LM4868MT National Semiconductor, LM4868MT Datasheet - Page 18

• Manufacturer Part Number: LM4868MT
• Description: Output-Transient-Free Dual 2.1W Audio Amplifier Plus No Coupling Capacitor Stereo Headphone Function
• Manufacturer: National Semiconductor

www.national.com
Application Information
V
1.0µF along with a small value of C
0.39µF), produces a click-less and pop-less shutdown func-
tion. As discussed above, choosing C
sary for the desired bandwidth helps minimize clicks and
pops. C
the value of C
eliminated when power is first applied or the LM4868 re-
sumes operation after shutdown.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4868 contains circuitry that eliminates turn-on and
shutdown transients (“clicks and pops“) and transients that
could occur when switching between BTL speakers and
single-ended headphones. For this discussion, turn-on re-
fers 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 LM4868’s internal amplifiers
are configured as unity gain buffers and are disconnected
from the -OUT and +OUT pins. 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 amplifi-
ers remains unity until the voltage on the bypass pin reaches
1/2 V
the device becomes fully operational and the amplifier out-
puts are reconnected to the -OUT and +OUT pins. Although
the BYPASS pin current cannot be modified, changing the
size of C
relationship between the size of C
Here are some typical 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“.
NO LOAD STABILITY
The LM4868 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:
DD
Power Output:
Load Impedance:
), the smaller the turn-on pop. Choosing C
DD
B
. As soon as the voltage on the bypass pin is stable,
’s value should be in the range of 5 times to 7 times
B
alters the device’s turn-on time. There is a linear
i
. This ensures that output transients are
C
0.01µF
0.22µF
0.47µF
0.1µF
1.0µF
2.2µF
B
T
i
134ms
300ms
630ms
ON
B
(in the range of 0.1µF to
30ms
63ms
i
3ms
and the turn-on time.
no larger than neces-
(Continued)
DD
B
1 W
equal to
may not
RMS
8Ω
B
:
18
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 is
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
LM4868 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 LM4868’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 LM4868’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
Bandwidth:
Input Level:
Input Impedance:
V
DD
i
) resistors. With the desired input impedance
f
≥ (V
is 30kΩ.
OUTPEAK
R
f
/R
+ (V
i
VD
= A
OD TOP
100 Hz−20 kHz
= 3.
VD
/2
+ V
OD BOT
))
±
0.25 dB
±
±
1 V
0.25dB
0.25dB
20 kΩ
i
) and
RMS
(10)
(11)
(8)
(9)