LM4756 datasheet(9/17 Pages) NSC

9 / 17 page

Application Information (Continued)

If the sequence described above and shown in Figure 4 is

not used, then the external circuitry shown in Figure 5 should

be used to minimize turn-on/off pops and protect the output

stage against SOA violations.

In Figure 5 there are only a few components that are differ-

ent than the ones described earlier for lowpass filtering the

pin voltages. The new components are Q1, R2, R3, D1 and

D2. All of the other components will perform the same func-

tions that were previously described.

The explanation of how the circuit in Figure 5 works will be

related to the timing waveforms in Figure 6. The circuit in

Figure 5 protects the LM4756 from SOA violations by ensur-

ing that the enabling of the standby function when music is

playing will not quickly bring the biasing to ground before the

input signal is smoothly attenuated through the volume func-

tion. Again, this is important because any quick changes in

output current when driving an inductive load will cause a

flyback voltage that may damage the IC.

As shown in Figure 6, first notice that music is playing at the

output. When the mechanical standby switch is toggled from

ground (play mode) to 5V (standby mode), transistor Q1 is

quickly turned on, discharging capacitor C7, bringing the

voltage at the volume pin, pin 3, to ground. This quickly

attenuates the audio signal at the output as shown in Figure

6. While the input signal is being attenuated, the diode D1

becomes reverse biased and the voltage at the standby pin

starts to charge through R4, C8 and C9. There is also a finite

amount of current flowing through R5 as well, but because of

its high resistance, we can neglect it in the charge-up timing

of pin 9. Note that when the standby switch was grounded,

the diode D1 was clamping the standby pin low, setting the

initial voltage condition of C8 at a low voltage. Once C8

starts charging up, diode D2 becomes forward biased and

C9 also starts charging up. This brings the standby and mute

pin voltages up simultaneously. By the time the standby pin

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FIGURE 4. Turn-On/Off Sequence

LM4756 Datasheet(PDF) 9 Page - National Semiconductor (TI)