LME49830TB/NOPB National Semiconductor, LME49830TB/NOPB Datasheet - Page 13

• Manufacturer Part Number: LME49830TB/NOPB
• Description: IC AMP AUDIO MONO HIFI TO247-15
• Manufacturer: National Semiconductor
• Type: Class ABr

Specifications of LME49830TB/NOPB

• Output Type: 1-Channel (Mono)
• Voltage - Supply: ±20 V ~ 100 V
• Features: Mute, Thermal Protection
• Mounting Type: Surface Mount
• Package / Case: TO-247-15 (Bent and Staggered Leads)
• For Use With: LME49830TBBD - BOARD EVALUATION FOR LME49830
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Max Output Power X Channels @ Load: -
• Other names: *LME49830TB/NOPB; LME49830TB; LME49830TB-5; LME49830TB

If the value of R
to increase in order to maintain the same -3dB frequency re-
sponse.
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
property of the transistor). As temperature increases, V
creases. In practice, current flowing through a bipolar tran-
sistor heats up the transistor, which lowers the V
turn increases the current again, and the cycle repeats. If the
system is not designed properly, this positive feedback mech-
anism can destroy the bipolar transistors used in the output
stage.
One of the recommended methods of preventing thermal run-
away is to use a heat sink on the bipolar output transistors.
This will keep the temperature of the transistors lower. A sec-
ond 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
bias the bipolar output stage. The V
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
circuit is called the V
sistor (Q
the bipolar output transistors, its temperature will track that of
the output transistors. The bias voltage will be reduced as the
Q
The bias circuit used in Figure 1 is a modified V
circuit. The additional resistor, R
pendent portion of the bias voltage while the rest of the V
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.
BIAS SETTING
Setting the bias voltage and resulting output stage bias cur-
rent is done by adjusting the R
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 maxi-
mum 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 stabil-
ity, THD+N, noise, slew rate and sonic performance. Opti-
mizing 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
VBE
heats up reducing bias current in the output stage.
VBE
in Figure 1) is mounted to the same heat sink as
IN
is decreased then the value of C
BIAS
BE
= V
multiplier. When V
BE
(1+R
B1
BIAS
E
CB
, sets a temperature inde-
resistors are often consid-
BE
/R
resistor. If temperature
multiplier consists of a
BE
), which is why this
BE
BE
BE
multiplier tran-
multiplier” to
(an inherent
BE
IN
BE
multiplier
will need
. This in
BE
de-
BE
13
ered optional. The R
purpose. As the output stage is scaled up in number of de-
vices the value of R
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
external components in value, placement and type. The ca-
pacitor should be placed close to the LME49830 and a silver
mica type will give good performance. The value of the ca-
pacitor 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 consid-
ered a starting value with optimization done on the bench and
in listening testing.
The input capacitor (C
against sources that may have a DC bias. For best audio per-
formance, the input capacitor should not be used. Without the
input capacitor, any DC bias from the source will be trans-
ferred to the load.
The feedback capacitor (C
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 capac-
itors 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 tanta-
lum 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 ad-
ditional 0.1μF - 1μF capacitor connected between the V
V
device should have adequate bypassing at each supply ter-
minal.
OUTPUT SENSING
The Output Sense pin Osense must be connected to the sys-
tem 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.
EE
pins of the LME49830 is recommended and each output
E
DS(on)
IN
will need to be optimized for best
) is shown in Figure 1 for protection
i
) is used to set the gain at DC to
of the devices serve a similar
C
) is one of the most critical
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to