參數(shù)資料
型號(hào)： LM4840MTX/NOPB
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 音頻控制
英文描述： 2 CHANNEL(S), VOLUME CONTROL CIRCUIT, PDSO28
封裝： TSSOP-28
文件頁(yè)數(shù)： 4/22頁(yè)
文件大?。?/td> 829K
代理商： LM4840MTX/NOPB
Application Information (Continued)
severe attenuation. In many cases the speakers used in
portable systems, whether internal or external, have little
ability to reproduce signals below 100 Hz–150 Hz. In this
case, usinga large input or output capacitor may not in-
crease system performance.
In addition to system cost and size, click and pop perfor-
mance is effected by the size of the input coupling capacitor,
C
i. A larger input coupling capacitor requires more charge to
reach its quiescent DC voltage (nominally 1/2 V
DD.) This
charge comes from the output through the feedback and is
apt to create pops once the device is enabled. By minimizing
the capacitor size based on necessary low frequency re-
sponse, turn-on pops can be minimized.
CLICK AND POP CIRCUITRY
The LM4840 contains circuitry to minimize turn-on transients
or “click and pops”. In this case, turn-on refers to either
power supply turn-on or the device coming out of shutdown
mode. When the device is turning on, the amplifiers are
internally muted. An internal current source ramps up the
voltage of the bypass pin. Both the inputs and outputs ideally
track the voltage at the bypass pin. The device will remain in
mute mode until the bypass pin has reached its half supply
voltage, 1/2 V
DD. As soon as the bypass node is stable, the
device will become fully operational.
Although the bypass pin current source cannot be modified,
the size of the bypass capacitor, C
B, can be changed to alter
the device turn-on time and the amount of “click and pop”. By
increasing C
B, the amount of turn-on pop can be reduced.
However, the trade-off for using a larger bypass capacitor is
an increase in the turn-on time for the device. Reducing C
B
will decrease turn-on time and increase “click and pop”.
There is a linear relationship between the size of C
B and the
turn-on time. Here are some typical turn-on times for differ-
ent values of C
B:
C
B
T
ON
0.01 F
2 ms
0.1 F
20 ms
0.22 F
42 ms
0.47 F
84 ms
1.0 F
200 ms
4.7 F
1sec
In order to eliminate “click and pop”, all capacitors must be
discharged before turn-on. Rapid on/off switching of the
device or shutdown function may cause the “click and pop”
circuitry to not operate fully, resulting in increased “click and
pop” noise.
DOCKING STATION
In an application such as a notebook computer, docking
station or line level outputs may be required. Pin 9 and Pin
13 can drive loads greater than 1k
rail to rail. These pins
are tied to the output of the input op-amp to drive powered
speakers and other high impedance loads. Output coupling
capacitors need to be placed in series with the load. The
recommended values of the capacitors are between 0.33F
to 1.0F with the positive side of the capacitors toward the
IC. The outputs of the docking station pins cannot be attenu-
ated with the DC volume control. However the gain of the
outputs can be configured by adjusting the feedback and
input resistors for the input op-amp. The input op-amp is in
an inverting configuration where the gain is:
R
F /Ri =- Av
Note that by adjusting the gain of the input op-amp the
overall gain of the output amplifiers are also affected. Al-
though the single ended outputs of the output amplifiers can
be used to drive line level outputs, it is recommended to use
Pins 9 and 13 to achieve better performance.
BEEP DETECT FUNCTION
The Beep Detect pin (Beep In) is a mono input that detects
the presence of a beep signal. When a signal greater than
2.5V
P-P
(or 1/2 V
DD) is present at Beep In, the Beep Detect
circuitry will enable the bridged amplifiers. Beep In signals
less than 2.5V
P-P (or 1/2 VDD) will not trigger the Beep
Detect circuitry. When triggered, the Beep Detect circuitry
will enable the bridged amplifiers regardless of the state of
the mute, mode, or HP sense pins. As shown in the Fig. 1, a
200k
resistor is placed in series with the input capacitor.
This 200k
resistor can be changed to vary the amplitude of
the beep in signal. Higher values of the resistor will reduce
the amplifier gain and attenuate the beep in signal. These
resistors are required in order for the beep signal to pass to
the output. The Beep Detect pin will not pass the beep signal
to the output. In cases where system beeps are required
when the system is in a suspended mode, the LM4840 must
be brought out of shutdown before the beep in signal is input.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4840 contains a shutdown pin to externally turn off the
bias circuitry. The LM4840 will shutdown when a logic high is
placed on the shutdown pin. The trigger point between a
logic low and logic high level is typically half supply. It is best
to switch between ground and the supply V
DD to provide
maximum device performance. By switching the shutdown
pin to V
DD, the LM4840 supply current draw will be mini-
mized. While the device will be disabled with shutdown pin
voltages less than V
DD, the idle current may be greater than
the typical value of 0.7 A.The shutdown pin should not be
floated, since this may result in an unwanted shutdown
condition.
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry which pro-
vides a quick, smooth transition into shutdown. Another so-
lution is to use a single-pole, single-throw switch in conjuc-
tion with an external pull-up resistor. When the switch is
closed,the shutdown pin is connected to ground and enables
the amplifier. If the switch is open, then the external pull-up
resistor will shutdown the LM4840. This scheme prevents
the shutdown pin from floating.
HP-IN FUNCTION
An internal pullup circuit is connected to the HPSense
headphone amplifier control pin. When this pin is left uncon-
nected, V
DD is applied to the HPSense. This turns off
Amp2B and switches Amp2A’s input signal from an audio
signal to the V
DD/2 voltage present on Bypass. The result is
muted bridge-connected loads. Quiescent current consump-
tion is reduced when the IC is in this singleended mode.
Figure 2 shows the implementation of the LM4840’s head-
phone control function. An internal comparator with a nomi-
nal 400mV offset monitors the signal present at the OUTB
output. It compares this signal against the signal applied to
the HPSense pin. When these signals are equal, as is the
LM4840
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