LME49830 Datasheet, PDF (14/23 Pages) National Semiconductor (TI) – Mono High Fidelity 200 Volt MOSFET Power Amplifier Input Stage with Mute

LME49830
SNAS396D – JANUARY 2008 – REVISED APRIL 2013
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BIAS SETTING
Setting the bias voltage and resulting output stage bias current is done by adjusting the R_BIAS resistor. If
temperature compensation is not needed for the bias stage, the bias stage can consist of just a resistor and a
sufficient capacitor. The output current from the two BIAS pins is typically 2mA and setting the output stage bias
voltage is a simple Ohm's Law calculation. The bias voltage can be set up to 16V for maximum flexibility for use
with a wide range of different MOSFET types. The wide range of bias voltage also allows for setting the output
stage bias current for different performance levels.
OPTIMIZING EXTERNAL COMPONENTS
External component values, types and placement are highly design dependent. Values affect performance such
as stability, THD+N, noise, slew rate and sonic performance. Optimizing the values can have a significant effect
on total audio performance.
In a simple output stage design with one MOSFET device per side, as shown in Figure 1, the R_E resistors are
often considered optional. The R_DS(on) of the devices serve a similar purpose. As the output stage is scaled up in
number of devices the value of R_E will need to be optimized for best performance. Typical values range from
0.1Ω to 0.5Ω.
The value of the gate resistors affect stability and slew rate. The capacitance of the output device should be
considered when determining the value of the gate resistor. The values shown in Figure 1 represent a typical
value or a starting value from which optimization can occur.
The compensation capacitor (C_C) is one of the most critical external components in value, placement and type.
The capacitor should be placed close to the LME49830 and a silver mica type will give good performance. The
value of the capacitor will affect slew rate and stability. The highest slew rate possible while also maintaining
stability through out the power and frequency range of operation results in the best audio performance. The value
shown in Figure 1 should be considered a starting value with optimization done on the bench and in listening
testing.
The input capacitor (C_IN) is shown in Figure 1 for protection against sources that may have a DC bias. For best
audio performance, the input capacitor should not be used. Without the input capacitor, any DC bias from the
source will be transferred to the load.
The feedback capacitor (C_i) is used to set the gain at DC to unity. Because a large value is required for a low
frequency -3dB point, the capacitor is an electrolytic type. An additional small value, high quality film capacitor
may be used in parallel to improve high frequency sonic performance. If DC offset in the output stage is
acceptable without the feedback capacitor, it may be removed but DC gain will now be equal to AC gain.
SUPPLY BYPASSING
The LME49830 has excellent power supply rejection and does not require a regulated supply. However, to
eliminate possible oscillations all op amps and power op amps should have their supply leads bypassed with low
inductance capacitors having short leads and located close to the package terminals. Inadequate power supply
bypassing will manifest itself by a low frequency oscillation known as “motorboating” or by high frequency
instabilities. These instabilities can be eliminated through multiple bypassing utilizing a large tantalum or
electrolytic capacitor (10μF minimum) which is used to absorb low frequency variations and a small capacitor
(0.1μF) to prevent any high frequency feedback through the power supply lines. These capacitors should be
located as close as possible to the supply pins of the LME49830. An additional 0.1μF - 1μF capacitor connected
between the V_CC to V_EE pins of the LME49830 is recommended and each output device should have adequate
bypassing at each supply terminal.
OUTPUT SENSING
The Output Sense pin Osense must be connected to the system output as shown in Figure 1. This connection
completes the return path to feedback the output voltage to the mute gain circuitry inside LME49830. If the
Osense pin is not connected to the output or it is floated, high voltage generated from the output stage may
cause damage to the speaker or load.
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