Line Out

The line out function for the LM4753 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 am-

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

Ω

and 10 k

Ω. Since the LM4753 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 20 Hz. The value

of capacitance is then calculated. For a 10 k

Ω impedance,

C = 1µF.

AC Short Circuit Protection

The LM4753 is AC short circuit protected with a current lim-

iting 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 LM4753 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 bi-

ases 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 audio at-

tenuation rather than abruptly pulling the outputs to ground.

When the outputs are being limited, the maximum voltage

swing will be reduced, creating a clipping effect as shown in

Figure 7. With further increases in die temperature the maxi-

mum voltage swing will be further reduced.

The thermal sensing mechanism monitors the global die

temperature and is not intended to operate quickly enough to

shutdown the IC for extremely high power dissipation pulses

created by driving very low impedance loads.

DS100043-30

FIGURE 6. Turn-On/Off External Circuitry Sequence

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