Datasheet

LM4995, LM4995TMBD
SNAS329G –APRIL 2006–REVISED APRIL 2013 www.ti.com
AUDIO POWER AMPLIFIER DESIGN
A 1W/8Ω AUDIO AMPLIFIER
Given:
Power Output 1 Wrms
Load Impedance 8Ω
Input Level 1 Vrms
Input Impedance 20 kΩ
Bandwidth 100 Hz–20 kHz ± 0.25 dB
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 LM4995 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 20 kΩ, 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
= 20 kΩ and R
f
= 30 kΩ. The final design step is to address the bandwidth requirements
which must be stated as a pair of −3 dB frequency points. Five times away from a −3 dB point is 0.17 dB down
from passband response which is better than the required ±0.25 dB 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*20 kΩ*20 Hz) = 0.397 µF; use 0.39 µF
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 LM4995
GBWP of 2.5MHz. This figure displays that if a designer has a need to design an amplifier with a higher
differential gain, the LM4995 can still be used without running into bandwidth limitations.
The LM4995 is unity-gain stable and requires no external components besides gain-setting resistors, an input
coupling capacitor, and proper supply bypassing in the typical application. However, if a closed-loop differential
gain of greater than 10 is required, a feedback capacitor (C4) may be needed as shown in Figure 24 to
bandwidth limit the amplifier. This feedback capacitor creates a low pass filter that eliminates possible high
frequency oscillations. Care should be taken when calculating the -3dB frequency in that an incorrect
combination of R
3
and C
4
will cause rolloff before 20kHz. A typical combination of feedback resistor and
capacitor that will not produce audio band high frequency rolloff is R
3
= 20kΩ and C
4
= 25pf. These components
result in a -3dB point of approximately 320kHz.
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