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參數(shù)資料
| 型號(hào)： | LM4917 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Ground-Referenced, 95mW Stereo Headphone Amplifier |
| 中文描述： | 地為參考，95mW立體聲耳機(jī)放大器 |
| 文件頁數(shù)： | 12/18頁 |
| 文件大?。?/td> | 1208K |
| 代理商： | LM4917 |
Application Information
(Continued)
operation by opening the switch. Closing the switch con-
nects the SD_LC/SD_RC pins to ground, activating micro-
power shutdown. The switch and resistor guarantee that the
SD_LC/SD_RC pins will not float. This prevents unwanted
state changes. In a system with a microprocessor or micro-
controller, use a digital output to apply the control voltage to
the SD_LC/SD_RC pins. Driving the SD_LC/SD_RC pins
with active circuitry eliminates the pull-up resistor.
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4917’s performance requires properly se-
lecting external components. Though the LM4917 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues.
The LM4917 is unity-gain stable, giving a designer maximum
design flexibility. The gain should be set to no more than a
given application requires. This allows the amplifier to
achieve minimum THD+N and maximum signal-to-noise ra-
tio. These parameters are compromised as the closed-loop
gain increases. However, low gain demands input signals
with greater voltage swings to achieve maximum output
power. Fortunately, many signal sources such as audio
CODECs have outputs of 1V
(2.83V
P-P
). Please refer to
the
Audio Power Amplifier Design
section for more infor-
mation on selecting the proper gain.
Charge Pump Capacitor Selection
Choose low ESR (
<
100m
) ceramic capacitors for optimum
performance. Low ESR capacitors keep the charge pump
output impedance to a minimum, extending the headroom
on the negative supply. Choose capacitors with an X7R
dielectric for best performance over temperature.
Charge pump load regulation and output resistance is af-
fected by the value of the flying capacitor (C1). A larger
valued C1 improves load regulation and minimizes charge
pump output resistance. The switch on-resistance and ca-
pacitor ESR dominates the output resistance for capacitor
values above 2.2μF.
The output ripple is affected by the value and ESR of the
output capacitor (C2). Larger valued capacitors reduce out-
put ripple on the negative power supply. Lower ESR capaci-
tors minimizes the output ripple and reduces the output
resistance of the charge pump.
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitor (C
in Figure 2). A high value capaci-
tor 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. Applications
using speakers with this limited frequency response reap
little improvement by using high value input and output ca-
pacitors.
Besides affecting system cost and size, C
i
has an effect on
the LM4917’s click and pop performance. The magnitude of
the pop is directly proportional to the input capacitor’s size.
Thus, pops can be minimized by selecting an input capacitor
value that is no higher than necessary to meet the desired
3dB frequency.
As shown in Figure 2, the input resistor, R
and the input
capacitor, C
, produce a -3dB high pass filter cutoff frequency
that is found using Equation (3).
f
i-3dB
= 1 / 2
π
R
i
C
i
(3)
Also, careful consideration must be taken in selecting a
certain type of capacitor to be used in the system. Different
types of capacitors (tantalum, electrolytic, ceramic) have
unique performance characteristics and may affect overall
system performance.
AUDIO POWER AMPLIFIER DESIGN
Design a Dual 90mW/16
Audio Amplifier
Given:
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
90mW
16
1Vrms (max)
20k
100Hz–20kHz
±
0.50dB
The design begins by specifying the minimum supply voltage
necessary to obtain the specified output power. One way to
find the minimum supply voltage is to use the Output Power
vs Supply Voltage curve in the
Typical Performance Char-
acteristics
section. Another way, using Equation (5), is to
calculate the peak output voltage necessary to achieve the
desired output power for a given load impedance. For a
single-ended application, the result is Equation (5).
(4)
V
DD
≥
[2V
OPEAK
+ (V
DOTOP
+ V
DOBOT
)]
(5)
The Output Power vs Supply Voltage graph for a 16
load
indicates a minimum supply voltage of 3.1V. This is easily
met by the commonly used 3.3V supply voltage. The addi-
tional voltage creates the benefit of headroom, allowing the
LM4917 to produce peak output power in excess of 90mW
without clipping or other audible distortion. The choice of
supply voltage must also not create a situation that violates
maximum power dissipation as explained above in the
Power Dissipation
section. Remember that the maximum
power dissipation point from Equation (1) must be multiplied
by two since there are two independent amplifiers inside the
package. Once the power dissipation equations have been
addressed, the required gain can be determined from Equa-
tion (6).
(6)
Thus, a minimum gain of 1.2 allows the LM4917 to reach full
output swing and maintain low noise and THD+N perfro-
mance. For this example, let A
V
= 1.5.
The amplifiers overall gain is set using the input (R
i
) and
feedback (R
f
) resistors. With the desired input impedance
set at 20k
, the feedback resistor is found using Equation
(7).
A
V
= R
f
/ R
i
(7)
L
www.national.com
12
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| LM4917MT | Ground-Referenced, 95mW Stereo Headphone Amplifier |
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM4917MT | 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 |
| LM4917MT/NOPB | 功能描述:音頻放大器 95mW St Headphone Amp 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 |
| LM4917MTBD | 功能描述:BOARD EVALUATION LM4917MT RoHS:否 類別:編程器，開發(fā)系統(tǒng) >> 評(píng)估板 - 音頻放大器 系列:Boomer® 產(chǎn)品培訓(xùn)模塊:Lead (SnPb) Finish for COTS Obsolescence Mitigation Program 標(biāo)準(zhǔn)包裝:1 系列:- |
| LM4917MTX | 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 |
| LM4917MTX/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 |
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