LM4756 datasheet(10/17 Pages) NSC

10 / 17 page

Application Information (Continued)

voltage enables the standby function, the voltage at the

volume pin will already have been ramped down to 0V and

the output signal will be close to 0V.

When the IC is in standby mode the biasing of the IC is

brought down to ground and the quiescent supply current is

around 7 mA. When the standby switch in Figure 5 is toggled

to ground for play mode, transistor Q1 is quickly cut off and

diode D1 is forward biased. When D1 is forward biased,

capacitor 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

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

control 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

microprocessor or COP microcontroller to interface with the

logic control functions of the LM4756. When a clean logic

signal is used, as from a microcontroller, RC lowpass filtering

is not required for the mute and standby functions.

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FIGURE 5. Turn-On/Off External Circuitry

LM4756 Datasheet(PDF) 10 Page - National Semiconductor (TI)