LME49830 Datasheet, PDF (13/23 Pages) National Semiconductor (TI) – Mono High Fidelity 200 Volt MOSFET Power Amplifier Input Stage with Mute

LME49830
www.ti.com
SNAS396D – JANUARY 2008 – REVISED APRIL 2013
Once the maximum package power dissipation has been calculated, the maximum thermal resistance, θ_SA, (heat
sink to ambient) in °C/W for a heat sink can be calculated. This calculation is made using Equation (2) which is
derived by solving for θ_SA in Equation (1).
θ_SA = [(T_JMAX−T_AMB)−P_DMAX(θ_JC +θ_CS)] / P_DMAX (°C/W)
(2)
Again it must be noted that the value of θ_SA is dependent upon the system designer's amplifier requirements. If
the ambient temperature that the audio amplifier is to be working under is higher, then the thermal resistance for
the heat sink, given all other things are equal, will need to be smaller (better heat sink).
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 is set by resistors R_f and R_i for the non-inverting configuration shown in Figure 1. The gain is found by
Equation 3 below:
A_V = 1 + R_f / R_i (V/V)
(3)
For best noise performance, lower values of resistors are used. For the LME49830 the gain should be set no
lower than 26dB. Gain settings below 26dB may experience instability.
The combination of R_i with C_i (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:
f_i = 1 / (2πR_iC_i) (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 C_IN and R_IN. When using a input coupling capacitor R_IN is needed
to set the DC bias point on the amplifier's input terminal. The resulting -3dB frequency response due to the
combination of C_IN and R_IN can be found from Equation 5 shown below:
f_IN = 1 / (2πR_INC_IN) (Hz)
(5)
With large values of R_IN oscillations may be observed on the outputs when the inputs are left floating. Decreasing
the value of R_IN or not letting the inputs float will remove the oscillations. If the value of R_IN is decreased then the
value of C_IN 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 LME49830, the designer must beware of thermal runaway. Thermal
runaway is a result of the temperature dependence of V_BE (an inherent property of the transistor). As temperature
increases, V_BE decreases. In practice, current flowing through a bipolar transistor heats up the transistor, which
lowers the V_BE. 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. 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.
A third recommended method is to use a “V_BE multiplier” to bias the bipolar output stage. The V_BE multiplier
consists of a bipolar transistor and two resistors, one from the base to the collector and one from the base to the
emitter. The voltage from the collector to the emitter (also the bias voltage of the output stage) is V_BIAS =
V_BE(1+R_CB/R_BE), which is why this circuit is called the V_BE multiplier. When V_BE multiplier transistor (Q_VBE in
Figure 1) is mounted to the same heat sink as the bipolar output transistors, its temperature will track that of the
output transistors. The bias voltage will be reduced as the Q_VBE heats up reducing bias current in the output
stage.
The bias circuit used in Figure 1 is a modified V_BE multiplier circuit. The additional resistor, R_B1, sets a
temperature independent portion of the bias voltage while the rest of the V_BE multiplier circuit will adjust bias
voltage with temperature. This reduces the amount of bias voltage change with heat sink temperature for steady
bias current with the output devices shown.
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