LM4756 National Semiconductor Corporation, LM4756 Datasheet - Page 7

• Manufacturer Part Number: LM4756
• Description: Dual 7w Audio Power Amplifier W/mute, Standby And Volume Control
• Manufacturer: National Semiconductor Corporation

Typical Performance Characteristics
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.
Supply Current vs
Standby Voltage
20064448
7
(Continued)
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.
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
0V or open
0V or open
5V
5V
TABLE 1. Mute and Standby Functional Conditions
FIGURE 2. Mute and Standby Pin Lowpass Filters
Standby
(Pin 9)
Supply Current vs
0V
5V or open
0V
5V or open
Mute Voltage
(Pin 10)
Mute
Music Plays
Mute Mode
Standby Mode
Standby Mode
Conditions
Operating
20064447
www.national.com
20064426