Datasheet

LM4766
www.ti.com SNAS031F –SEPTEMBER 1998–REVISED MARCH 2013
AUDIO POWER AMPLlFIER DESIGN
Design a 30W/8Ω Audio Amplifier
Given:
Power Output 30Wrms
Load Impedance 8Ω
Input Level 1Vrms(max)
Input Impedance 47kΩ
Bandwidth 20Hz−20kHz ±0.25dB
A designer must first determine the power supply requirements in terms of both voltage and current needed to
obtain the specified output power. V
OPEAK
can be determined from Equation 5 and I
OPEAK
from Equation 6.
(5)
(6)
To determine the maximum supply voltage the following conditions must be considered. Add the dropout voltage
to the peak output swing V
OPEAK
, to get the supply rail at a current of I
OPEAK
. The regulation of the supply
determines the unloaded voltage which is usually about 15% higher. The supply voltage will also rise 10% during
high line conditions. Therefore the maximum supply voltage is obtained from the following equation.
Max supplies ≈ ± (V
OPEAK
+ V
OD
) (1 + regulation) (1.1) (7)
For 30W of output power into an 8Ω load, the required V
OPEAK
is 21.91V. A minimum supply rail of 25.4V results
from adding V
OPEAK
and V
OD
. With regulation, the maximum supplies are ±32V and the required I
OPEAK
is 2.74A
from Equation 6. It should be noted that for a dual 30W amplifier into an 8Ω load the I
OPEAK
drawn from the
supplies is twice 2.74A
PK
or 5.48A
PK
. At this point it is a good idea to check the Power Output vs Supply Voltage
to ensure that the required output power is obtainable from the device while maintaining low THD+N. In addition,
the designer should verify that with the required power supply voltage and load impedance, that the required
heatsink value θ
SA
is feasible given system cost and size constraints. Once the heatsink issues have been
addressed, the required gain can be determined from Equation 8.
(8)
From Equation 8, the minimum A
V
is: A
V
≥ 15.5.
By selecting a gain of 21, and with a feedback resistor, R
f
= 20kΩ, the value of R
i
follows from Equation 9.
R
i
= R
f
(A
V
− 1) (9)
Thus with R
i
= 1kΩ a non-inverting gain of 21 will result. Since the desired input impedance was 47kΩ, a value of
47kΩ was selected for R
IN
. 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. This fact results in a low and high frequency pole of 4Hz and
100kHz respectively. As stated in the External Components Description section, R
i
in conjunction with C
i
create a
high-pass filter.
C
i
≥ 1/(2π * 1kΩ * 4Hz) = 39.8μF; use 39μF. (10)
The high frequency pole is determined by the product of the desired high frequency pole, f
H
, and the gain, A
V
.
With a A
V
= 21 and f
H
= 100kHz, the resulting GBWP is 2.1MHz, which is less than the ensured minimum GBWP
of the LM4766 of 8MHz. This will ensure that the high frequency response of the amplifier will be no worse than
0.17dB down at 20kHz which is well within the bandwidth requirements of the design.
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