lm4873iblx National Semiconductor Corporation, lm4873iblx Datasheet - Page 17

• Manufacturer Part Number: lm4873iblx
• Description: Dual 2.1w Audio Amplifier Plus Stereo Headphone Function
• Manufacturer: National Semiconductor Corporation

Application Information
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitor (C
tor can be expensive and may compromise space efficiency
in portable designs. In many cases, however, the speakers
used in portable systems, whether internal or external, have
little ability to reproduce signals below 150 Hz. Applications
using speakers with this limited frequency response reap
little improvement by using large input capacitor.
Besides effecting system cost and size, C
the LM4873’s click and pop performance. When the supply
voltage is first applied, a transient (pop) is created as the
charge on the input capacitor changes from zero to a quies-
cent state. The magnitude of the pop is directly proportional
to the input capacitor’s size. Higher value capacitors need
more time to reach a quiescent DC voltage (usually V
when charged with a fixed current. The amplifier’s output
charges the input capacitor through the feedback resistor,
R
capacitor value that is no higher than necessary to meet the
desired −3dB frequency.
A shown in Figure 2, the input resistor (R
capacitor, C
that is found using Equation (7).
As an example when using a speaker with a low frequency
limit of 150Hz, C
C
ciency, full range speaker whose response extends below
30Hz.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to value of C
nected to the BYPASS pin. Since C
the LM4873 settles to quiescent operation, its value is critical
when minimizing turn-on pops. The slower the LM4873’s
outputs ramp to their quiescent DC voltage (nominally 1/2
V
1.0 µF along with a small value of C
to 0.39 µF), produces a click-less and pop-less shutdown
function. As discussed above, choosing C
necessary for the desired bandwith helps minimize clicks
and pops.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4873 contains circuitry that minimizes turn-on and
shutdown transients or “clicks and pop”. For this discussion,
turn-on refers to either applying the power supply voltage or
when the shutdown mode is deactivated. While the power
supply is ramping to its final value, the LM4873’s internal
amplifiers are configured as unity gain buffers. An internal
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
DD
f
i
. Thus, pops can be minimized by selecting an input
shown in Figure 2 allows the LM4873 to drive high effi-
), the smaller the turn-on pop. Choosing C
I
produce a −3dB high pass filter cutoff frequency
i
, using Equation (4) is 0.063µF. The 1.0µF
B
alters the device’s turn-on time and
DD
i
in Figure 2). A high value capaci-
. As soon as the voltage on the
i
B
(in the range of 0.1 µF
B
determines how fast
, the capacitor con-
(Continued)
i
has an affect on
i
I
) and the input
no larger than
B
equal to
DD
(7)
/2)
17
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 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 par-
allel with the internal 20kΩ resistor. The tradeoff for using
this resistor is increased quiescent current.
NO LOAD STABILITY
The LM4873 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
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
LM4873 to produce peak output power in excess of 1W
B
Power Output:
Load Impedance:
Input Level:
Input Impedance:
Bandwidth:
reduces the magnitude of turn-on pops. However, this
B
V
DD
and the turn-on time. Here are some typical
OUT
≥ (V
discharges through internal 20kΩ resistors.
OUTPEAK
C
0.01µF
0.22µF
0.47µF
0.1µF
1.0µF
B
OUT
OUT
+ (V
. This capacitor usually has a
, the discharge time constant
OD TOP
T
100Hz−20kHz
200ms
440ms
940ms
ON
B
20ms
B
2sec
increases, the turn-on
:
+ V
OD BOT
DD
))
www.national.com
±
may not
1W
0.25dB
1V
20kΩ
RMS
8Ω
rms
(8)
(9)