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参数资料
| 型号： | LM4865IBPX |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 音频控制 |
| 英文描述： | 1 CHANNEL(S), VOLUME CONTROL CIRCUIT, PBGA8 |
| 封装： | MICRO SMD-8 |
| 文件页数： | 2/16页 |
| 文件大小： | 512K |
| 代理商： | LM4865IBPX |
Application Information (Continued)
voltage applied to the HP-Sense pin (pin 3) at approximately
50mV. This 50mV enables the LM4865 and places it in
bridged mode operation.
While the LM4865 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
O1 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.
PROPERLY SELECTING EXTERNAL COMPONENTS
Optimizing the LM4865’s performance requires properly se-
lecting external components. Though the LM4865 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 RIN, 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
i for lowest desired frequency response
should be calculated with R
IN = 10k
. As an example when
using a speaker with a low frequency limit of 150Hz, C
i,
using Equation (4) is 0.1F. The 0.22F 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 LM4865 turns on, its value
is the most critical when minimizing turn-on pops. The slower
the LM4865’s outputs ramp to their quiescent DC voltage
(nominally V
DD/2), the smaller the turn-on pop. Choosing CB
equal to 1.0F, along with a small value of C
i (in the range of
0.1F to 0.39F), 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 LM4865 contains circuitry that minimizes turn-on and
shutdown transients or ’clicks and pops’. For this discussion,
turn-on refers to either applying the power supply voltage or
when the shutdown mode is deactivated. While the power
supply is ramping to its final value, the LM4865’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 operational 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
B 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
T
ON
0.01F
20ms
0.1F
200ms
0.22F
420ms
0.47F
840ms
1.0F
2sec
In order eliminate ’clicks and pops’, all capacitors must be
discharged before turn-on. Rapidly switching V
DD may not
allow the capacitors to fully discharge, which may cause
’clicks and pops’. In a single-ended configuration, the output
coupling capacitor, C
OUT, is of particular concern. This ca-
pacitor discharges through an internal 20k
resistor. De-
pending on the size of C
OUT, the time constant can be
relatively large. To reduce transients in single-ended mode,
10102534
FIGURE 3. Headphone Circuit
LM4865
www.national.com
10
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LM4865M | 功能描述:音频放大器 RoHS:否 制造商:STMicroelectronics 产品:General Purpose Audio Amplifiers 输出类型:Digital 输出功率: THD + 噪声: 工作电源电压:3.3 V 电源电流: 最大功率耗散: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:TQFP-64 封装:Reel |
| LM4865M | 制造商:Texas Instruments 功能描述:AMP AUDIO/VOLT CONTROL 0.75W 4865 |
| LM4865M/NOPB | 功能描述:音频放大器 RoHS:否 制造商:STMicroelectronics 产品:General Purpose Audio Amplifiers 输出类型:Digital 输出功率: THD + 噪声: 工作电源电压:3.3 V 电源电流: 最大功率耗散: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:TQFP-64 封装:Reel |
| LM4865MM | 功能描述:音频放大器 RoHS:否 制造商:STMicroelectronics 产品:General Purpose Audio Amplifiers 输出类型:Digital 输出功率: THD + 噪声: 工作电源电压:3.3 V 电源电流: 最大功率耗散: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:TQFP-64 封装:Reel |
| LM4865MM/NOPB | 功能描述:音频放大器 RoHS:否 制造商:STMicroelectronics 产品:General Purpose Audio Amplifiers 输出类型:Digital 输出功率: THD + 噪声: 工作电源电压:3.3 V 电源电流: 最大功率耗散: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:TQFP-64 封装:Reel |
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