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 5

μF to 10μF tantalum capacitor

and a ceramic 0.1

μF capacitor as close as possible to the

device supply pins.

Ceramic capacitor have very lower ESR (typically less than

10m

Ω) 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 0.01

μF 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

the power supplies should be bypassed with capacitors con-

nected close to the LME49610’s power supply pins. Capacitor

values as low as 0.01

μF 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 5

μF to 10μF range and a

ceramic 0.1

μF 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

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

LOAD IMPEDANCE

The LME49610 is stable under any capacitive load when driv-

en by a source that has an impedance of 50

Ω 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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