參數(shù)資料
型號： LM4838ITL
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 音頻控制
英文描述： Stereo 2W Audio Power Amplifiers with DC Volume Control and Selectable Gain
中文描述： 2 CHANNEL(S), VOLUME CONTROL CIRCUIT, PBGA36
封裝： MICRO, SMD-36
文件頁數(shù)： 17/36頁
文件大?。?/td> 2085K
代理商： LM4838ITL
Application Information
(Continued)
INPUT CAPACITOR VALUE SELECTION
Amplifying the lowest audio frequencies requires a high
value input coupling capacitor (0.33μF in
Figure 2
), but high
value capacitors can be expensive and may compromise
space efficiency in portable designs. In many cases, how-
ever, the speakers used in portable systems, whether inter-
nal or external, have little ability to reproduce signals below
150 Hz. Applications using speakers with this limited fre-
quency response reap little improvement by using a large
input capacitor.
Besides effecting system cost and size, the input coupling
capacitor has an affect on the LM4838’s click and pop per-
formance. When the supply voltage is first applied, a tran-
sient (pop) is created as the charge on the input capacitor
changes from zero to a quiescent state. The magnitude of
the pop is directly proportional to the input capacitor’s size.
Higher value capacitors need more time to reach a quiescent
DC voltage (usually V
/2) when charged with a fixed cur-
rent. The amplifier’s output charges the input capacitor
through the feedback resistor, R
. Thus, pops can be mini-
mized by selecting an input capacitor value that is no higher
than necessary to meet the desired 6dB frequency.
As shown in
Figure 2
, the input resistor (R
IR
, R
IL
= 20k) ( and
the input capacitor (C
IR
, C
IL
= 0.33μF) produce a 6dB high
pass filter cutoff frequency that is found using Equation (7).
(7)
As an example when using a speaker with a low frequency
limit of 150Hz, the input coupling capacitor, using Equation
(7), is 0.053μF. The 0.33μF input coupling capacitor shown
in
Figure 2
allows the LM4838 to drive a high efficiency, full
range speaker whose response extends below 30Hz.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4838 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 LM4838’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
on the BYPASS pin is stable, the device becomes fully
operational. Although the BYPASS pin current cannot be
modified, changing the size of C
alters the device’s turn-on
time and the magnitude of “clicks and pops”. Increasing the
value of C
reduces the magnitude of turn-on pops. How-
ever, this presents a tradeoff: as the size of C
increases, the
turn-on time increases. There is a linear relationship be-
tween the size of C
and the turn-on time. Here are some
typical turn-on times for various values of C
B
:
C
B
0.01μF
T
ON
2ms
C
B
0.1μF
0.22μF
0.47μF
1.0μF
T
ON
20ms
44ms
94ms
200ms
DOCKING STATION INTERFACE
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 LM4838 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
V
DD
/2, coupling capacitors should be connected in series
with the load when using these outputs. Typical values for
the output coupling capacitors are 0.33μF to 1.0μF. If polar-
ized coupling capacitors are used, connect their "+" termi-
nals to the respective output pin, see Figure 2.
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 (R
, R
) are shown in
Fig-
ure 2
and they set each input amplifier’s gain to -1. Use
Equation 7 to determine the input and feedback resistor
values for a desired gain.
- A
VR
= R
FR
/R
IR
and - A
VL
= R
FL
/R
IL
Adjusting the input amplifier’s gain sets the minimum gain for
that channel. Although the single ended output 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.
(8)
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 LM4838’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
DD
/2 is detected on the BEEP IN pin, the bridge output
amplifiers are enabled. The beep signal is amplified and
applied to the load connected to the output amplifiers.Avalid
beep signal will be applied to the load even when MUTE is
active. Use the input resistors connected between the BEEP
IN pin and the stereo input pins to accommodate different
beep signal amplitudes. These resistors (R
) are shown
as 200k
devices in
Figure 2
. Use higher value resistors to
reduce the gain applied to the beep signal. The resistors
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
magnitude: it does not pass the signal to the output amplifi-
ers. The LM4838’s shutdown mode must be deactivated
before a system alert signal is applied to BEEP IN pin.
L
www.national.com
17
相關PDF資料
PDF描述
LM4838ITLX Stereo 2W Audio Power Amplifiers with DC Volume Control and Selectable Gain
LM4839 Stereo 2W Audio Power Amplifiers with DC Volume Control, Bass Boost, and Input Mux
LM4839MT Stereo 2W Audio Power Amplifiers with DC Volume Control, Bass Boost, and Input Mux
LM4839LQ Stereo 2W Audio Power Amplifiers with DC Volume Control, Bass Boost, and Input Mux
LM4839MTE Stereo 2W Audio Power Amplifiers with DC Volume Control, Bass Boost, and Input Mux
相關代理商/技術(shù)參數(shù)
參數(shù)描述
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