LME49610 Datasheet, PDF (11/18 Pages) National Semiconductor (TI) – High Performance, High Fidelity, High Current Audio Buffer

AUDIO BUFFERS
Audio buffers or unity-gain followers, have large current gain
and a voltage gain of one. Audio buffers serve many applica-
tions that require high input impedance, low output
impedance and high output current. They also offer constant
gain over a very wide bandwidth.
Buffers serve several useful functions, either in stand-alone
applications or in tandem with operational amplifiers. In stand-
alone applications, their high input impedance and low output
impedance isolates a high impedance source from a low
impedance load.
SUPPLY BYPASSING
The LME49610 will place great demands on the power supply
voltage source when operating in applications that require
fast slewing and driving heavy loads. These conditions can
create high amplitude transient currents. A power supply’s
limited bandwidth can reduce the supply’s ability to supply the
needed current demands during these high slew rate condi-
tions. This inability to supply the current demand is further
exacerbated by PCB trace or interconnecting wire induc-
tance. The transient current flowing through the inductance
can produce voltage transients.
For example, the LME49610’s output voltage can slew at a
typical 2000V/μs. When driving a 100Ω load, the di/dt current
demand is 20 A/μs. This current flowing through an induc-
tance of 50nH (approximately 1.5” of 22 gage wire) will pro-
duce a 1V transient. In these and similar situations, place the
parallel combination of a solid 5μF to 10μF tantalum capacitor
and a ceramic 0.1μF capacitor as close as possible to the
device supply pins.
Ceramic capacitor have very lower ESR (typically less than
10mΩ) and low ESL when compared to the same valued tan-
talum capacitor. The ceramic capacitors, therefore, have su-
perior AC performance for bypassing high frequency noise.
In less demanding applications that have lighter loads or low-
er slew rates, the supply bypassing is not as critical. Capacitor
values in the range of 0.01μF to 0.1μF are adequate.
SIMPLIFIED LME49610 CIRCUIT DIAGRAM
The LME49610’s simplified circuit diagram is shown in Figure
4. The diagram shows the LME49610’s complementary emit-
ter follower design, bias circuit and bandwidth adjustment
node.
30042559
FIGURE 4. Simplified Circuit Diagram
Figure 5 shows the LME49610 connected as an open-loop
buffer. The source impedance and optional input resistor,
R_S, can alter the frequency response. As previously stated,
the power supplies should be bypassed with capacitors con-
nected close to the LME49610’s power supply pins. Capacitor
values as low as 0.01μF to 0.1μF will ensure stable operation
in lightly loaded applications, but high output current and fast
output slewing can demand large current transients from the
power supplies. Place a recommended parallel combination
of a solid tantalum capacitor in the 5μF to 10μF range and a
ceramic 0.1μF capacitor as close as possible to the device
supply pins.
30042560
FIGURE 5. Buffer Connections
OUTPUT CURRENT
The LME49610 can continuously source or sink 250mA. In-
ternal circuitry limits the short circuit output current to approx-
imately ±450mA. For many applications that fully utilize the
LME49610’s current source and sink capabilities, thermal dis-
sipation may be the factor that limits the continuous output
current.
The maximum output voltage swing magnitude varies with
junction temperature and output current. Using sufficient PCB
copper area as a heatsink when the metal tab of the
LME49610’s surface mount TO–263 package is soldered di-
rectly to the circuit board reduces thermal impedance. This in
turn reduces junction temperature. The PCB copper area
should be in the range of 2in² to 6in².
THERMAL PROTECTION
LME49610 power dissipated will cause the buffer’s junction
temperature to rise. A thermal protection circuit in the
LME49610 will disable the output when the junction temper-
ature exceeds 150°C. When the thermal protection is activat-
ed, the output stage is disabled, allowing the device to cool.
The output circuitry is enabled when the junction temperature
drops below 150°C.
The TO–263 package has excellent thermal characteristics.
To minimize thermal impedance, its exposed die attach pad-
dle should be soldered to a circuit board copper area for good
heat dissipation. Figure 6 shows typical thermal resistance
from junction to ambient as a function of the copper area. The
TO–263’s exposed die attach paddle is electrically connected
to the V_EE power supply pin.
LOAD IMPEDANCE
The LME49610 is stable under any capacitive load when driv-
en by a source that has an impedance of 50Ω or less. When
driving capacitive loads, any overshoot that is present on the
output signal can be reduced by shunting the load capaci-
tance with a resistor.
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