lm4863mtx National Semiconductor Corporation, lm4863mtx Datasheet - Page 15

• Manufacturer Part Number: lm4863mtx
• Description: Dual 2.2w Audio Amplifier Plus Stereo Headphone Function
• Manufacturer: National Semiconductor Corporation

Application Information
sents a tradeoff: as the size of C
increases. There is a linear relationship between the size of
C
for various values of C
In order eliminate "clicks and pops", all capacitors must be
discharged before turn-on. Rapidly switching V
allow the capacitors to fully discharge, which may cause
"clicks and pops". In a single-ended configuration, the output
is coupled to the load by C
high value. C
Depending on the size of C
can be relatively large. To reduce transients in single-ended
mode, an external 1kΩ - 5kΩ resistor can be placed in
parallel with the internal 20kΩ resistor. The tradeoff for using
this resistor is increased quiescent current.
NO LOAD STABILITY
The LM4863 may exhibit low level oscillation when the load
resistance is greater than 10kΩ. This oscillation only occurs
as the output signal swings near the supply voltages. Pre-
vent this oscillation by connecting a 5kΩ between the output
pins and ground.
AUDIO POWER AMPLIFIER DESIGN
Audio Amplifier Design: Driving 1W into an 8Ω Load
The following are the desired operational parameters:
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-
ages, based on the Dropout Voltage vs Supply Voltage in the
Typical Performance Characteristics curves, must be
added to the result obtained by Equation (8). The result in
Equation (9).
The Output Power vs Supply Voltage graph for an 8Ω 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
B
Power Output:
Load Impedance:
Input Level:
Input Impedance:
and the turn-on time. Here are some typical turn-on times
Bandwidth:
V
DD
OUT
≥ (V
discharges through internal 20kΩ resistors.
OUTPEAK
C
0.01µF
0.22µF
0.47µF
B
0.1µF
1.0µF
B
:
OUT
OUT
+ (V
. This capacitor usually has a
, the discharge time constant
B
OD TOP
100Hz−20 kHz
increases, the turn-on time
T
200 ms
440 ms
940 ms
ON
20 ms
2 Sec
+ V
(Continued)
OD BOT
DD
))
±
0.25 dB
may not
1Wrms
1Vrms
20kΩ
8Ω
(8)
(9)
15
LM4863 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
maximum power dissipation as explained above in the
Power Dissipation section.
After satisfying the LM4863’s power dissipation require-
ments, the minimum differential gain is found using Equation
(10).
Thus, a minimum gain of 2.83 allows the LM4863’s to reach
full output swing and maintain low noise and THD+N perfor-
mance. For this example, let A
The amplifier’s overall gain is set using the input (R
feedback (R
set at 20kΩ, the feedback resistor is found using Equation
(11).
The value of R
The last step in this design example is setting the amplifier’s
−3dB frequency bandwidth. To achieve the desired
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. The gain variation for
both response limits is 0.17dB, well within the
desired limit. The results are an
and an
and C
bandpass frequency limit. Find the coupling capacitor’s
value using Equation (12).
the result is
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
upper passband response limit. With A
100kHz, the closed-loop gain bandwidth product (GBWP) is
300kHz. This is less than the LM4863’s 3.5MHz GBWP. With
this margin, the amplifier can be used in designs that require
more differential gain while avoiding performance-lrestricting
bandwidth limitations.
RECOMMENDED PRINTED CIRCUIT BOARD LAYOUT
Figures 3 through 6 show the recommended two-layer PC
board layout that is optimized for the 20-pin MTE-packaged
LM4863 and associated external components. Figures 7
through 11 show the recommended four-layer PC board
layout that is optimized for the 24-pin LQ-packaged LM4863
and associated external components. These circuits are de-
signed for use with an external 5V supply and 4Ω speakers.
These circuit boards are easy to use. Apply 5V and ground to
the board’s V
speakers between the board’s -OUTA and +OUTA and
OUTB and +OUTB pads.
As mentioned in the External Components section, R
i
create a highpass filter that sets the amplifier’s lower
f
) resistors. With the desired input impedance
DD
f
1/(2π*20kΩ*20Hz) = 0.398µF
is 30kΩ.
and GND pads, respectively. Connect 4Ω
F
H
f
L
= 20kHzx5 = 100kHz
= 100Hz/5 = 20Hz
R
f
/R
i
= A
VD
VD
= 3.
/2
VD
VD
, determines the
= 3 and f
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±
±
0.25dB
0.25dB
i
) and
(10)
(12)
(13)
(14)
(11)
H
=
i