lm4838mtx National Semiconductor Corporation, lm4838mtx Datasheet - Page 20

• Manufacturer Part Number: lm4838mtx
• Description: Stereo 2w Audio Power Amplifiers With Dc Volume Control And Selectable Gain
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
GAIN SELECT FUNCTION (Bass Boost)
The LM4838 features selectable gain, using either internal or
external feedback resistors. Either set of feedback resistors
set the gain of the output amplifiers. The voltage applied to
the GAIN SELECT pin controls which gain is selected. Ap-
plying V
mode. Applying 0V to the GAIN SELECT pin selects the
internally set unity gain.
At low, frequencies C
frequencies, its nearly zero ohm impedance shorts R
The result is increased bridge-amplifier gain at low frequen-
cies. The combination of R
frequency at
The bridged-amplifier low frequency differential gain is:
Using the component values shown in Figure 1 (R
R
created at 120Hz. Assuming R
differential gain is 4. The input (C
values must be selected for a low frequency response that
covers the range of frequencies affected by the desired
bass-boost operation.
LFE
= 20kΩ, and C
DD
to the GAIN SELECT pin selects the external gain
A
VD
f
LFE
C
LFE
= 1/(2πR
= 2(R
= 0.068µF), a first-order, -6dB pole is
is a virtual open circuit and at high
LFE
F
+ R
and C
LFE
i
i
= 20kΩ, the low frequency
) and output (C
LFE
C
LFE
) / R
LFE
)
form a -6dB corner
FIGURE 4. Low Frequency Enhancement
i
(Continued)
O
) capacitor
F
= 20kΩ,
(10)
LFE
(9)
.
20
In some cases a designer may want to improve the low
frequency response of the bridged amplifier or incorporate a
bass boost feature. This bass boost can be useful in systems
where speakers are housed in small enclosures. A resistor,
R
series with the feedback resistor of the bridged amplifier as
seen in Figure 4.
DC VOLUME CONTROL
The LM4838 has an internal stereo volume control whose
setting is a function of the DC voltage applied to the DC VOL
CONTROL pin.
The LM4838 volume control consists of 31 steps that are
individually selected by a variable DC voltage level on the
volume control pin. The range of the steps, controlled by the
DC voltage, are from 0dB - 78dB. Each gain step corre-
sponds to a specific input voltage range, as shown in table 2.
To minimize the effect of noise on the volume control pin,
which can affect the selected gain level, hysteresis has been
implemented. The amount of hysteresis corresponds to half
of the step width, as shown in Volume Control Characteriza-
tion Graph (DS200133-40).
For highest accuracy, the voltage shown in the ’recom-
mended voltage’ column of the table is used to select a
desired gain. This recommended voltage is exactly halfway
between the two nearest transitions to the next highest or
next lowest gain levels.
The gain levels are 1dB/step from 0dB to -6dB, 2dB/step
from -6dB to -36dB, 3dB/step from -36dB to -47dB, 4dB/step
from -47db to -51dB, 5dB/step from -51dB to -66dB, and
12dB to the last step at -78dB.
LFE
, and a capacitor, C
20013311
LFE
, in parallel, can be placed in