LM4838LQ datasheet(19/36 Pages) NSC

19 / 36 page

Application Information (Continued)

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

DD to the GAIN SELECT pin selects the external gain

mode. Applying 0V to the GAIN SELECT pin selects the

internally set unity gain.

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,

LFE, and a capacitor, CLFE, in parallel, can be placed in

series with the feedback resistor of the bridged amplifier as

seen in Figure 4.

At low, frequencies C

LFE is a virtual open circuit and at high

frequencies, its nearly zero ohm impedance shorts R

LFE.

The result is increased bridge-amplifier gain at low frequen-

cies. The combination of R

LFE and CLFE form a -6dB corner

frequency at

C = 1/(2

πR

LFEC LFE)

(9)

The bridged-amplifier low frequency differential gain is:

VD = 2(RF +RLFE)/R i

(10)

Using the component values shown in Figure 1 (R

F = 20k

Ω,

LFE = 20k

Ω, and C

LFE = 0.068µF), a first-order, -6dB pole is

created at 120Hz. Assuming R

i = 20k

Ω, the low frequency

differential gain is 4. The input (C

i) and output (CO) capacitor

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 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.

20013311

FIGURE 4. Low Frequency Enhancement

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