LM3875 Datasheet, PDF (21/29 Pages) National Semiconductor (TI) – OverturTM Audio Power Amplifier Series High-Performance 56W Audio Power Amplifier

LM3875
www.ti.com
SNAS083D – JUNE 1999 – REVISED APRIL 2013
Normally the gain is set between 20 and 200; for a 40W, 8Ω audio amplifier this results in a sensitivity of 894 mV
and 89 mV, respectively. Although higher gain amplifiers provide greater output power and dynamic headroom
capabilities, there are certain shortcomings that go along with the so called “gain”. The input referred noise floor
is increased and hence the SNR is worse. With the increase in gain, there is also a reduction of the power
bandwidth which results in a decrease in feedback thus not allowing the amplifier to respond as quickly to
nonlinearities. This decreased ability to respond to nonlinearities increases the THD + N specification.
The desired input impedance is set by R_IN. Very high values can cause board layout problems and DC offsets at
the output. The value for the feedback resistance, R_f1, should be chosen to be a relatively large value (10
kΩ–100 kΩ), and the other feedback resistance, Ri, is calculated using standard op amp configuration gain
equations. Most audio amplifiers are designed from the non-inverting amplifier configuration.
DESIGN A 40W/8Ω AUDIO AMPLIFIER
Given:
Power Output
Load Impedance
Input Level
Input Impedance
Bandwidth
40W
8Ω
1V_(max)
100 kΩ
20 Hz–20 kHz ±0.25 dB
Equation 6 and Equation 7 give:
40W/8Ω V_opeak = 25.3V I_opeak = 3.16A
Therefore the supply required is: ±30.3V @3.16A
With 15% regulation and high line the final supply voltage is ±38.3V using Equation 8. 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. It is also good to check the Power Dissipation vs Supply Voltage to
ensure that the device can handle the internal power dissipation. At the same time designing in a relatively
practical sized heat sink with a low thermal resistance is also important. Refer to Typical Performance
Characteristics graphs and the THERMAL CONSIDERATIONS section for more information.
The minimum gain from Equation 9 is: A_V ≥ 18
We select a gain of 21 (Non-Inverting Amplifier); resulting in a sensitivity of 894 mV.
Letting R_IN equal 100 kΩ gives the required input impedance, however, this would eliminate the “volume control”
unless an additional input impedance was placed in series with the 10 kΩ potentiometer that is depicted in
Figure 1. Adding the additional 100 kΩ resistor would ensure the minimum required input impedance.
For low DC offsets at the output we let R_f1 = 100 kΩ. Solving for Ri (Non-Inverting Amplifier) gives the following:
Ri = R_f1/(A_V − 1) = 100k/(21 − 1) = 5 kΩ; use 5.1 kΩ
(10)
The bandwidth requirement must be stated as a pole, i.e., the 3 dB frequency. Five times away from a pole give
0.17 dB down, which is better than the required 0.25 dB. Therefore:
f_L = 20 Hz/5 = 4 Hz
(11)
f_H = 20 kHz × 5 = 100 kHz
(12)
At this point, it is a good idea to ensure that the Gain Bandwidth Product for the part will provide the designed
gain out to the upper 3 dB point of 100 kHz. This is why the minimum GBWP of the LM3875 is important.
GBWP = A_V × f3 dB = 21 × 100 kHz = 2.1 MHz
(13)
GBWP = 2.0 MHz (min) for LM3875
(14)
Solving for the low frequency roll-off capacitor, Ci, we have:
Ci > 1/(2π Ri f_L) = 7.8 μF; use 10 μF.
(15)
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