參數(shù)資料
型號(hào)： LM4843MHX/NOPB
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 音頻控制
英文描述： 2 CHANNEL(S), VOLUME CONTROL CIRCUIT, PDSO20
封裝： TSSOP-20
文件頁(yè)數(shù)： 5/19頁(yè)
文件大小： 597K
代理商： LM4843MHX/NOPB
Application Information (Continued)
Higher value capacitors need more time to reach a quiescent
DC voltage (usually V
DD/2) when charged with a fixed cur-
rent. The amplifier’s output charges the input capacitor
through the feedback resistor, R
f. 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,RIL = 20k) ( and
the input capacitor (C
IR,CIL = 0.33F) 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 0.063F. The 0.33F input coupling capacitor shown
in Figure 2 allows the LM4843 to drive a high efficiency, full
range speaker whose response extends below 30Hz.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4843 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 LM4843’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
DD . As soon as the voltage
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
B alters the device’s turn-on time and
the magnitude of “clicks and pops”. Increasing the value of
C
B reduces the magnitude of turn-on pops. However, this
presents a tradeoff: as the size of C
B increases, the turn-on
time increases. There is a linear relationship between the
size of C
B and the turn-on time. Here are some typical
turn-on times for various values of C
B:
C
B
T
ON
0.01F
2ms
0.1F
20ms
0.22F
44ms
0.47F
94ms
1.0F
200ms
MICRO-POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the
LM4843’s shutdown function. Activate micro-power shut-
down by applying V
DD to the SHUTDOWN pin. When active,
the LM4843’s micro-power shutdown feature turns off the
amplifier’s bias circuitry, reducing the supply current. The
logic threshold is typically V
DD/2. The low 0.7 A typical
shutdown current is achieved by applying a voltage that is as
near as V
DD as possible to the SHUTDOWN pin. A voltage
that is less than V
DD may increase the shutdown current.
There are a few ways to control the micro-power shutdown.
These include using a single-pole, single-throw switch, a
microprocessor, or a microcontroller. When using a switch,
connect an external 10k
pull-up resistor between the
SHUTDOWN pin and V
DD. Connect the switch between the
SHUTDOWN pin and ground. Select normal amplifier opera-
tion by closing the switch. Opening the switch connects the
SHUTDOWN pin to V
DD through the pull-up resistor, activat-
ing micro-power shutdown. The switch and resistor guaran-
tee that the SHUTDOWN pin will not float. This prevents
unwanted state changes. In a system with a microprocessor
or a microcontroller, use a digital output to apply the control
voltage to the SHUTDOWN pin. Driving the SHUTDOWN pin
with active circuitry eliminates the need for a pull up resistor.
DC VOLUME CONTROL
The LM4843 has an internal stereo volume control whose
setting is a function of the DC voltage applied to the DC VOL
CONTROL pin.
The LM4843 volume control consists of 31 steps that are
individually selected by a variable DC voltage level on the
volume control pin. The range of the steps, controlled by the
DC voltage, are from 0dB - 78dB. Each attenuation step
corresponds to a specific input voltage range, as shown in
table 2.
To minimize the effect of noise on the volume control pin,
which can affect the selected attenuation level, hysteresis
has been implemented. The amount of hysteresis corre-
sponds to half of the step width, as shown in Volume Control
Characterization Graph (DS200133-40).
For highest accuracy, the voltage shown in the ’recom-
mended voltage’ column of the table is used to select a
desired attenuation level. This recommended voltage is ex-
actly halfway between the two nearest transitions to the next
highest or next lowest attenuation levels.
The attenuation levels are 1dB/step from 0dB to -6dB, 2dB/
step from -6dB to -36dB, 3dB/step from -36dB to -47dB,
4dB/step from -47db to -51dB, 5dB/step from -51dB to
-66dB, and 12dB to the last step at -78dB.
LM4843
www.national.com
13
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