LM4753 National Semiconductor, LM4753 Datasheet - Page 10

• Manufacturer Part Number: LM4753
• Description: Dual 10W Audio Power Amplifier w/Mute/ Standby and Volume Control
• Manufacturer: National Semiconductor

www.national.com
Application Information
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
and 10 k . Since the LM4753 utilizes a single +22V power
supply, the output of the line out amplifier is biased at
V
coupled to any other processing IC. The value of the capaci-
tor is chosen by using Equation (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
C = 1 µF.
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
CC
or +11V. Because of this, its output should be capacitor
f = 1/2 RC
FIGURE 6. Turn-On/Off External Circuitry Sequence
(Continued)
impedance,
1
⁄
2
(1)
of
10
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