LM4867MT National Semiconductor, LM4867MT Datasheet - Page 16

• Manufacturer Part Number: LM4867MT
• Description: Output-Transient-Free Dual 2.1W Audio Amplifier Plus No Coupling Capacitor Stereo Headphone Function
• Manufacturer: National Semiconductor

www.national.com
Application Information
NO LOAD STABILITY
The LM4867 may exhibit low level oscillation when the load
resistance is greater than 10k . This oscillation only occurs
as the output signal swings near the supply voltages. Pre-
vent this oscillation by connecting a 5k between the output
pins and ground.
AUDIO POWER AMPLIFIER DESIGN
Audio Amplifier Design: Driving 1W into an 8
The following are the desired operational parameters:
Bandwidth:
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 (8), 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 is
Equation (9).
The Output Power vs Supply Voltage graph for an 8
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
LM4867 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
of maximum power dissipation as explained above in the
Power Dissipation section.
After satisfying the LM4867’s power dissipation require-
ments, the minimum differential gain needed to achieve 1W
dissipation in an 8
Thus, a minimum gain of 2.83 allows the LM4867’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 20k , the feedback resistor is found using Equation
(11).
Power Output:
Load Impedance:
Input Level:
Input Impedance:
V
DD
i
) resistors. With the desired input impedance
(V
OUTPEAK
load is found using Equation (10).
+ (V
VD
OD TOP
100 Hz−20 kHz
= 3.
+ V
(Continued)
OD BOT
))
±
Load
1 W
0.25 dB
1 V
20 k
i
) and
RMS
RMS
load
(10)
8
(8)
(9)
16
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
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
As mentioned in the Selecting Proper External Compo-
nents section, R
amplifier’s lower bandpass frequency limit. Find the coupling
capacitor’s value using Equation (12).
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 LM4867’s 3.5MHz GBWP. With
this margin, the amplifier can be used in designs that require
more differential gain while avoiding performance,restricting
bandwidth limitations.
RECOMMENDED PRINTED CIRCUIT BOARD LAYOUT
Figures 8 through 12 show the recommended four-layer PC
board layout that is optimized for the 24-pin LQ-packaged
LM4867 and associated external components. Figures 13
through 17 show the recommended two-layer PC board
layout that is optimized for the 24-pin MTE-packaged
LM4867 and associated external components. Figures 18
through 20 show the recommended two-layer PC board
layout that is optimized for the 20-pin MT-packaged LM4867
and associated external components. These circuits are de-
signed for use with an external 5V supply and 4
These circuit boards are easy to use. Apply 5V and ground to
the board’s V
speakers between the board’s −OUTA and +OUTA and
OUTB and +OUTB pads.
DD
f
1/(2
is 30k .
i
and GND pads, respectively. Connect 4
and C
f
H
*
f
= 20kHz x 5 = 100kHz
L
20k
i
= 100Hz/5 = 20Hz
create a highpass filter that sets the
C
R
i
f
/R
*
20Hz) = 0.397µF
1/(2 R
i
= A
VD
i
f
/2
L
)
VD
VD
, determines the
= 3 and f
speakers.
±
±
0.25dB
0.25dB
(12)
(13)
(14)
(15)
H
(11)
=