lm4835lq National Semiconductor Corporation, lm4835lq Datasheet - Page 20

• Manufacturer Part Number: lm4835lq
• Description: Stereo 2w Audio Power Amplifiers With Dc Volume Control And Selectable Gain
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
stereo channels, and the step size is small enough to reach
a desired volume that is not too loud or too soft. Since gain
accuracy is not critical, there will be volume variation from
part-to-part even with the same applied DC control voltage.
The gain of a given LM4835 can be set with a fixed external
voltage, but another LM4835 may require a different control
voltage to achieve the same gain. The typical part-to-part
variation can be as large as 8dB for the same control volt-
age.
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 (10), 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 (11). The result is
Equation (12).
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
LM4835 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 LM4835’s power dissipation require-
ments, the minimum differential gain needed to achieve 1W
dissipation in an 8Ω load is found using Equation (13).
Thus, a minimum gain of 2.83 allows the LM4835’s to reach
full output swing and maintain low noise and THD+N perfor-
mance. For this example, let A
Bandwidth:
Power Output:
Load Impedance:
Input Level:
Input Impedance:
V
DD
≥ (V
OUTPEAK
+ (V
VD
100 Hz−20 kHz
OD TOP
= 3.
+ V
OD BOT
(Continued)
))
±
1 W
0.25 dB
1 V
20 kΩ
RMS
RMS
(12)
(13)
8Ω
(11)
20
The amplifier’s overall gain is set using the input (R
feedback (R
set at 20kΩ, the feedback resistor is found using Equation
(14).
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
As mentioned in the Selecting Proper External Compo-
nents section, R
amplifier’s lower bandpass frequency limit. Find the input
coupling capacitor’s value using Equation (17).
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 LM4835’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
Figure (6) through (10) show the recommended four-layer
PC board layout that is optimized for the 24-pin LQ-
packaged LM4835 and associated external components.
This circuit is designed for use with an external 5V supply
and 4Ω speakers.
This circuit board is 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.
and an
DD
i
) resistors. With the desired input impedance
1 / (2π
and GND pads, respectively. Connect 4Ω
f
is 30kΩ.
i
f
H
and C
f
L
= 20kHz x 5 = 100kHz
*
= 100Hz / 5 = 20Hz
R
20kΩ
C
f
i
i
≥ 1 / (2πR
/ R
create a highpass filter that sets the
*
i
20Hz) = 0.397µF
= A
VD
i
f
/ 2
L
)
VD
VD
, determines the
= 3 and f
±
±
0.25dB
0.25dB
i
) and
(14)
(15)
(16)
(17)
(18)
H
=