
that is less than V
Logic Level Truth Table shows the logic signal levels that
activate and deactivate micro-power shutdown and head-
phone amplifier operation.
There are a few ways to control the micro-power shutdown.
These include using a single-pole, single-throw switch, a
16
- A
= R
/ R
v
F
i
to the SHUTDOWN pin. When active,
DD
/2. The low 0.7 µA typical
DD
as possible, to the SHUTDOWN pin. A voltage
DD
may increase the shutdown current.
DD
(8)
to the
DD