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參數(shù)資料
| 型號(hào)： | LM4780 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Audio Power Amplifier Series Stereo 60W, Mono 120W Audio Power Amplifier with Mute |
| 中文描述： | 音頻功率放大器系列立體聲60瓦，120瓦單聲道音頻功率放大器與靜音 |
| 文件頁數(shù)： | 16/24頁 |
| 文件大小： | 1030K |
| 代理商： | LM4780 |
Application Information
(Continued)
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
(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 LM4780T is 0.8C/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 LM4780 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
JA
=
θ
JC
+
θ
CS
θ
SA
.
20058652
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 LM4780 has excellent power supply rejection and does
not require a regulated supply. However, to improve system
performance as well as eliminate possible oscillations, the
LM4780 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 LM4780 has two operational amplifiers internally, allow-
ing for a few different amplifier configurations. One of these
configurations is referred to as “bridged mode” and involves
driving the load differentially through the LM4780’s outputs.
This configuration is shown in
Figure 2
. Bridged mode op-
eration 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-
sible as compared to a single-ended amplifier under the
same conditions. This increase in attainable output power
assumes that the amplifier is not current limited or clipped.
A direct consequence of the increased power delivered to
the load by a bridge amplifier is an increase in internal power
dissipation. For each operational amplifier in a bridge con-
figuration, the internal power dissipation will increase by a
factor of two over the single ended dissipation. Thus, for an
audio power amplifier such as the LM4780, which has two
operational amplifiers in one package, the package dissipa-
tion will increase by a factor of four. To calculate the
LM4780’s maximum power dissipation point for a bridged
load, multiply
Equation (2)
by a factor of four.
This value of P
can be used to calculate the correct size
heat sink for a bridged amplifier application. Since the inter-
nal dissipation for a given power supply and load is in-
creased by using bridged-mode, the heatsink’s
θ
will have
to decrease accordingly as shown by
Equation (4)
. Refer to
the section,
Determining the Correct Heat Sink,
for a more
detailed discussion of proper heat sinking for a given appli-
cation.
PARALLEL AMPLIFIER APPLICATION
Parallel configuration is normally used when higher output
current is needed for driving lower impedance loads (i.e. 4
or lower) to obtain higher output power levels. As shown in
Figure 3
, the parallel amplifier configuration consist of de-
signing the amplifiers in the IC to have identical gain, con-
necting the inputs in parallel and then connecting the outputs
in parallel through a small external output resistor. Any num-
ber of amplifiers can be connected in parallel to obtain the
needed output current or to divide the power dissipation
across multiple IC packages. Ideally, each amplifier shares
the output current equally. Due to slight differences in gain
the current sharing will not be equal among all channels. If
current is not shared equally among all channels then the
power dissipation will also not be equal among all channels.
It is recommended that 0.1% tolerance resistors be used to
set the gain (R
i
and R
f
) for a minimal amount of difference in
current sharing.
When operating two or more amplifiers in parallel mode the
impedance seen by each amplifier is equal to the total load
impedance multiplied by the number of amplifiers driving the
load in parallel as shown by
Equation (5)
below:
R
L(parallel)
= R
L(total)
* Number of amplifiers
Once the impedance seen by each amplifier in the parallel
configuration is known then Equation (2) can be used with
this calculated impedance to find the amount of power dis-
sipation for each amplifier. Total power dissipation (P
DMAX
)
within an IC package is found by adding up the power
dissipation for each amplifier in the IC package. Using the
calculated P
DMAX
the correct heat sink size can be deter-
(5)
L
www.national.com
16
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參數(shù)描述 |
|---|---|
| LM4780TA | 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 |
| LM4780TA | 制造商:Texas Instruments 功能描述:IC AMP 120W MONO/60W STEREO 4780 |
| LM4780TA/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 |
| LM4780TABD | 功能描述:音頻 IC 開發(fā)工具 LM4780 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 |
| LM4780TABD/NOPB | 功能描述:BOARD EVAL FOR LM4780A RoHS:是 類別:編程器，開發(fā)系統(tǒng) >> 評(píng)估板 - 音頻放大器 系列:Overture™ 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:- |
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