LM4842MH NSC [National Semiconductor], LM4842MH Datasheet - Page 24

• Manufacturer Part Number: LM4842MH
• Description: Stereo 2W Amplifiers with DC Volume Control Transient Free Outputs, and Cap-less Headphone Drive
• Manufacturer: NSC [National Semiconductor]

www.national.com
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 (9), 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 (9). The result is
Equation (10).
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
LM4842 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 LM4842’s power dissipation require-
ments, the minimum differential gain needed to achieve 1W
dissipation in an 8Ω load is found using Equation (11).
Thus, a minimum overall gain of 2.83 allows the LM4842’s to
reach full output swing and maintain low noise and THD+N
performance.
Bandwidth:
Power Output:
Load Impedance:
Input Level:
Input Impedance:
V
DD
≥ (V
OUTPEAK
+ (V
100 Hz−20 kHz
OD TOP
+ V
OD BOT
))
±
1 W
0.25 dB
1 V
20 kΩ
RMS
RMS
(10)
8Ω
(11)
(9)
24
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
a highpass filter that sets the amplifier’s lower 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 LM4842’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 7 through 9 show the recommended 2 later PC
board layout that is optimized for the MT/MH packaged
LM4842 and associated external components. Figures 10
through 12 show the recommended 2 layer PC board layout
that is optimized for the LQ packaged LM4842 and associ-
ated external components. These circuits are designed for
use with an external 5V supply and 4Ω or 8Ω speakers.
These circuit boards are easy to use. Apply 5V and ground to
the board’s V
8Ω speakers between the board’s −OUTA and +OUTA and
OUTB and +OUTB pads.
DD
and GND pads, respectively. Connect 4Ω or
i
(Right & Left) and C
1/(2π
f
H
f
= 20kHz x 5 = 100kHz
L
*
C
20kΩ
= 100Hz/5 = 20Hz
i
≥ 1/(2πR
*
20Hz) = 0.397µF
i
f
L
i
)
(Right & Left) create
VD
VD
, determines the
= 3 and f
±
±
0.25dB
0.25dB
(12)
(13)
(14)
(15)
H
=