LM4854 National Semiconductor, LM4854 Datasheet - Page 23

• Manufacturer Part Number: LM4854
• Description: 1.9W Monaural/ 85mW Stereo Headphone Audio Amplifier
• Manufacturer: National Semiconductor

Application Information
OPTIMIZING CLICK AND POP REDUCTION PERFOR-
MANCE
The LM4854 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
micro-power shutdown mode is deactivated.
As the V
final value, the LM4854’s internal amplifiers are configured
as unity gain buffers and are disconnected from the L-OUT,
BTL-OUT, and R-OUT pins. An internal current source
charges the capacitor connected between the BYPASS pin
and GND 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 V
bypass pin is stable and after a fixed nominal delay of
120ms, the device becomes fully operational and the ampli-
fier outputs are reconnected to their respective output pins.
Although the BYPASS pin current cannot be modified,
changing the size of CB alters the device’s turn-on time.
There is a linear relationship between the size of CB and the
turn-on time. Here are some typical turn-on times for various
values of CB:
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".
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 Charac-
teristics section. Another way, using Equation (8), is to cal-
culate 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).
• Power Output:
• Load Impedance
• Input Level:
• Input Impedance:
• Bandwidth:
DD
/2 voltage present at the BYPASS pin ramps to its
C
B
0.01
0.22
0.47
0.1
1.0
2.2
(µF)
DD
/2. Once the voltage on the
100Hz - 20kHz
T
ON
120
130
140
160
200
300
(Continued)
(ms)
DD
±
may not
1W
0.25dB
1V
20kΩ
RMS
RMS
8Ω
23
The Output Power vs. Supply Voltage graph for an 8Ω load
indicates a minimum supply voltage of 4.6V. The commonly
used 5V supply voltage easily meets this. The additional
voltage creates the benefit of headroom, allowing the
LM4854 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 LM4854’s
power dissipation requirements, 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 LM4854’s to reach
full output swing and maintain low noise and THD+N perfor-
mance. For this example, let AVD = 3. The amplifier’s overall
gain is set using the input (R
With the desired input impedance set at 20kΩ, the feedback
resistor is found using Equation (11).
The value of R
TThe last step in this design example is setting the amplifi-
er’s -3dB frequency bandwidth. To achieve the desired
quency response must extend to at least one-fifth the lower
bandwidth limit and the high frequency response must ex-
tend to at least five times the upper bandwidth limit. The gain
variation for both response limits is 0.17dB, well within the
and an
As mentioned in the SELECTING EXTERNAL COMPO-
NENTS section, R
the amplifier’s lower bandpass frequency limit. Find the cou-
pling capacitor’s value using Equation (14).
The result is
Use a 0.39µF capacitor, the closest standard value.
±
±
0.25dB pass band magnitude variation limit, the low fre-
0.25dB-desired limit. The results are an
V
1 / ( 2π x 20kΩ x 20Hz) = 0.397µF
DD
f
is 30kΩ. The nominal output power is 1.13W.
= V
f
L
f
i
L
OUTPEAK
= 20kHz x 5 = 100kHz
and C
= 100Hz / 5 = 20Hz
R
C
f
i
/ R
= 1 / (2πR
i
create a highpass filter that sets
i
+ V
= A
i
) and feedback (R
ODTOP
VD
i
/ 2
f
L
)
+ V
ODBOT
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f
) resistors.
(10)
(12)
(13)
(14)
(15)
(11)
(8)
(9)