Datasheet

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 V
DD
may
not allow the capacitors to fully discharge, which may cause "clicks and pops".
There is a relationship between the value of C
IN
and C
BYPASS
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 C
IN
and R
i
+ R
f
to a value less than the turn-on time for a given value of C
BYPASS
as shown in the table
above.
AUDIO POWER AMPLIFIER DESIGN
Audio Amplifier Design: Driving 3W into a 4Ω load
The following are the desired operational parameters:
Power Output 3W
RMS
Load Impedance 4Ω
Input Level 0.3V
RMS
(max)
Input Impedance 20kΩ
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)
V
DD
= V
OUTPEAK
+ V
ODTOP
+ V
ODBOT
(7)
The Figure 13 graph for an 8Ω 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 4Ω 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 A
V
= 12. The amplifier's overall BTL gain is set using the input (RIN
A
) and
feedback (R) resistors of the first amplifier in the series BTL configuration. Additionaly, A
V-BTL
is twice the gain set
by the first amplifier's R
IN
and R
f
. With the desired input impedance set at 20kΩ, the feedback resistor is found
using Equation 9.
R
f
/ R
IN
= A
V
(9)
The value of R
f
is 240kΩ. The nominal output power is 3W.
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