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參數(shù)資料
| 型號： | LM4834MWC |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻控制 |
| 英文描述： | 2 CHANNEL(S), VOLUME CONTROL CIRCUIT, UUC |
| 封裝： | WAFER |
| 文件頁數(shù)： | 3/15頁 |
| 文件大?。?/td> | 479K |
| 代理商： | LM4834MWC |
Application Information (Continued)
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 LM4834 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 inter-
nally configured as unity gain buffers. An internal current
source ramps up the voltage of the bypass pin. Both the in-
puts and outputs ideally track the voltage at the bypass pin.
The device will remain in buffer 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 opera-
tional.
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
20 ms
0.1 F
200 ms
0.22 F
420 ms
0.47 F
840 ms
1.0 F
2 sec
In order to eliminate “click and pop”, all capacitors must be
discharged before turn-on. Rapid on/off switching of the de-
vice or shutdown function may cause the “click and pop” cir-
cuitry to not operate fully, resulting in increased “click and
pop” noise.
In systems where the line out and headphone jack are the
same, the output coupling cap, C
O, is of particular concern.
C
Ois chosen for a desired cutoff frequency with a headphone
load. This desired cutoff frequency will change when the
headphone load is replaced by a high impedance line out
load(powered speakers). The input impedance of head-
phones are typically between 32
and 64. Whereas, the
input impedance of powered speakers can vary from 1k
top 100k
. As the RC time constant of the load and the out-
put coupling capacitor increases, the turn off transients are
increased.
To improve click and pop performance in this situation, exter-
nal resistors R6 and R7 should be added. The recom-
mended value for R6 is between 150
to 1k. The recom-
mended value for R7 is between 100
to 500. To achieve
virtually clickless and popless performance R6 = 150
,R7=
100
,C
O = 220F, and CB = 0.47F should be used. Lower
values of R6 will result in better click and pop performance.
However, it should be understood that lower resistance val-
ues of R6 will increase quiescent current.
LOW FREQUENCY ENHANCEMENT
In some cases a designer may want to improve the low fre-
quency response of the bridged amplifier. This low frequency
boost can be useful in systems where speakers are housed
in small enclosures. A resistor, R
LFE, and a capacitor, CLFE,
in parallel, can be placed in series with the feedback resistor
of the bridged amplifier as seen in
Figure 5.
At low frequencies the capacitor will be virtually an open cir-
cuit. At high frequencies the capacitor will be virtually a short
circuit. As a result of this, the gain of the bridge amplifier is
increased at low frequencies. A first order pole is formed with
a corner frequency at:
f
c = 1/(2πRLFECLFE)
The resulting low frequency differential gain of this bridged
amplifier becomes:
2(R
f +RLFE)/Ri =Avd
With R
F = 20k,RLFE = 20k, and CLFE = 0.068 F, a first
order pole is formed with a corner frequency of 120 Hz. At
low frequencies the differential gain will be 4, assuming R
S =
20k. The low frequency boost formulas assume that C
O,Ci,
f
IC,fOC allow the appropriate low frequency response.
DS100015-33
FIGURE 4. Resistors for Varying Output Loads
DS100015-32
FIGURE 5. Low Frequency Enhancement
LM4834
www.national.com
11
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