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参数资料
| 型号： | LM4857GR/NOPB |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 消费家电 |
| 英文描述： | SPECIALTY CONSUMER CIRCUIT, PBGA49 |
| 文件页数： | 24/40页 |
| 文件大小： | 1948K |
| 代理商： | LM4857GR/NOPB |
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Application Information (Continued)
Best thermal performance is achieved with the largest prac-
tical copper heat sink area. If the heatsink and amplifier
share the same PCB layer, a nominal 2in
2 area is necessary
for 5V operation with a 4
load. Heatsink areas not placed
on the same PCB layer as the LM4857 should be 4in
2 for the
same supply voltage and load resistance. The last two area
recommendations apply for 25C ambient temperature. In-
crease the area to compensate for ambient temperatures
above 25C. In all circumstances and under all conditions,
the junction temperature must be held below 150C to pre-
vent activating the LM4857’s thermal shutdown protection.
An example PCB layout for the exposed-DAP SP package is
shown in the Demonstration Board Layout section. Further
detailed and specific information concerning PCB layout and
fabrication and mounting an SP (LLP) is found in National
Semiconductor’s AN1187.
PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS FOR DRIVING 3
AND 4 LOADS
Power dissipated by a load is a function of the voltage swing
across the load and the load’s impedance. As load imped-
ance decreases, load dissipation becomes increasingly de-
pendent on the interconnect (PCB trace and wire) resistance
between the amplifier output pins and the load’s connec-
tions. Residual trace resistance causes a voltage drop,
which results in power dissipated in the trace and not in the
load as desired. For example, 0.1
trace resistance reduces
the output power dissipated by a 4
load from 1.6W to 1.5W.
The problem of decreased load dissipation is exacerbated
as load impedance decreases. Therefore, to maintain the
highest load dissipation and widest output voltage swing,
PCB traces that connect the output pins to a load must be as
wide as possible.
Poor power supply regulation adversely affects maximum
output power. A poorly regulated supply’s output voltage
decreases with increasing load current. Reduced supply
voltage causes decreased headroom, output signal clipping,
and reduced output power. Even with tightly regulated sup-
plies, trace resistance creates the same effects as poor
supply regulation. Therefore, making the power supply
traces as wide as possible helps maintain full output voltage
swing.
BRIDGE CONFIGURATION EXPLANATION
The LM4857 consists of three sets of a bridged-tied amplifier
pairs that drive the left loudspeaker (LLS), the right loud-
speaker (RLS), and the mono earpiece (EP). For this discus-
sion, only the LLS bridge-tied amplifier pair will be referred
to. The LM4857 drives a load, such as a speaker, connected
between outputs, LLS+ and LLS-. In the LLS amplifier block,
the output of the amplifier that drives LLS- serves as the
input to the unity gain inverting amplifier that drives LLS+.
This results in both amplifiers producing signals identical in
magnitude, but 180 out of phase. Taking advantage of this
phase difference, a load is placed between LLS- and LLS+
and driven differentially (commonly referred to as ’bridge
mode’). This results in a differential or BTL gain of:
A
VD = 2(Rf /Ri)=2
(2)
Both the feedback resistor, R
f, and the input resistor, Ri, are
internally set.
Bridge mode amplifiers are different from single-ended am-
plifiers that drive loads connected between a single amplifi-
er’s output and ground. For a given supply voltage, bridge
mode has a distinct advantage over the single-ended con-
figuration: its differential output doubles the voltage swing
across the load. Theoretically, this produces four times the
output power when compared to a single-ended amplifier
under the same conditions. This increase in attainable output
power assumes that the amplifier is not current limited and
that the output signal is not clipped.
Another advantage of the differential bridge output is no net
DC voltage across the load. This is accomplished by biasing
LLS- and LLS+ outputs at half-supply. This eliminates the
coupling capacitor that single supply, single-ended amplifiers
require. Eliminating an output coupling capacitor in a typical
single-ended configuration forces a single-supply amplifier’s
half-supply bias voltage across the load. This increases
internal IC power dissipation and may permanently damage
loads such as speakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful single-ended or bridged amplifier.
A direct consequence of the increased power delivered to
the load by a bridge amplifier is higher internal power dissi-
pation. The LM4857 has 3 sets of bridged-tied amplifier pairs
driving LLS, RLS, and EP. The maximum internal power
dissipation operating in the bridge mode is twice that of a
single-ended amplifier. From Equation (3) and (4), assuming
a 5V power supply and an 8
load, the maximum power
dissipation for LLS and RLS is 634mW per channel. From
equation (5), assuming a 5V power supply and a 32
load,
the maximum power dissipation for EP is 158mW.
P
DMAX-LLS = 4(VDD)
2 / (2
π2 R
L): Bridged
(3)
P
DMAX-RLS = 4(VDD)
2 / (2
π2 R
L): Bridged
(4)
P
DMAX-EP = 4(VDD)
2 / (2
π2 R
L): Bridged
(5)
The LM4857 also has 3 sets of single-ended amplifiers
driving LHP, RHP, and LINEOUT. The maximum internal
power dissipation for ROUT and LOUT is given by equation
(6) and (7). From Equations (6) and (7), assuming a 5V
power supply and a 32
load, the maximum power dissipa-
tion for LOUT and ROUT is 40mW per channel. From equa-
tion (8), assuming a 5V power supply and a 5k
load, the
maximum power dissipation for LINEOUT is negligible.
P
DMAX-LHP =(VDD)
2 /(2
π2 R
L): Single-ended
(6)
P
DMAX-RHP =(VDD)
2 /(2
π2 R
L): Single-ended
(7)
P
DMAX-LINE =(VDD)
2 /(2
π2 R
L): Single-ended
(8)
The maximum internal power dissipation of the LM4857
occurs during output modes 3, 8, and 13 when both loud-
speaker and headphone amplifiers are simultaneously on;
and is given by Equation (9).
P
DMAX-TOTAL =
P
DMAX-LLS +PDMAX-RLS +PDMAX-LHP +PDMAX-RHP
(9)
LM4857
www.national.com
30
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LM4857ITL | 制造商:Texas Instruments 功能描述:1.2W STEREO SUBSYST 3D SMD 4857 |
| LM4857ITL/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 |
| LM4857ITLBD | 功能描述:BOARD EVALUATION LM4857ITL RoHS:否 类别:编程器，开发系统 >> 评估板 - 音频放大器 系列:Boomer® 产品培训模块:Lead (SnPb) Finish for COTS Obsolescence Mitigation Program 标准包装:1 系列:- |
| LM4857ITLX/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 |
| LM4857SP | 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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