LM4839 datasheet(18/34 Pages) NSC

18 / 34 page

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:

c = 1/(2π10kΩCBS)

(9)

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.

LM4839 Datasheet(PDF) 18 Page - National Semiconductor (TI)