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參數資料
| 型號： | LM4801MH/NOPB |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | 1.5 W, 2 CHANNEL, AUDIO AMPLIFIER, PDSO28 |
| 封裝： | 4.40 MM, TSSOP-28 |
| 文件頁數： | 6/20頁 |
| 文件大小： | 725K |
| 代理商： | LM4801MH/NOPB |
Application Information (Continued)
The LM4801 is unity-gain stable, giving a designer maximum
design flexibility. The gain should be set to no more than a
given application requires. This allows the amplifier to
achieve minimum THD+N and maximum signal-to-noise ra-
tio. These parameters are compromised as the closed-loop
gain increases. However, low gain demands input signals
with greater voltage swings to achieve maximum output
power. Fortunately, many signal sources such as audio CO-
DECs have outputs of 1V
RMS (2.83VP-P). Please refer to the
Audio Power Amplifier Design section for more informa-
tion on selecting the proper gain.
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitor (C
i in Figure 1). A high value capaci-
tor can be expensive and may compromise space efficiency
in portable designs. In many cases, however, the speakers
used in portable systems, whether internal or external, have
little ability to reproduce signals below 150Hz. Applications
using speakers with this limited frequency response reap
little improvement by using large input capacitor.
Besides effecting system cost and size, C
i has an affect on
the LM4801’s click and pop performance. When the supply
voltage is first applied, a transient (pop) is created as the
charge on the input capacitor changes from zero to a quies-
cent state. The magnitude of the pop is directly proportional
to the input capacitor’s size. Higher value capacitors need
more time to reach a quiescent DC voltage (usually V
DD/2)
when charged with a fixed current. The amplifier’s output
charges the input capacitor through the feedback resistor,
R
f. Thus, pops can be minimized by selecting an input
capacitor value that is no higher than necessary to meet the
desired -3dB frequency.
A shown in Figure 1, the input resistor (R
I) and the input
capacitor, C
I produce a 3dB high pass filter cutoff frequency
that is found using Equation (7).
(7)
As an example when using a speaker with a low frequency
limit of 150Hz, C
I, using Equation (4), is 0.063F. The 1.0F
C
I shown inFigure 1 allows the LM4801 to drive high effi-
ciency, full range speaker whose response extends below
30Hz.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to value of C
B, the capacitor con-
nected to the BYPASS pin. Since C
B determines how fast
the LM4801 settles to quiescent operation, its value is critical
when minimizing turnon pops. The slower the LM4801’s
outputs ramp to their quiescent DC voltage (nominally 1/2
V
DD), the smaller the turnon pop. Choosing CB equal to
1.0F along with a small value of C
i (in the range of 0.1F to
0.39F), produces a click-less and pop-less shutdown func-
tion. As discussed above, choosing C
i no larger than neces-
sary for the desired bandwidth helps minimize clicks and
pops.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4801 contains circuitry to minimize turn-on and shut-
down transients or "clicks and pop". For this discussion,
turn-on refers to either applying the power supply voltage or
when the shutdown mode is deactivated. While the power
supply is ramping to its final value, the LM4801’s internal
amplifiers are configured as unity gain buffers. An internal
current source changes the voltage of the BYPASS pin in a
controlled, linear manner. Ideally, the input and outputs track
the voltage applied to the BYPASS pin. The gain of the
internal amplifiers remains unity until the voltage on the
bypass pin reaches 1/2 V
DD. As soon as the voltage on the
BYPASS pin is stable, the device becomes fully operational.
Although the bypass pin current cannot be modified, chang-
ing the size of C
B alters the device’s turn-on time and the
magnitude of "clicks and pops". Increasing the value of C
B
reduces the magnitude of turn-on pops. However, this pre-
sents a tradeoff: as the size of C
B increases, the turn-on time
increases. There is a linear relationship between the size of
C
B and the turn-on time. Here are some typical turn-on times
for various values of C
B:
C
B
T
ON
0.01F
20 ms
0.1F
200 ms
0.22F
440 ms
0.47F
940 ms
1.0F
2 Sec
In order eliminate "clicks and pops", all capacitors must be
discharged before turn-on. Rapidly switching V
DD may not
allow the capacitors to fully discharge, which may cause
"clicks and pops".
NO LOAD STABILITY
The LM4801 may exhibit low level oscillation when the load
resistance is greater than 10k
. This oscillation only occurs
as the output signal swings near the supply voltages. Pre-
vent this oscillation by connecting a 5k
between the output
pins and ground.
AUDIO POWER AMPLIFIER DESIGN
Audio Amplifier Design: Driving 1W into an 8
Load
The following are the desired operational parameters:
Power Output:
1W
RMS
Load Impedance:
8
Input Level:
1V
RMS
Input Impedance:
20k
Bandwidth:
100Hz20 kHz ± 0.25 dB
The design begins by specifying the minimum supply voltage
necessary to obtain the specified output power. One way to
find the minimum supply voltage is to use the Output Power
vs Supply Voltage curve in the Typical Performance Char-
acteristics section. Another way, using Equation (4), is to
calculate the peak output voltage necessary to achieve the
desired output power for a given load impedance. To ac-
count for the amplifier’s dropout voltage, two additional volt-
LM4801
www.national.com
14
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