LM4839LQ NSC [National Semiconductor], LM4839LQ Datasheet - Page 18

• Manufacturer Part Number: LM4839LQ
• Description: Stereo 2W Audio Power Amplifiers with DC Volume Control, Bass Boost, and Input Mux
• Manufacturer: NSC [National Semiconductor]

Application Information
(Continued)
When the bass boost is enabled, the output amplifiers will be
internally set at a gain of 2 at low frequencies (gain of 4 in
bridged mode). As shown in Figure 1, CBS sets the cutoff
frequency for the bass boost. At low frequencies, the capaci-
tor will be virtually an open circuit. At high frequencies, the
capacitor will be virtually a short circuit. As a result of this, the
gain of the bridge amplifier is increased as low frequencies.
A first order pole is formed with a corner frequency at:
f
= 1/(2 10k CBS)
(9)
c
With CBS = 0.1uF, a first order pole is formed with a corner
frequency of 160Hz.
DC VOLUME CONTROL
The LM4839 has an internal stereo volume control whose
setting is a function of the DC voltage applied to the DC VOL
CONTROL pin.
The LM4839 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.
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