Datasheet

LM4990
www.ti.com SNAS184E –DECEMBER 2002–REVISED MAY 2013
AUDIO POWER AMPLIFIER DESIGN
A 1W/8Ω Audio Amplifier
Power Output 1Wrms
Load Impedance 8Ω
Given: Input Level 1Vrms
Input Impedance 20kΩ
Bandwidth 100Hz–20kHz ± 0.25dB
A designer must first determine the minimum supply rail to obtain the specified output power. By extrapolating
from the Output Power vs Supply Voltage graphs in the Typical Performance Characteristics section, the supply
rail can be easily found.
5V is a standard voltage in most applications, it is chosen for the supply rail. Extra supply voltage creates
headroom that allows the LM4990 to reproduce peaks in excess of 1W without producing audible distortion. At
this time, the designer must make sure that the power supply choice along with the output impedance does not
violate the conditions explained in the POWER DISSIPATION section.
Once the power dissipation equations have been addressed, the required differential gain can be determined
from Equation 3.
(3)
R
f
/R
i
= A
VD
/2 (4)
From Equation 3, the minimum A
VD
is 2.83; use A
VD
= 3.
Since the desired input impedance was 20kΩ, and with a A
VD
impedance of 2, a ratio of 1.5:1 of R
f
to R
i
results
in an allocation of R
i
= 20kΩ and R
f
= 30kΩ. The final design step is to address the bandwidth requirements
which must be stated as a pair of −3dB frequency points. Five times away from a −3dB point is 0.17dB down
from passband response which is better than the required ±0.25dB specified.
f
L
= 100Hz/5 = 20Hz
f
H
= 20kHz * 5 = 100kHz
As stated in the External Components Description section, R
i
in conjunction with C
i
create a highpass filter.
C
i
≥ 1/(2π*20kΩ*20Hz) = 0.397µF; use 0.39µF (5)
The high frequency pole is determined by the product of the desired frequency pole, f
H
, and the differential gain,
A
VD
. With a A
VD
= 3 and f
H
= 100kHz, the resulting GBWP = 300kHz which is much smaller than the LM4990
GBWP of 2.5MHz. This figure displays that if a designer has a need to design an amplifier with a higher
differential gain, the LM4990 can still be used without running into bandwidth limitations.
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