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参数资料
| 型号： | LM4731TA/NOPB |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 音频／视频放大 |
| 英文描述： | 25 W, 2 CHANNEL, AUDIO AMPLIFIER, PZFM15 |
| 封装： | PLASTIC, TO-220, 15 PIN |
| 文件页数： | 3/16页 |
| 文件大小： | 640K |
| 代理商： | LM4731TA/NOPB |
Application Information (Continued)
These instabilities can be eliminated through multiple by-
passing utilizing a large tantalum or electrolytic capacitor (10
F or larger) which is used to absorb low frequency varia-
tions and a small ceramic capacitor (0.1 F) 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 470 F or more.
BRIDGED AMPLIFIER APPLICATION
The LM4731 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 LM4731’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. Consequently, theoretically four times the output
power is possible 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 LM4731, which has two
operational amplifiers in one package, the package dissipa-
tion will increase by a factor of four. To calculate the
LM4731’s maximum power dissipation point for a bridged
load, multiply equation (1) by a factor of four.
This value of P
DMAX 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
SA will have
to decrease accordingly as shown by equation (3). Refer to
the section, Determining the Correct Heat Sink, for a more
detailed discussion of proper heat sinking for a given appli-
cation.
SINGLE-SUPPLY AMPLIFIER APPLICATION
The typical application of the LM4731 is a split supply am-
plifier. But as shown in Figure 3, the LM4731 can also be
used in a single power supply configuration. This involves
using some external components to create a half-supply bias
which is used as the reference for the inputs and outputs.
Thus, the signal will swing around half-supply much like it
swings around ground in a split-supply application. Along
with proper circuit biasing, a few other considerations must
be accounted for to take advantage of all of the LM4731
functions.
The LM4731 possesses a mute and standby function with
internal logic gates that are half-supply referenced. Thus, to
enable either the Mute or Standby function, the voltage at
these pins must be a minimum of 2.5V above half-supply. In
single-supply systems, devices such as microprocessors
and simple logic circuits used to control the mute and
standby functions, are usually referenced to ground, not
half-supply. Thus, to use these devices to control the logic
circuitry of the LM4731, a “level shifter,” like the one shown in
Figure 5, must be employed. A level shifter is not needed in
a split-supply configuration since ground is also half-supply.
When the voltage at the Logic Input node is 0V, the 2N3904
is “off” and thus resistor R
c pulls up mute or standby input to
the supply. This enables the mute or standby function. When
the Logic Input is 5V, the 2N3904 is “on” and consequently,
the voltage at the collector is essentially 0V. This will disable
the mute or standby function, and thus the amplifier will be in
its normal mode of operation. R
shift, along with Cshift, creates
an RC time constant that reduces transients when the mute
or standby functions are enabled or disabled. Additionally,
R
shift limits the current supplied by the internal logic gates of
the LM4731 which insures device reliability. Refer to the
Mute Mode and Standby Mode sections in the Application
Information section for a more detailed description of these
functions.
CLICKS AND POPS
In the typical application of the LM4731 as a split-supply
audio power amplifier, the IC exhibits excellent “click” and
“pop” performance when utilizing the mute and standby
modes. In addition, the device employs Under-Voltage Pro-
tection, which eliminates unwanted power-up and power-
down transients. The basis for these functions are a stable
and constant half-supply potential. In a split-supply applica-
tion, ground is the stable half-supply potential. But in a
single-supply application, the half-supply needs to charge up
just like the supply rail, V
CC. This makes the task of attaining
a clickless and popless turn-on more challenging. Any un-
even charging of the amplifier inputs will result in output
clicks and pops due to the differential input topology of the
LM4731.
20060354
FIGURE 5. Level Shift Circuit
LM4731
www.national.com
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