LM4841 datasheet(20/31 Pages) NSC

20 / 31 page

Application Information (Continued)

At low, frequencies C

BA is a virtual open circuit and at high

frequencies, its nearly zero ohm impedance shorts R

BA. The

result is increased bridge-amplifier gain at low frequencies.

The combination of R

BA and CBA form a -6dB corner fre-

quency at

C = 1/(2

πR

BACBA)

(9)

The bridged-amplifier low frequency differential gain is:

VD = 2(RGFA +RBA)/RGIA

(10)

Using the component values shown in Figure 2 (R

GFA =

20k

Ω,R

BA = 20k

Ω, and C

BA = 0.068µF), a first-order, -6dB

pole is created at 120Hz. Assuming R

GIA = 20k

Ω, the low

frequency differential gain is 4. The input (C

in A and B) capaci-

tor values must be selected for a low frequency response

that covers the range of frequencies affected by the desired

bass-boost operation.

DC VOLUME CONTROL

The LM4841 has an internal stereo volume control whose

setting is a function of the DC voltage applied to the DC VOL

CONTROL pin.

The LM4841 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,

(on the following page.)

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.

20028011

FIGURE 7. Low Frequency Enhancement ( MT/MH PINOUT )

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