參數(shù)資料
型號(hào)： LM4781
廠商： National Semiconductor Corporation
英文描述： Overture⑩ Audio Power Amplifier Series 3 Channel 35W Audio Power Amplifier with Mute
中文描述： 序曲⑩音頻功率放大器系列三通道35W的音頻功率放大器靜音
文件頁(yè)數(shù)： 16/25頁(yè)
文件大?。?/td> 1125K
代理商： LM4781
Application Information
(Continued)
tics
section shows the area of device operation where
SPiKe
Protection Circuitry is not enabled. The SPiKe Pro-
tection Response waveform graph shows the waveform dis-
tortion when SPiKe is enabled. Please refer to AN-898 for
more detailed information.
THERMAL PROTECTION
The LM4781 has a sophisticated thermal protection scheme
to prevent long-term thermal stress of the device. When the
temperature on the die exceeds 150C, the LM4781 shuts
down. It starts operating again when the die temperature
drops to about 145C, but if the temperature again begins to
rise, shutdown will occur again above 150C. Therefore, the
device is allowed to heat up to a relatively high temperature
if the fault condition is temporary, but a sustained fault will
cause the device to cycle in a Schmitt Trigger fashion be-
tween the thermal shutdown temperature limits of 150C and
145C. This greatly reduces the stress imposed on the IC by
thermal cycling, which in turn improves its reliability under
sustained fault conditions.
Since the die temperature is directly dependent upon the
heat sink used, the heat sink should be chosen so that
thermal shutdown is not activated during normal operation.
Using the best heat sink possible within the cost and space
constraints of the system will improve the long-term reliability
of any power semiconductor device, as discussed in the
Determining the Correct Heat Sink
section.
DETERMlNlNG MAXIMUM POWER DISSIPATION
Power dissipation within the integrated circuit package is a
very important parameter requiring a thorough understand-
ing if optimum power output is to be obtained. An incorrect
maximum power dissipation calculation may result in inad-
equate heat sinking causing thermal shutdown and thus
limiting the output power.
Equation (2)
shows the theoretical maximum power dissipa-
tion point for each amplifier in a single-ended configuration
where V
CC
is the total supply voltage.
P
DMAX
= (V
CC
)
2
/ 2
π
2
R
L
(2)
Thus by knowing the total supply voltage and rated output
load, the maximum power dissipation point can be calcu-
lated. The package dissipation is three times the number
which results from
Equation (2)
since there are three ampli-
fiers in each LM4781. Refer to the graphs of Power Dissipa-
tion versus Output Power in the
Typical Performance Char-
acteristics
section which show the actual full range of power
dissipation not just the maximum theoretical point that re-
sults from
Equation (2)
.
DETERMINING THE CORRECT HEAT SINK
The choice of a heat sink for a high-power audio amplifier is
made entirely to keep the die temperature at a level such
that the thermal protection circuitry is not activated under
normal circumstances.
The thermal resistance from the die to the outside air,
θ
JA
(junction to ambient), is a combination of three thermal re-
sistances,
θ
(junction to case),
θ
(case to sink), and
θ
(sink to ambient). The thermal resistance,
θ
(junction to
case), of the LM4781T is 0.9C/W. Using Thermalloy Ther-
macote thermal compound, the thermal resistance,
θ
(case to sink), is about 0.2C/W. Since convection heat flow
(power dissipation) is analogous to current flow, thermal
resistance is analogous to electrical resistance, and tem-
perature drops are analogous to voltage drops, the power
dissipation out of the LM4781 is equal to the following:
P
DMAX
= (T
JMAX
T
AMB
) /
θ
JA
(3)
where T
JMAX
= 150C, T
AMB
is the system ambient tempera-
ture and
θ
JA
=
θ
JC
+
θ
CS
+
θ
SA
.
200673B8
Once the maximum package power dissipation has been
calculated using Equation 2, the maximum thermal resis-
tance,
θ
, (heat sink to ambient) in C/W for a heat sink can
be calculated. This calculation is made using Equation 4
which is derived by solving for
θ
SA
in Equation 3.
θ
SA
= [(T
JMAX
T
AMB
)P
DMAX
(
θ
JC
+
θ
CS
)] / P
DMAX
(4)
Again it must be noted that the value of
θ
is dependent
upon the system designer’s amplifier requirements. If the
ambient temperature that the audio amplifier is to be working
under is higher than 25C, then the thermal resistance for the
heat sink, given all other things are equal, will need to be
smaller.
SUPPLY BYPASSING
The LM4781 has excellent power supply rejection and does
not require a regulated supply. However, to improve system
performance as well as eliminate possible oscillations, the
LM4781 should have its supply leads bypassed with low-
inductance capacitors having short leads that are located
close to the package terminals. Inadequate power supply
bypassing will manifest itself by a low frequency oscillation
known as “motorboating” or by high frequency instabilities.
These instabilities can be eliminated through multiple by-
passing utilizing a large tantalum or electrolytic capacitor
(10μF or larger) which is used to absorb low frequency
variations and a small ceramic capacitor (0.1μF) to prevent
any high frequency feedback through the power supply lines.
If adequate bypassing is not provided, the current in the
supply leads which is a rectified component of the load
current may be fed back into internal circuitry. This signal
causes distortion at high frequencies requiring that the sup-
plies be bypassed at the package terminals with an electro-
lytic capacitor of 470μF or more.
BRIDGED AMPLIFIER APPLICATION
The LM4781 has three operational amplifiers internally, al-
lowing for a few different amplifier configurations. One of
these configurations is referred to as “bridged mode” and
involves driving the load differentially through two of the
LM4781’s outputs. This configuration is shown in
Figure 2
.
Bridged mode operation is different from the classical single-
ended amplifier configuration where one side of its load is
connected to ground.
A bridge amplifier design has a distinct advantage over the
single-ended configuration, as it provides differential drive to
the load, thus doubling output swing for a specified supply
voltage. Theoretically, four times the output power is pos-
L
www.national.com
16
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參數(shù)描述
LM4781TA 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
LM4781TA/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
LM4781TABD 功能描述:音頻 IC 開發(fā)工具 LM4871 EVAL BOARD RoHS:否 制造商:Texas Instruments 產(chǎn)品:Evaluation Kits 類型:Audio Amplifiers 工具用于評(píng)估:TAS5614L 工作電源電壓:12 V to 38 V
LM4781TABD/NOPB 功能描述:音頻 IC 開發(fā)工具 LM4781TABD EVAL MOD RoHS:否 制造商:Texas Instruments 產(chǎn)品:Evaluation Kits 類型:Audio Amplifiers 工具用于評(píng)估:TAS5614L 工作電源電壓:12 V to 38 V
LM4782 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Overture Audio Power Amplifier Series 3 Channel 25W Audio Power Amplifier with Mute and Standby
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