LME49810 Datasheet, PDF (14/21 Pages) National Semiconductor (TI) – 200V Audio Power Amplifier Driver with Baker Clamp

LME49810
SNAS391C – MAY 2007 – REVISED APRIL 2013
www.ti.com
OUTPUT STAGE USING BIPOLAR TRANSISTORS
With a properly designed output stage and supply voltage of ±100V, an output power up to 500W can be
generated at 0.05% THD+N into an 8Ω speaker load. With an output current of several amperes, the output
transistors need substantial base current drive because power transistors usually have quite low current
gain—typical h_fe of 50 or so. To increase the current gain, audio amplifiers commonly use Darlington style
devices. Power transistors should be mounted together with the V_BE multiplier transistor on the same heat sink to
avoid thermal run away. Please see the section BIASING TECHNIQUES AND AVOIDING THERMAL
RUNAWAY for additional information.
BIASING TECHNIQUES AND AVOIDING THERMAL RUNAWAY
A class AB amplifier has some amount of distortion called Crossover distortion. To effectively minimize the
crossover distortion from the output, a V_BE multiplier may be used instead of two biasing diodes. The LME49810
has two dedicated pins (BIAS_M and BIAS_P) for Bias setup and provide a constant current source of about 2.8mA.
A V_BE multiplier normally consists of a bipolar transistor (Q_MULT, see Figure 1) and two resistors (R_B1 and R_B2,
see Figure 1). A trim pot can also be added in series with R_B1 for optional bias adjustment. A properly designed
output stage, combine with a V_BE multiplier, can eliminate the trim pot and virtually eliminate crossover distortion.
The V_CE voltage of Q_MULT (also called BIAS of the output stage) can be set by following formula:
V_BIAS = V_BE(1+R_B2/R_B1) (V)
(7)
When using a bipolar output stage with the LME49810 (as in Figure 1), 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 gain, 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 the same heat sink on
the bipolar output stage transistor together with V_BE multiplier transistor. When the V_BE multiplier transistor is
mounted to the same heat sink as the bipolar output stage transistors, it temperature will track that of the output
transistors. Its V_BE is dependent upon temperature as well, and so it will draw more current as the output
transistors heat up, reducing the bias voltage to compensate. This will limit the base current into the output
transistors, which counteracts thermal runaway. Another widely popular method of preventing thermal runaway is
to use low value emitter degeneration resistors (R_E1 and R_E2). As current increases, the voltage at the emitter
also increases, which decreases the voltage across the base and emitter. This mechanism helps to limit the
current and counteracts thermal runaway.
LAYOUT CONSIDERATION AND AVOIDING GROUND LOOPS
A proper layout is virtually essential for a high performance audio amplifier. It is very important to return the load
ground, supply grounds of output transistors, and the low level (feedback and input) grounds to the circuit board
common ground point through separate paths. When ground is routed in this fashion, it is called a star ground or
a single point ground. It is advisable to keep the supply decoupling capacitors of 0.1μF close as possible to
LME49810 to reduce the effects of PCB trace resistance and inductance. Following the general rules will
optimize the PCB layout and avoid ground loops problems:
a. Make use of symmetrical placement of components.
b. Make high current traces, such as output path traces, as wide as possible to accomodate output stage
current requirement.
c. To reduce the PCB trace resistance and inductance, same ground returns paths should be as short as
possible. If possible, make the output traces short and equal in length.
d. To reduce the PCB trace resistance and inductance, ground returns paths should be as short as possible.
e. If possible, star ground or a single point ground should be observed. Advanced planning before starting the
PCB can improve audio performance.
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