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參數(shù)資料
| 型號： | LA49032H |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | 3 W, 2 CHANNEL, AUDIO AMPLIFIER, PDSO36 |
| 封裝： | 0.375 INCH, HSOP-36 |
| 文件頁數(shù)： | 2/12頁 |
| 文件大?。?/td> | 809K |
| 代理商： | LA49032H |
needed to couple in low frequencies without severe attenua-
tion. But in many cases the speakers used in portable sys-
tems, whether internal or external, have little ability to
reproduce signals below 100 Hz to 150 Hz. Thus, using a
large input capacitor may not increase actual system perfor-
mance.
In addition to system cost and size, click and pop performance
is effected by the size of the input coupling capacitor, C
i. A
larger input coupling capacitor requires more charge to reach
its quiescent DC voltage (nominally 1/2 V
DD). This charge
comes from the output via the feedback and is apt to create
pops upon device enable. Thus, by minimizing the capacitor
size based on necessary low frequency response, turn-on
pops can be minimized.
Besides minimizing the input capacitor size, careful consid-
eration should be paid to the bypass capacitor value. Bypass
capacitor, C
B, is the most critical component to minimize turn-
on pops since it determines how fast the LM4889 turns on.
The slower the LM4889's outputs ramp to their quiescent DC
voltage (nominally 1/2 V
DD), the smaller the turn-on pop.
Choosing C
B equal to 1.0 F along with a small value of Ci (in
the range of 0.1 F to 0.39 F), should produce a virtually
clickless and popless shutdown function. While the device will
function properly, (no oscillations or motorboating), with C
B
equal to 0.1 F, the device will be much more susceptible to
turn-on clicks and pops. Thus, a value of C
B equal to 1.0 F
is recommended in all but the most cost sensitive designs.
AUDIO POWER AMPLIFIER DESIGN
A 1W/8
Audio Amplifier
Given:
Power Output
1 Wrms
Load Impedance
8
Input Level
1 Vrms
Input Impedance
20 k
Bandwidth
100 Hz–20 kHz ± 0.25 dB
A designer must first determine the minimum supply rail to
obtain the specified output power. By extrapolating from the
Output Power vs Supply Voltage graphs in the Typical Per-
formance Characteristics
section, the supply rail can be
easily found. A second way to determine the minimum supply
rail is to calculate the required V
opeak using Equation 2 and
add the output voltage. Using this method, the minimum sup-
ply voltage would be (V
opeak + (VOD
TOP
+ V
OD
BOT
)), where
V
OD
BOT
and V
OD
TOP
are extrapolated from the Dropout Voltage
vs Supply Voltage curve in the Typical Performance Char-
acteristics
section.
(2)
5V is a standard voltage in most applications, it is chosen for
the supply rail. Extra supply voltage creates headroom that
allows the LM4889 to reproduce peaks in excess of 1W with-
out producing audible distortion. At this time, the designer
must make sure that the power supply choice along with the
output impedance does not violate the conditions explained
in the Power Dissipation section.
Once the power dissipation equations have been addressed,
the required differential gain can be determined from Equa-
tion 3.
(3)
R
f/Ri = AVD/2
From Equation 3, the minimum A
VD is 2.83; use AVD = 3.
Since the desired input impedance was 20 k
, and with a
A
VD impedance of 2, a ratio of 1.5:1 of Rf to Ri results in an
allocation of R
i = 20 k and Rf = 30 k. The final design step
is to address the bandwidth requirements which must be stat-
ed as a pair of 3 dB frequency points. Five times away from
a 3 dB point is 0.17 dB down from passband response which
is better than the required ±0.25 dB specified.
f
L = 100 Hz/5 = 20 Hz
f
H = 20 kHz * 5 = 100 kHz
As stated in the External Components section, R
i in con-
junction with C
i create a highpass filter.
C
i ≥ 1/(2π*20 k*20 Hz) = 0.397 F; use 0.39 F
The high frequency pole is determined by the product of the
desired frequency pole, f
H, and the differential gain, AVD. With
a A
VD = 3 and fH = 100 kHz, the resulting GBWP = 300kHz
which is much smaller than the LM4889 GBWP of 2.5MHz.
This calculation shows that if a designer has a need to design
an amplifier with a higher differential gain, the LM4889 can
still be used without running into bandwidth limitations.
www.national.com
10
LM4889
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