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参数资料
| 型号： | LM4990ITL/NOPB |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 音频／视频放大 |
| 英文描述： | 2 W, 1 CHANNEL, AUDIO AMPLIFIER, PBGA9 |
| 封装： | MICRO SMD-9 |
| 文件页数： | 5/19页 |
| 文件大小： | 1124K |
| 代理商： | LM4990ITL/NOPB |
Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4990 has two internal opera-
tional amplifiers. The first amplifier’s gain is externally con-
figurable, while the second amplifier is internally fixed in a
unity-gain, inverting configuration. The closed-loop gain of
the first amplifier is set by selecting the ratio of R
f to Ri while
the second amplifier’s gain is fixed by the two internal 20k
resistors. Figure 1 shows that the output of amplifier one
serves as the input to amplifier two which results in both
amplifiers producing signals identical in magnitude, but out
of phase by 180. Consequently, the differential gain for the
IC is
A
VD= 2 *(Rf/Ri)
By driving the load differentially through outputs Vo1 and
Vo2, an amplifier configuration commonly referred to as
“bridged mode” is established. Bridged mode operation is
different from the classical single-ended amplifier configura-
tion where one side of the load is connected to ground.
A bridge amplifier design has a few distinct advantages over
the single-ended configuration, as it provides differential
drive to the load, thus doubling output swing for a specified
supply voltage. Four times the output power is possible as
compared to a single-ended amplifier under the same con-
ditions. This increase in attainable output power assumes
that the amplifier is not current limited or clipped. In order to
choose an amplifier’s closed-loop gain without causing ex-
cessive clipping, please refer to the Audio Power Amplifier
Design section.
A bridge configuration, such as the one used in LM4990,
also creates a second advantage over single-ended amplifi-
ers. Since the differential outputs, Vo1 and Vo2, are biased
at half-supply, no net DC voltage exists across the load. This
eliminates the need for an output coupling capacitor which is
required in a single supply, single-ended amplifier configura-
tion. Without an output coupling capacitor, the half-supply
bias across the load would result in both increased internal
IC power dissipation and also possible loudspeaker damage.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful amplifier, whether the amplifier is bridged or
single-ended. A direct consequence of the increased power
delivered to the load by a bridge amplifier is an increase in
internal power dissipation. Since the LM4990 has two opera-
tional amplifiers in one package, the maximum internal
power dissipation is 4 times that of a single-ended amplifier.
The maximum power dissipation for a given application can
be derived from the power dissipation graphs or from Equa-
tion 1.
P
DMAX = 4*(VDD)
2/(2
π2R
L)
(1)
It is critical that the maximum junction temperature T
JMAX of
150C is not exceeded. T
JMAX can be determined from the
power derating curves by using P
DMAX and the PC board foil
area. By adding copper foil, the thermal resistance of the
application can be reduced from the free air value of
θ
JA,
resulting in higher P
DMAX values without thermal shutdown
protection circuitry being activated. Additional copper foil can
be added to any of the leads connected to the LM4990. It is
especially effective when connected to V
DD, GND, and the
output pins. Refer to the application information on the
LM4990 reference design board for an example of good heat
sinking. If T
JMAX
still exceeds 150C, then additional
changes must be made. These changes can include re-
duced supply voltage, higher load impedance, or reduced
ambient temperature. Internal power dissipation is a function
of output power. Refer to the Typical Performance Charac-
teristics curves for power dissipation information for differ-
ent output powers and output loading.
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is critical for
low noise performance and high power supply rejection. The
capacitor location on both the bypass and power supply pins
should be as close to the device as possible. Typical appli-
cations employ a 5V regulator with 10F tantalum or elec-
trolytic capacitor and a ceramic bypass capacitor which aid
in supply stability. This does not eliminate the need for
bypassing the supply nodes of the LM4990. The selection of
a bypass capacitor, especially C
B, is dependent upon PSRR
requirements, click and pop performance (as explained in
the section, Proper Selection of External Components),
system cost, and size constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4990 contains shutdown circuitry that is used to turn off
the amplifier’s bias circuitry. In addition, the LM4990 con-
tains a Shutdown Mode pin (LD and MH packages only),
allowing the designer to designate whether the part will be
driven into shutdown with a high level logic signal or a low
level logic signal. This allows the designer maximum flexibil-
ity in device use, as the Shutdown Mode pin may simply be
tied permanently to either V
DD or GND to set the LM4990 as
either a "shutdown-high" device or a "shutdown-low" device,
respectively. The device may then be placed into shutdown
mode by toggling the Shutdown pin to the same state as the
Shutdown Mode pin. For simplicity’s sake, this is called
"shutdown same", as the LM4990 enters shutdown mode
whenever the two pins are in the same logic state. The MM
package lacks this Shutdown Mode feature, and is perma-
nently fixed as a ‘shutdown-low’ device. The trigger point for
either shutdown high or shutdown low is shown as a typical
value in the Supply Current vs Shutdown Voltage graphs in
the Typical Performance Characteristics section. It is best
to switch between ground and supply for maximum perfor-
mance. While the device may be disabled with shutdown
voltages in between ground and supply, the idle current may
be greater than the typical value of 0.1A. In either case, the
shutdown pin should be tied to a definite voltage to avoid
unwanted state changes.
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry, which pro-
vides a quick, smooth transition to shutdown. Another solu-
tion is to use a single-throw switch in conjunction with an
external pull-up resistor (or pull-down, depending on shut-
down high or low application). This scheme guarantees that
the shutdown pin will not float, thus preventing unwanted
state changes.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4990 is tolerant of
LM4990
www.national.com
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相关PDF资料 |
PDF描述 |
|---|---|
| LM4990ITLX/NOPB | 2 W, 1 CHANNEL, AUDIO AMPLIFIER, PBGA9 |
| LM4995SDX/NOPB | 1.25 W, 1 CHANNEL, AUDIO AMPLIFIER, DSO8 |
| LM5007MMEP/NOPB | SWITCHING REGULATOR, PDSO8 |
| LM5007MMEP | SWITCHING REGULATOR, PDSO8 |
| LM5060MMX | 1-CHANNEL POWER SUPPLY SUPPORT CKT, PDSO10 |
相关代理商/技术参数 |
参数描述 |
|---|---|
| LM4990ITLX | 制造商:NSC 制造商全称:National Semiconductor 功能描述:2 Watt Audio Power Amplifier with Selectable Shutdown Logic Level |
| LM4990ITLX/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 |
| LM4990LD | 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 |
| LM4990LD/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 |
| LM4990LDX | 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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