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參數(shù)資料
| 型號： | LM4875MM |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻控制 |
| 英文描述： | 750 mW Audio Power Amplifier with DC Volume Control and Headphone Switch |
| 中文描述： | 1 CHANNEL(S), VOLUME CONTROL CIRCUIT, PDSO8 |
| 封裝： | 0.118 INCH, MSOP-8 |
| 文件頁數(shù)： | 9/13頁 |
| 文件大?。?/td> | 673K |
| 代理商： | LM4875MM |
Application Information
(Continued)
While the LM4875 operates in bridged mode, the DC poten-
tial across the load is essentially 0V. Since the HP-Sense
threshold is set at 4V, even in an ideal situation, the output
swing cannot cause a false single-ended trigger. Connecting
headphones to the headphone jack disconnects the head-
phone jack contact pin from V
1 and allows R1 to pull the
HP Sense pin up to V
CC
. This enables the headphone func-
tion, turns off Amp2, and mutes the bridged speaker. The
amplifier then drives the headphones, whose impedance is
in parallel with resistor R2. Resistor R2 has negligible effect
on output drive capability since the typical impedance of
headphones is 32
. The output coupling capacitor blocks
the amplifier’s half supply DC voltage, protecting the head-
phones.
A microprocessor or a switch can replace the headphone
jack contact pin. When a microprocessor or switch applies a
voltage greater than 4V to the HP Sense pin, a bridge-
connected speaker is muted and Amp1 drives the head-
phones.
SELECTING EXTERNAL COMPONENTS
Optimizing the LM4875’s performance requires properly se-
lecting external components. Though the LM4875 operates
well when using external components having wide toler-
ances, the best performance is achieved by optimizing com-
ponent values.
Input Capacitor Value Selection
Amplification of the lowest audio frequencies requires high
value input coupling capacitors. These high value capacitors
can be expensive and may compromise space efficiency in
portable designs. In many cases, however, the speakers
used in portable systems, whether internal or external, have
little ability to reproduce signals below 150Hz. In application
5 using speakers with this limited frequency response, a
large input capacitor will offer little improvement in system
performance.
Figure 1
shows that the nominal input impedance (R
IN
) is
10k
at maximum volume and 110k
at minimum volume.
Together, the input capacitor, C
i
, and R
IN
, produce a -3dB
high pass filter cutoff frequency that is found using Equation
(4).
(4)
As the volume changes from minimum to maximum, R
IN
decrease from 110k
to 10k
. Equation (4) reveals that the
-3dB frequency will increase as the volume increases. The
nominal value of C
for lowest desired frequency response
should be calculated with R
= 10k
. As an example when
using a speaker with a low frequency limit of 150Hz, C
i
,
using Equation (4) is 0.1μF. The 0.22μF C
i
shown in
Figure 1
is optimized for a speaker whose response extends down to
75Hz.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to value of the bypass capacitor C
B
.
Since C
B
determines how fast the LM4875 turns on, its value
is the most critical when minimizing turn-on pops. The slower
the LM4875’s outputs ramp to their quiescent DC voltage
(nominally V
/2), the smaller the turn-on pop. Choosing C
B
equal to 1.0μF, along with a small value of C
(in the range of
0.1μF to 0.39μF), produces a clickless and popless shut-
down function. Choosing C
i
as small as possible helps mini-
mize clicks and pops.
CLICK AND POP CIRCUITRY
The LM4875 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 LM4875’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 and the gain is set by the external voltage applied to
the DC Vol/SD pin.
Although the bypass pin current cannot be modified, chang-
ing 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 pre-
sents a tradeoff: as the size of C
increases, the turn-on time
increases. There is a linear relationship between the size of
CB and the turn-on time. Shown below are some typical
turn-on times for various values of C
B
:
C
B
0.01μF
0.1μF
0.22μF
0.47μF
1.0μF
T
ON
3ms
30ms
65ms
135ms
280ms
In order eliminate "clicks and pops", all capacitors must be
discharged before turn-on. Rapidly switching V
may not
allow the capacitors to fully discharge, which may cause
"clicks and pops". In a single-ended configuration, the output
coupling capacitor, C
, is of particular concern. This ca-
pacitor discharges through an internal 20k
resistor. De-
pending on the size of C
, the time constant can be
relatively large. To reduce transients in single-ended mode,
10104234
FIGURE 3. Headphone Circuit
L
www.national.com
9
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| LM4875MM/NOPB | 功能描述:音頻放大器 RoHS:否 制造商:STMicroelectronics 產(chǎn)品:General Purpose Audio Amplifiers 輸出類型:Digital 輸出功率: THD + 噪聲: 工作電源電壓:3.3 V 電源電流: 最大功率耗散: 最大工作溫度: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:TQFP-64 封裝:Reel |
| LM4875MX | 功能描述:音頻放大器 RoHS:否 制造商:STMicroelectronics 產(chǎn)品:General Purpose Audio Amplifiers 輸出類型:Digital 輸出功率: THD + 噪聲: 工作電源電壓:3.3 V 電源電流: 最大功率耗散: 最大工作溫度: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:TQFP-64 封裝:Reel |
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| LM4876 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:1.1W Audio Power Amplifier with Logic Low Shutdown |
| LM4876_03 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:1.1W Audio Power Amplifier with Logic Low Shutdown |
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