LM4756TALB10 National Semiconductor, LM4756TALB10 Datasheet - Page 9

• Manufacturer Part Number: LM4756TALB10
• Manufacturer: National Semiconductor

Specifications of LM4756TALB10

• Operational Class: Class-AB
• Audio Amplifier Output Configuration: 2-Channel Stereo
• Output Power (typ): 7x2@8OhmW
• Audio Amplifier Function: Speaker
• Total Harmonic Distortion: 0.4@8Ohm@1W%
• Single Supply Voltage (typ): 18/24V
• Dual Supply Voltage (typ): Not RequiredV
• Power Supply Requirement: Single
• Power Dissipation: 22W
• Unity Gain Bandwidth Product (typ): 2MHz
• Rail/rail I/o Type: No
• Power Supply Rejection Ratio: 50dB
• Single Supply Voltage (min): 15V
• Single Supply Voltage (max): 28V
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Operating Temp Range: -40C to 85C
• Operating Temperature Classification: Industrial
• Mounting: Through Hole
• Pin Count: 15 +Tab
• Package Type: TO-220
• Lead Free Status / RoHS Status: Not Compliant

Application Information
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.
(Continued)
FIGURE 4. Turn-On/Off Sequence
9
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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