lm4809mmx National Semiconductor Corporation, lm4809mmx Datasheet - Page 13

• Manufacturer Part Number: lm4809mmx
• Description: Dual 105mw Headphone Amplifier With Active-low Shutdown Mode
• Manufacturer: National Semiconductor Corporation

Application Information
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 (5), 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 (5). For a single-
ended application, the result is Equation (6).
The Output Power vs Supply Voltage graph for a 32Ω load
indicates a minimum supply voltage of 4.8V. This is easily
met by the commonly used 5V supply voltage. The additional
voltage creates the benefit of headroom, allowing the
LM4809 to produce peak output power in excess of 70mW
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. Remember that the maximum
power dissipation point from Equation (1) must be multiplied
by two since there are two independent amplifiers inside the
package. Once the power dissipation equations have been
addressed, the required gain can be determined from Equa-
tion (7).
Thus, a minimum gain of 1.497 allows the LM4809 to reach
full output swing and maintain low noise and THD+N perfro-
mance. For this example, let A
The amplifiers overall gain is set using the input (R
feedback (R
set at 20kΩ, the feedback resistor is found using Equation
(8).
V
DD
f
) resistors. With the desired input impedance
≥ (2V
OPEAK
+ (V
V
=1.5.
OD TOP
+ V
OD BOT
(Continued)
))
i
) and
(5)
(6)
(7)
13
The value of R
The last step in this design is setting the amplifier’s −3db
frequency bandwidth. To achieve the desired
band magnitude variation limit, the low frequency response
must extend to at lease 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
limit. The results are an
and a
As stated in the External Components section, both R
conjunction with C
pass filters. Thus to obtain the desired low frequency re-
sponse of 100Hz within
into consideration. The combination of two single order filters
at the same frequency forms a second order response. This
results in a signal which is down 0.34dB at five times away
from the single order filter −3dB point. Thus, a frequency of
20Hz is used in the following equations to ensure that the
response is better than 0.5dB down at 100Hz.
The high frequency pole is determined by the product of the
desired high frequency pole, f
A
resulting GBWP = 150kHz which is much smaller than the
LM4809’s GBWP of 900kHz. This figure displays that if a
designer has a need to design an amplifier with a higher
gain, the LM4809 can still be used without running into
bandwidth limitations.
V
. With a closed-loop gain of 1.5 and f
C
C
i
≥ 1 / (2π * 20kΩ * 20Hz) = 0.397µF; use 0.39µF.(11)
o
≥ 1 / (2π * 32Ω * 20Hz) = 249µF; use 330µF. (12)
f
is 30kΩ.
f
i
H
, and C
f
L
= 20kHz
= 100Hz/5 = 20Hz
A
±
V
0.5dB, both poles must be taken
o
= R
with R
*
5 = 100kHz
H
f
, and the closed-loop gain,
/R
i
L
, create first order high-
H
±
0.25dB desired
= 100kHz, the
±
0.25dB pass
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(10)
i
(8)
(9)
in