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參數(shù)資料
| 型號(hào)： | LM4919MM/NOPB |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類(lèi)： | 音頻／視頻放大 |
| 英文描述： | 0.014 W, 2 CHANNEL, AUDIO AMPLIFIER, PDSO10 |
| 封裝： | MSOP-10 |
| 文件頁(yè)數(shù)： | 5/15頁(yè) |
| 文件大?。?/td> | 568K |
| 代理商： | LM4919MM/NOPB |
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Application Information (Continued)
during the shutdown transitions or any other time the part is
in normal operation. Failure to operate mute correctly may
result in much higher click and pop values or failure of the
device to mute at all.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4919 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity. The LM4919 is unity-gain stable that gives the designer
maximum system flexibility. The LM4919 should be used in
low gain configurations to minimize THD+N values, and
maximize the signal to noise ratio. Low gain configurations
require large input signals to obtain a given output power.
Input signals equal to or greater than 1V
rms are available
from sources such as audio codecs. Very large values
should not be used for the gain-setting resistors. Values for
R
i and Rf should be less than 1M
. Please refer to the
section, Audio Power Amplifier Design, for a more complete
explanation of proper gain selection. Besides gain, one of
the major considerations is the closed-loop bandwidth of the
amplifier. To a large extent, the bandwidth is dictated by the
choice of external components shown in Figures 2 and 3.
The input coupling capacitor, C
i, forms a first order high pass
filter that limits low frequency response. This value should be
chosen based on needed frequency response and turn-on
time.
SELECTION OF INPUT CAPACITOR SIZE
Amplifying the lowest audio frequencies requires a high
value input coupling capacitor, C
i. A high value capacitor can
be expensive and may compromise space efficiency in por-
table designs. In many cases, however, the headphones
used in portable systems have little ability to reproduce
signals below 60Hz. Applications using headphones with this
limited frequency response reap little improvement by using
a high value input capacitor. In addition to system cost and
size, turn on time is affected by the size of the input coupling
capacitor C
i. A larger input coupling capacitor requires more
charge to reach its quiescent DC voltage. This charge
comes from the output via the feedback. Thus, by minimizing
the capacitor size based on necessary low frequency re-
sponse, turn-on time can be minimized. A small value of C
i
(in the range of 0.1F to 0.47F), is recommended.
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 LM4919 settles to quiescent operation, its value is critical
when minimizing turn-on pops. The slower the LM4919’s
outputs ramp to their quiescent DC voltage (nominally V
DD/
2), the smaller the turn-on pop. Choosing C
B equal to 4.7F
along with a small value of C
i (in the range of 0.1F to
0.47F), 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. This ensures that output transients are eliminated
when power is first applied or the LM4919 resumes opera-
tion after shutdown.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4919 contains circuitry that eliminates turn-on and
shutdown transients ("clicks and pops"). For this discussion,
turn-on refers to either applying the power supply voltage or
when the micro-power shutdown mode is deactivated.
As the V
DD/2 voltage present at the BYPASS pin ramps to its
final value, the LM4919’s internal amplifiers are configured
as unity gain buffers. An internal current source charges the
capacitor connected between the BYPASS pin and GND 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 V
DD/2. As soon as the voltage on the
bypass pin is stable, the device becomes fully operational
and the amplifier outputs are reconnected to their respective
output pins. Although the BYPASS pin current cannot be
modified, changing the size of C
B alters the device’s turn-on
time. 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:
Single-Ended
C
B(F)
T
ON
0.1
117ms
0.22
179ms
0.47
310ms
1.0
552ms
2.2
1.14s
4.7
2.4s
BTL
C
B(F)
T
ON (ms)
0.1
72
0.22
79
0.47
89
1.0
112
2.2
163
4.7
283
In order to 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".
AUDIO POWER AMPLIFIER DESIGN
A 25mW/32
Audio Amplifier
Given:
Power Output
10mWrms
Load Impedance
16
Input Level
0.4Vrms
Input Impedance
20k
A designer must first choose a mode of operation (SE or
BTL) and determine the minimum supply rail to obtain the
specified output power. By extrapolating from the Output
Power vs. Supply Voltage graphs in the Typical Performance
Characteristics section, the supply rail can be easily found.
1.5V is a standard voltage in most applications, it is chosen
LM4919
www.national.com
13
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