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LM4884 Datasheet(Fiches technique) 18 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Numéro de pièce LM4884
Description  2.1W Differential Input, BTL Output Stereo Audio Amplifier with RF Suppression and Shutdown
Télécharger  22 Pages
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Fabricant  NSC [National Semiconductor (TI)]
Site Internet  http://www.national.com
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Application Information (Continued)
Input Level:
1 V
RMS
Input Impedance:
20 k
Bandwidth:
100 Hz−20 kHz ± 0.25 dB
The design begins by specifying the minimum supply voltage
necessary to obtain the desired 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 (6), 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-
ages, based on the Dropout Voltage vs Supply Voltage in the
Typical Performance Characteristics curves, must be
added to the result obtained by Equation (6). The result is
Equation (7).
(6)
V
DD
≥ (V
OUTPEAK+(VODTOP +VODBOT))
(7)
The Output Power vs Supply Voltage graph for an 8
Ω load
indicates a minimum supply voltage of 4.6V. This is easily
met by the commonly used 5V supply voltage. The additional
voltage creates the benefit of headroom, allowing the
LM4884 to produce peak output power in excess of 1W
without clipping or other audible distortion. The choice of
supply voltage must also not create a situation that violates
of maximum power dissipation as explained above in the
Power Dissipation section.
After satisfying the LM4884’s power dissipation require-
ments, the minimum differential gain is found using Equation
(8).
(8)
Thus, a minimum gain of 2.83 allows the LM4884’s to reach
full output swing and maintain low noise and THD+N perfor-
mance. For this example, let A
VD = 3. In the example design,
the gain will be set to 10dB (A
VD = 3.2) by applying a logic
low to GAIN 0 and a logic high to GAIN 1.
The last step in this design example is setting the amplifier’s
-3dB frequency bandwidth. To achieve the desired ±0.25dB
pass band magnitude variation limit, the low frequency re-
sponse must extend to at least one-fifth the lower bandwidth
limit and the high frequency response must extend to at least
five times the upper bandwidth limit. This extended bandwith
produces a gain variation of -0.17dB at the bandwith’s limits,
well within the ±0.25dB desired limit. The results are an
f
L = 100Hz/5 = 20Hz
(9)
and an
f
H = 20kHzx5= 100kHz
(10)
As mentioned in the External Components section, the inter-
nal input resistor and C
i create a high pass filter that sets the
amplifier’s lower bandpass frequency limit. Find the coupling
capacitor’s value using Equation (11).
f
-3dB = 1/2
π(20kΩ)C
I
(11)
The result is (using the minimum R
IN resistor value to ensure
correct magnitude response at 20Hz)
1/(2
π*20kΩ*20Hz) = 0.398µF
(12)
Use a 0.39µF capacitor, the closest standard value. The
product of the desired high frequency cutoff (100kHz in this
example) and the differential gain, A
VD, determines the up-
per passband response limit. With A
VD = 3.2 and fH =
100kHz, the closed-loop gain bandwidth product (GBWP) is
320kHz. This is less than the LM4884’s 3.5MHz GBWP. With
this margin, the amplifier can be used in designs that require
more differential gain while avoiding performance-restricting
bandwidth limitations.
Recommended Printed Circuit
Board Layout
Figures 2 through 6 show the recommended four-layer PC
board layout that is optimized for the 20-pin MH-packaged
LM4884 and associated external components. This circuit is
designed for use with an external 5V supply and 3
Ω (or
higher) speakers (or load resistors).
This circuit board is easy to use. Apply 5V and ground to the
board’s V
DD
and GND terminals, respectively. Connect
speakers (or load resistors) between the board’s -OUTA and
+OUTA and -OUTB and +OUTB pads. Apply balanced dif-
ferential stereo input signals to the input pins labeled "-INA,"
"+INA," "-INB," and "+INB."
www.national.com
18




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