lm4883sqx National Semiconductor Corporation, lm4883sqx Datasheet - Page 18

• Manufacturer Part Number: lm4883sqx
• Description: Dual 2.1w Audio Amplifier Plus Stereo Headphone
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
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
bypass pin is stable, the device becomes fully operational.
Although the BYPASS pin current cannot be modified,
changing the size of C
the magnitude of “clicks and pops”. Increasing the value of
C
presents a tradeoff: as the size of C
time increases. There is a linear relationship between the
size of C
turn-on times 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 C7,8. These capacitors usually
have a high value. C7,8 discharges through internal 20kΩ
resistors. Depending on the size of C7,8, the discharge time
constant 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 LM4883 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 (8), 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
5
Power Output:
Load Impedance:
Input Level:
Input Impedance:
Bandwidth:
reduces the magnitude of turn-on pops. However, this
5
and the turn-on time. Here are some typical
C
0.01µF
0.22µF
0.47µF
5
0.1µF
1.0µF
5
alters the device’s turn-on time and
DD
. As soon as the voltage on the
T
140 ms
100Hz−20kHz
ON
5
30ms
40ms
60ms
80ms
5
increases, the turn-on
:
(Continued)
DD
±
may not
1W
0.25dB
1V
20kΩ
RMS
8Ω
rms
18
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
LM4883 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 LM4883’s power dissipation require-
ments, the minimum differential gain needed to achieve 1W
dissipation in an 8Ω load is found using Equation (10).
Thus, a minimum gain of 2.83 allows the LM4883’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
As mentioned in the External Components section, R
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, AVD, determines the
upper passband response limit. With A
100kHz, the closed-loop gain bandwidth product (GBWP) is
300kHz. This is less than the LM4883’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.
1
create a highpass filter that sets the amplifier’s lower
V
DD
3
) resistors. With the desired input impedance
≥ (V
f
1/(2π*20kΩ*20Hz) = 0.398µF.
is 30kΩ.
f
H
OUTPEAK
f
L
= 20kHz*5 = 100kHz.
= 100Hz/5 = 20Hz
C
R
1
3
/R
≥ 1/(2πR
1
+ (V
= A
VD
OD TOP
VD
= 3.
1
/2
f
L
)
+ V
VD
OD BOT
= 3 and f
))
±
±
0.25dB
0.25dB
1
) and
1
(10)
(12)
(11)
H
and
(8)
(9)
=