LM4940

www.ti.com

SNAS219C – OCTOBER 2003 – REVISED MAY 2013

In order eliminate "clicks and pops", all capacitors must be discharged before turn-on. Rapidly switching VDD may

not allow the capacitors to fully discharge, which may cause "clicks and pops".

There is a relationship between the value of CIN and CBYPASS that ensures minimum output transient when power

is applied or the shutdown mode is deactivated. Best performance is achieved by setting the time constant

created by CIN and Ri + Rf to a value less than the turn-on time for a given value of CBYPASS as shown in the table

above.

AUDIO POWER AMPLIFIER DESIGN

Audio Amplifier Design: Driving 3W into a 4

Ω load

The following are the desired operational parameters:

Power Output

3WRMS

Load Impedance

4

Ω

Input Level

0.3VRMS (max)

Input Impedance

20k

Ω

Bandwidth

100Hz–20kHz ± 0.25dB

The design begins by specifying the minimum supply voltage necessary to obtain the specified output power.

One way to find the minimum supply voltage is to use the Output Power vs Power Supply Voltage curve in the

TYPICAL PERFORMANCE CHARACTERISTICS section. Another way, using Equation 6, is to calculate the

peak output voltage necessary to achieve the desired output power for a given load impedance. To account for

the amplifier's dropout voltage, two additional voltages, based on the Clipping Dropout Voltage vs Power Supply

Voltage in the TYPICAL PERFORMANCE CHARACTERISTICS curves, must be added to the result obtained by

Equation 6. The result is Equation 7.

(6)

VDD = VOUTPEAK + VODTOP + VODBOT

(7)

The Figure 13 graph for an 8

Ω load indicates a minimum supply voltage of 11.8V. The commonly used 12V

supply voltage easily meets this. The additional voltage creates the benefit of headroom, allowing the LM4940 to

produce an output power of 3W without clipping or other audible distortion. The choice of supply voltage must

also not create a situation that violates of maximum power dissipation as explained above in the POWER

DISSIPATION section. After satisfying the LM4940's power dissipation requirements, the minimum differential

gain needed to achieve 3W dissipation in a 4

Ω BTL load is found using Equation 8.

(8)

Thus, a minimum gain of 11.6 allows the LM4940's to reach full output swing and maintain low noise and THD+N

performance. For this example, let AV = 12. The amplifier's overall BTL gain is set using the input (RINA) and

feedback (R) resistors of the first amplifier in the series BTL configuration. Additionaly, AV-BTL is twice the gain set

by the first amplifier's RIN and Rf. With the desired input impedance set at 20kΩ, the feedback resistor is found

using Equation 9.

Rf/ RIN = AV

(9)

The value of Rf is 240kΩ. The nominal output power is 3W.

Copyright © 2003–2013, Texas Instruments Incorporated

Submit Documentation Feedback

13

Product Folder Links: LM4940