LM4753 datasheet(9/13 Pages) NSC

9 / 13 page

Application Information (Continued)

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

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

FIGURE 5. Turn-On/Off External Circuitry

LM4753 bảng dữ liệu(PDF) 9 Page - National Semiconductor (TI)