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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： | 4/16页 |
| 文件大小： | 640K |
| 代理商： | LM4731TA/NOPB |
Application Information (Continued)
To achieve a transient free power-up and power-down, the
voltage seen at the input terminals should be ideally the
same. Such a signal will be common-mode in nature, and
will be rejected by the LM4731. In Figure 3, the resistor R
INP
serves to keep the inputs at the same potential by limiting the
voltage difference possible between the two nodes. This
should significantly reduce any type of turn-on pop, due to an
uneven charging of the amplifier inputs. This charging is
based on a specific application loading and thus, the system
designer may need to adjust these values for optimal perfor-
mance.
As shown in Figure 3, the resistors labeled R
BI help bias up
the LM4731 off the half-supply node at the emitter of the
2N3904. But due to the input and output coupling capacitors
in the circuit, along with the negative feedback, there are two
different values of R
BI, namely 10 k
and 200 k. These
resistors bring up the inputs at the same rate resulting in a
popless turn-on. Adjusting these resistors values slightly
may reduce pops resulting from power supplies that ramp
extremely quick or exhibit overshoot during system turn-on.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components is required to meet
the design targets of an application. The choice of external
component values that will affect gain and low frequency
response are discussed below.
The gain of each amplifier is set by resistors R
f and Ri for the
non-inverting configuration shown in Figure 1. The gain is
found by Equation (4) below:
A
V =1+Rf /Ri (V/V)
(4)
For best noise performance, lower values of resistors are
used. A value of 1k
is commonly used for R
i and then
setting the value of R
f for the desired gain. For the LM4731
the gain should be set no lower than 10V/V and no higher
than 50V/V. Gain settings below 10V/V may experience
instability and using the LM4731 for gains higher than 50V/V
will see an increase in noise and THD.
The combination of R
pass filter. The low frequency response is determined by
these two components. The -3dB point can be found from
Equation (5) shown below:
f
i =1/(2
πR
iCi) (Hz)
(5)
If an input coupling capacitor is used to block DC from the
inputs as shown in Figure 4, there will be another high pass
filter created with the combination of C
IN and RIN. When
using a input coupling capacitor R
IN is needed to set the DC
bias point on the amplifier’s input terminal. The resulting
-3dB frequency response due to the combination of C
IN and
R
IN can be found from Equation (6) shown below:
f
IN =1/(2
πR
INCIN) (Hz)
(6)
PHYSICAL IC MOUNTING CONSIDERATIONS
Mounting of the TO-220 package to a heat sink must be
done such that there is sufficient pressure from the mounting
screw to insure good contact with the heat sink for efficient
heat flow. Over tightening the mounting screw will cause the
TO-220 package to warp reducing contact area with the heat
sink. Less contact with the heat sink will increase the thermal
resistance from the TO-220 package case to the heat sink
(
θ
CS) resulting in higher operating die temperatures and
possible unwanted thermal shut down activation. Extreme
over tightening of the mounting screw will cause severe
physical stress resulting in cracked die and catastrophic IC
failure. The recommended maximum mounting screw torque
is 40 inch-lbs or 3.3 foot-lbs (4.5 newton-meter).
Additionally, if the mounting screw is used to force the TO-
220 package into correct alignment with the heat sink, pack-
age stress will be increased. This increase in package stress
will result in reduced contact area with the heat sink increas-
ing die operating temperature and possible catastrophic IC
failure.
LM4731
www.national.com
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