Datasheet

LM2876
SNAS088C –AUGUST 1995–REVISED MARCH 2013 www.ti.com
DESIGN A 25W/8Ω AUDIO AMPLIFIER
Given:
Power Output 25W
Load Impedance 8Ω
Input Level 1V(max)
Input Impedance 100 kΩ
Bandwidth 20 Hz–20 kHz ± 0.25 dB
Equation 5 and Equation 6 give:
25W/8Ω V
opeak
= 20.0V I
opeak
= 2.5A
Therefore the supply required is: ±24.0V @ 2.5A
With 15% regulation and high line the final supply voltage is ±30.36V using Equation 7. 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 8 is: A
V
≥ 14
We select a gain of 15 (Non-Inverting Amplifier); resulting in a sensitivity of 942.8 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/(15 − 1) = 7.1 kΩ; use 6.8 kΩ
The bandwidth requirement must be stated as a pole, i.e., the 3 dB frequency. Five times away from a pole gives
0.17 dB down, which is better than the required 0.25 dB. Therefore:
f
L
= 20 Hz/5 = 4 Hz
f
H
= 20 kHz × 5 = 100 kHz
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 LM2876 is important.
GBWP ≥ A
V
× f3 dB = 15 × 100 kHz = 1.5 MHz
GBWP = 2.0 MHz (min) for the LM2876
Solving for the low frequency roll-off capacitor, Ci, we have:
Ci ≥ 1/(2π Ri f
L
) = 5.9 μF; use 10 μF.
Definition of Terms
Input Offset Voltage: The absolute value of the voltage which must be applied between the input terminals
through two equal resistances to obtain zero output voltage and current.
Input Bias Current: The absolute value of the average of the two input currents with the output voltage and
current at zero.
Input Offset Current: The absolute value of the difference in the two input currents with the output voltage and
current at zero.
Input Common-Mode Voltage Range (or Input Voltage Range): The range of voltages on the input terminals
for which the amplifier is operational. Note that the specifications are not ensured over the full common-mode
voltage range unless specifically stated.
Common-Mode Rejection: The ratio of the input common-mode voltage range to the peak-to-peak change in
input offset voltage over this range.
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