Line Out

The line out function for the LM4756 is intended to provide

preamp output control for signal routing to an external power

amplifier. An example of this would be in a TV where the TV’s

remote control provides volume control on the audio signals

that may be sent to a home theater receiver. The line out

amplifier is only able to drive high impedance loads like 2k

Ω

and 10k

Ω. Since the LM4756 utilizes a single +22V power

V

coupled to any other processing IC. The value of the capaci-

tor is chosen by using Equation (1).

f = 1/2

πRC

(1)

where R is the processing IC input impedance and f is the

lowest audio frequency to be passed, like 20Hz. The value of

capacitance is then calculated. For a 10k

Ω impedance,

C = 1µF.

AC Short Circuit Protection

The LM4756 is AC short circuit protected with a current

limiting setting minimum of 2.0A. Current limiting protection

works on AC waveforms only. DC shorts from the output to

ground are not protected. Generally this is not a concern as

there is a DC blocking capacitor on the output to protect the

speaker from single-supply DC bias.

Thermal Shutdown Protection

The LM4756 has a thermal shutdown protection scheme that

limits the drive capability of each amplifier output when the

internal die temperature reaches the temperature trip point

of 150˚C. The limiting of the output current drive capability is

proportional to increasing die temperature.

When the IC is in thermal shutdown mode, all of the DC

biases of the IC remain unchanged. It is only the current

drive capability of the output power transistors that is limited.

This thermal shutdown mechanism provides for smooth au-

dio attenuation rather than abruptly pulling the outputs to

ground. When the outputs are being limited, the maximum

20064430

FIGURE 6. Turn-On/Off External Circuitry Sequence

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