LM4835MT datasheet(13/16 Pages) NSC

13 / 16 page

Application Information (Continued)

At low frequencies the capacitor will be virtually an open cir-

cuit. At high frequencies the capacitor will be virtually a short

circuit. As a result of this, the gain of the bridge amplifier is

increased at low frequencies. A first order pole is formed with

a corner frequency at:

c = 1/(2πRLFEC LFE)

The resulting low frequency differential gain of this bridged

amplifier becomes:

2(R

f +RLFE)/R i =Avd

With R

F = 20kΩ,RLFE = 20kΩ, and CLFE = 0.068 µF, a first

order pole is formed with a corner frequency of 120 Hz. At

low frequencies the differential gain will be 4, assuming R

S =

20k

Ω. The low frequency boost formulas assume that C

i,fIC,fOC allow the appropriate low frequency response.

The bass boost feature is enabled/disable by toggling the

logic at pin 3.

DC VOLUME CONTROL

The DC voltage at the DC Volume Control pin (pin 7) deter-

mines the attenuation of output of the amplifiers. If the DC

potential of pin 7 is at 4V (or 80% V

DD) the internal amplifiers

are set at unity gain. The attenuator range is from 0dB (pin7

=80% V

DD) to -81dB (pin7 = 0V). Any DC voltage greater

than 4V (or 80% V

DD) will result in a gain of unity. When the

mode pin is given a logic low,amplifiers will be fixed at a gain

of unity regardless of the voltage of pin 7. Refer to the Typi-

cal Performance Characteristics for detailed information of

the attenuation characteristics of the DC Volume Control pin.

DS100139-11

FIGURE 5. Low Frequency Enhancement

LM4835MT Datasheet(PDF) 13 Page - National Semiconductor (TI)