LM4756 datasheet(7/17 Pages) NSC

7 / 17 page

Typical Performance Characteristics (Continued)

Supply Current vs

Standby Voltage

Supply Current vs

Mute Voltage

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Application Information

GENERAL FEATURES

The LM4756 has a number of valuable functions that make

this audio amplifier IC an all-in-one solution. The IC has a

stereo audio path from input to output with a DC voltage

controlled volume attenuator in the preamp section. After the

volume attenuator is a line-out connection for preamp-out

control. The attenuation curve versus DC voltage can be

found by referring to the Volume Attenuation vs DC Voltage

graph in the Typical Performance Characteristics section.

The IC also possesses a mute function to provide audio

attenuation as used on a remote control for a TV, as well as

a standby function for power conservation when not being

used. The IC is well protected with thermal shutdown and

output AC short circuit protection.

Mute Function

The muting function of the LM4756 allows the user to mute

the music going into the amplifier, providing over 60 dB of

attenuation from input to output. The function is enabled by

placing a logic “1” or 5V onto the mute pin, pin 10. To disable

the function, allowing music to be passed to the output, a

logic “0” or 0V should be placed on the mute pin. By placing

the device into mute mode, each of the power amplifier

outputs are simultaneously muted. The DC volume control

and line-out amplifiers are not affected by the mute function.

Please refer to Table 1 for each input condition.

To prevent mechanical switch bouncing from adversely af-

fecting the functionality of the IC, an RC lowpass filter should

be used as shown in Figure 2. This circuit replaces the need

for a debounce circuit when using a mechanical switch to

control the IC logic functions. However, most systems typi-

cally utilize a microprocessor or COP microcontroller to in-

terface with the logic control functions of the LM4756. When

a clean logic signal is used, as from a microcontroller, the RC

lowpass filter is not required.

Standby Function

The standby function allows the user to place the LM4756

into a power conserving mode that draws less than 10 mA of

quiescent power supply current. With the IC in this mode,

while using +22V for the supply voltage, the IC draws about

150mW of power.

The standby function is enabled by placing a logic “1” or 5V

onto the standby pin, pin 9. To disable the function allowing

music to be passed to the output, a logic “0” or 0V should be

placed on the standby pin. When the standby function is

enabled, it overrides the mute function and places the IC in

its power conserving mode. If the mute function is enabled

while in standby mode, the IC will continue to remain in

standby mode. After the standby function is disabled, the IC

will be placed into mute mode. Please refer to the Table 1 for

each input condition.

TABLE 1. Mute and Standby Functional Conditions

Standby

(Pin 9)

Mute

(Pin 10)

Operating

Conditions

0V or open

Music Plays

0V or open

5V or open

Mute Mode

Standby Mode

5V or open

Standby Mode

To prevent mechanical switch bouncing from adversely af-

fecting the functionality of an IC, an RC lowpass filter should

be used as shown in Figure 2. This circuit replaces the need

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FIGURE 2. Mute and Standby Pin Lowpass Filters

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