Datasheet

LM4755

SNAS010E –FEBRUARY 1999–REVISED APRIL 2013

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PREVENTING OSCILLATIONS

With the integration of the feedback and bias resistors on-chip, the LM4755 fits into a very compact package.

However, due to the close proximity of the non-inverting input pins to the corresponding output pins, the inputs

should be AC terminated at all times. If the inputs are left floating, the amplifier will have a positive feedback path

through high impedance coupling, resulting in a high frequency oscillation. In most applications, this termination

is typically provided by the previous stage's source impedance. If the application will require an external signal,

the inputs should be terminated to ground with a resistance of 50 kΩ or less on the AC side of the input coupling

capacitors.

UNDERVOLTAGE SHUTDOWN

If the power supply voltage drops below the minimum operating supply voltage, the internal under-voltage

detection circuitry pulls down the half-supply bias line, shutting down the preamp section of the LM4755. Due to

the wide operating supply range of the LM4755, the threshold is set to just under 9V. There may be certain

applications where a higher threshold voltage is desired. One example is a design requiring a high operating

supply voltage, with large supply and bias capacitors, and there is little or no other circuitry connected to the

main power supply rail. In this circuit, when the power is disconnected, the supply and bias capacitors will

discharge at a slower rate, possibly resulting in audible output distortion as the decaying voltage begins to clip

the output signal. An external circuit may be used to sense for the desired threshold, and pull the bias line (pin 6)

to ground to disable the input preamp. Figure 45 shows an example of such a circuit. When the voltage across

the zener diode drops below its threshold, current flow into the base of Q1 is interrupted. Q2 then turns on,

discharging the bias capacitor. This discharge rate is governed by several factors, including the bias capacitor

value, the bias voltage, and the resistor at the emitter of Q2. An equation for approximating the value of the

emitter discharge resistor, R, is given below:

R = (0.7v) / (Cb • (V

/2) / 0.1s) (1)

Note that this is only a linearized approximation based on a discharge time of 0.1s. The circuit should be

evaluated and adjusted for each application.

As mentioned earlier in the Built-in Mute Circuit section, when using an external circuit to pull down the bias line,

the rate of discharge will have an effect on the turn-off induced distortions. Please refer to the Table 1 section for

more information.

Figure 45. External Undervoltage Pull-Down

THERMAL CONSIDERATIONS

Heat Sinking

Proper heatsinking is necessary to ensure that the amplifier will function correctly under all operating conditions.

A heatsink that is too small will cause the die to heat excessively and will result in a degraded output signal as

the thermal protection circuitry begins to operate.

Product Folder Links: LM4755