Datasheet

LM4876
www.ti.com SNAS054E –FEBRUARY 2000–REVISED MAY 2013
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 Supply Voltage curve in the Typical
Performance Characteristics section. Another way, using Equation 8, 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 Dropout Voltage vs Supply Voltage in the Typical Performance
Characteristics curves, must be added to the result obtained by Equation 8. This results in Equation 9.
(8)
V
CC
≥ (V
OUTPEAK
+ (V
OD
TOP
+ V
OD
BOT
)) (9)
The Output Power vs Supply Voltage graph for an 8Ω load indicates a minimum supply voltage of 4.6V. This is
easily met by the commonly used 5V supply voltage. The additional voltage creates the benefit of headroom,
allowing the LM4876 to produce peak output power in excess of 1W without clipping or other audible distortion.
The choice of supply voltage must also not create a violation of maximum power dissipation as explained above
in the POWER DISSIPATION section.
After satisfying the LM4876's power dissipation requirements, the minimum differential gain is found using
Equation 10.
(10)
Thus, a minimum gain of 2.83 allows the LM4876's to reach full output swing and maintain low noise and THD+N
performance. For this example, let A
VD
= 3.
The amplifier's overall gain is set using the input (R
i
) and feedback (R
f
) resistors. With the desired input
impedance set at 20kΩ, the feedback resistor is found using Equation 11.
R
f
/R
i
= A
VD
/2
where
• The value of R
f
is 30kΩ. (11)
The last step in this design example is setting the amplifier's -3dB low frequency bandwidth. To achieve the
desired ±0.25dB pass band magnitude variation limit, the low frequency response must extend to at least one-
fifth the lower bandwidth limit and the high frequency response must extend to at least five times the upper
bandwidth limit. The results is an
f
L
= 100 Hz/5 = 20Hz
and an
F
H
= 20 kHz*5 = 100kHz
As mentioned in the External Components Description section, R
i
and C
i
create a highpass filter that sets the
amplifier's lower bandpass frequency limit. Find the coupling capacitor's value using Equation 12.
Ci ≥ 1/(2πRif
L
) (12)
The result is
1/(2π*20kΩ*20Hz) = 0.398µF.
Use a 0.39µF capacitor, the closest standard value.
The product of the desired high frequency cutoff (100kHz in this example) and the differential gain, A
VD
,
determines the upper passband response limit. With A
VD
= 3 and f
H
= 100kHz, the closed-loop gain bandwidth
product (GBWP) is 150kHz. This is less than the LM4876's 4MHz GBWP. With this margin, the amplifier can be
used in designs that require more differential gain and avoid performance-restricting bandwidth limitations.
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