LM4753 National Semiconductor, LM4753 Datasheet - Page 9

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

Application Information
to ground for play mode, transistor Q1 is quickly cut off and
diode D1 is forward biased. When D1 is forward biased, ca-
pacitor C8 is quickly discharged to ground, bringing the
standby pin voltage to 0V. When C8 is discharged, diode D2
becomes reverse biased allowing capacitor C9 to discharge
to ground through R5. Diode D2 was clamping the voltage
on C9 to the same voltage as C8. Because R5 is 10 times
R4 it takes longer for the mute function to be disabled. While
the mute voltage is decreasing, the biasing of the amplifier is
charging up, since the standby function has already been
disabled. While the mute pin voltage is decreasing the vol-
ume pin voltage is slowly increasing through the charge-up
capacitor C7. Charging of the volume pin is from the 5V
(Continued)
FIGURE 5. Turn-On/Off External Circuitry
9
through the volume control pot, R6 and C7. Notice that the
time constant of the volume pin charging is greater than the
mute pin discharging. As shown in Figure 6 , the volume con-
trol function finally ramps up the input signal, allowing music
to be amplified at the output.
Please notice that with this circuit the standby switch will
override the mute switch as required in the IC’s functional
truth table in Table 1 .
Also note once again that most systems typically utilize a mi-
croprocessor or COP microcontroller to interface with the
logic control functions of the LM4753. When a clean logic
signal is used, as from a microcontroller, RC lowpass filtering
is not required for the mute and standby functions.
DS100043-29
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