LM4866LQBD National Semiconductor, LM4866LQBD Datasheet - Page 14

• Manufacturer Part Number: LM4866LQBD
• Description: BOARD EVALUATION LM4866LQ
• Manufacturer: National Semiconductor
• Series: Boomer®r

Specifications of LM4866LQBD

• Amplifier Type: Class AB
• Output Type: 2-Channel (Stereo)
• Max Output Power X Channels @ Load: 3.2W x 2 @ 3 Ohm
• Voltage - Supply: 2 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Board Type: Fully Populated
• Utilized Ic / Part: LM4866
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Application Information
AUDIO POWER AMPLIFIER DESIGN
Audio Amplifier Design: Driving 1W into an 8Ω Load
The following are the desired operational parameters:
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 Char-
acteristics section. Another way, using Equation (4), is to
calculate the peak output voltage necessary to achieve the
desired output power for a given load impedance. To ac-
count for the amplifier’s dropout voltage, two additional volt-
ages, based on the Dropout Voltage vs Supply Voltage in the
Typical Performance Characteristics curves, must be
added to the result obtained by Equation (8). The result in
Equation (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
LM4866 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 situation that violates
maximum power dissipation as explained above in the
Power Dissipation section.
After satisfying the LM4866’s power dissipation require-
ments, the minimum differential gain is found using Equation
(10).
Thus, a minimum gain of 2.83 allows the LM4866’s to reach
full output swing and maintain low noise and THD+N perfor-
mance. For this example, let A
The amplifier’s overall gain is set using the input (R
feedback (R
set at 20kΩ, the feedback resistor is found using Equation
(11).
The value of R
The last step in this design example is setting the amplifier’s
−3dB frequency bandwidth. To achieve the desired
pass band magnitude variation limit, the low frequency re-
sponse must extend to at least one−fifth the lower bandwidth
limit and the high frequency response must extend to at least
Power Output:
Load Impedance:
Input Level:
Input Impedance:
Bandwidth:
V
DD
f
) resistors. With the desired input impedance
≥ (V
f
is 30kΩ.
OUTPEAK
R
f
/R
i
+ (V
= A
VD
VD
OD TOP
100Hz−20 kHz
= 3.
/2
+ V
(Continued)
OD BOT
))
±
0.25 dB
±
1W
1V
0.25dB
20kΩ
i
) and
RMS
RMS
(10)
8Ω
(11)
(8)
(9)
14
five times the upper bandwidth limit. The gain variation for
both response limits is 0.17dB, well within the
desired limit. The results are an
and an
and C
bandpass frequency limit. Find the coupling capacitor’s
value using Equation (14).
the result is
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
upper passband response limit. With A
100kHz, the closed-loop gain bandwidth product (GBWP) is
300kHz. This is less than the LM4866’s 3.5MHz GBWP. With
this margin, the amplifier can be used in designs that require
more differential gain while avoiding performance-lrestricting
bandwidth limitations.
RECOMMENDED PRINTED CIRCUIT BOARD LAYOUT
Figures 2 through 6 show the recommended four-layer PC
board layout that is optimized for the 24-pin LQ-packaged
LM4866 and associated external components. Figures 7
through 11 show the recommended four-layer PC board
layout that is optimized for the 20-pin MTE-packaged
LM4866 and associated components. Figures 12 through 14
show the recommended two-layer PC board layout that is
optimized for the 20-pin MT-packaged LM4866 and associ-
ated components. These circuits are designed for use with
an external 5V supply and 3Ω (or greater) speakers for the
LQ- and MTE-packaged LM4866 and 4Ω (or greater) speak-
ers for the MT-packaged LM4866.
This circuit board is easy to use. Apply 5V and ground to the
board’s V
between the board’s -OUTA and +OUTA and OUTB and
+OUTB pads. Apply the stereo input signal to the input pins
labeled "-INA" and "-INB." The stereo input signal’s ground
references are connected to the respective input channel’s
"GND" pin, adjacent to the input pins.
As mentioned in the External Components section, R
i
create a highpass filter that sets the amplifier’s lower
DD
and GND pads, respectively. Connect speakers
1/(2π*20kΩ*20Hz) = 0.398µF
F
H
f
L
= 20kHzx5 = 100kHz
= 100Hz/5 = 20Hz
VD
VD
, determines the
= 3 and f
±
0.25dB
(12)
(13)
(14)
(15)
H
=
i