LM4702CTASLASHNOPB datasheet(14/20 Pages) TI1

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LM4702

SNAS328I – AUGUST 2005 – REVISED APRIL 2013

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Figure 20.

Once the maximum package power dissipation has been calculated using Equation 2, the maximum thermal

resistance,

θSA, (heat sink to ambient) in °C/W for a heat sink can be calculated. This calculation is made using

Equation 4 which is derived by solving for

θSA Equation 3.

θSA = [(TJMAX−TAMB)−PDMAX(θJC +θCS)] / PDMAX

(2)

Again it must be noted that the value of

θSA is dependent upon the system designer's amplifier requirements. If

the ambient temperature that the audio amplifier is to be working under is higher than 25°C, then the thermal

resistance for the heat sink, given all other things are equal, will need to be smaller.

PROPER SELECTION OF EXTERNAL COMPONENTS

Proper selection of external components is required to meet the design targets of an application. The choice of

external component values that will affect gain and low frequency response are discussed below.

The gain of each amplifier is set by resistors Rf and Ri for the non-inverting configuration shown in Figure 1. The

gain is found by Equation 3 below:

AV = 1 + Rf / Ri (V/V)

(3)

For best noise performance, lower values of resistors are used. A value of 1k

Ω is commonly used for Ri and then

setting the value of Rf for the desired gain. For the LM4702 the gain should be set no lower than 26dB. Gain

settings below 26dB may experience instability.

The combination of Ri with Ci (see Figure 1) creates a high pass filter. The low frequency response is determined

by these two components. The -3dB point can be found from Equation 4 shown below:

fi = 1 / (2πRiCi) (Hz)

(4)

If an input coupling capacitor is used to block DC from the inputs as shown in Figure 1, there will be another high

pass filter created with the combination of CIN and RIN. When using a input coupling capacitor RIN is needed to

set the DC bias point on the amplifier's input terminal. The resulting -3dB frequency response due to the

combination of CIN and RIN can be found from Equation 5 shown below:

fIN = 1 / (2πRINCIN) (Hz)

(5)

With large values of RIN oscillations may be observed on the outputs when the inputs are left floating. Decreasing

the value of RIN or not letting the inputs float will remove the oscillations. If the value of RIN is decreased then the

value of CIN will need to increase in order to maintain the same -3dB frequency response.

AVOIDING THERMAL RUNAWAY WHEN USING BIPOLAR OUTPUT STAGES

When using a bipolar output stage with the LM4702 (as in Figure 1), the designer must beware of thermal

runaway. Thermal runaway is a result of the temperature dependence of Vbe (an inherent property of the

transistor). As temperature increases, Vbe decreases. In practice, current flowing through a bipolar transistor

heats up the transistor, which lowers the Vbe. This in turn increases the current again, and the cycle repeats. If

the system is not designed properly, this positive feedback mechanism can destroy the bipolar transistors used in

the output stage.

One of the recommended methods of preventing thermal runaway is to use a heat sink on the bipolar output

transistors. This will keep the temperature of the transistors lower. A second recommended method is to use

emitter degeneration resistors (see Re1, Re2, Re3, Re4 in Figure 1). As current increases, the voltage across

the emitter degeneration resistor also increases, which decreases the voltage across the base and emitter. This

mechanism helps to limit the current and counteracts thermal runaway.

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