LME49610 Datasheet, PDF (12/18 Pages) National Semiconductor (TI) – High Performance, High Fidelity, High Current Audio Buffer

OVERVOLTAGE PROTECTION
If the input-to-output differential voltage exceeds the
LME49610’s Absolute Maximum Rating of 3V, the internal
diode clamps shown in Figure 1 conduct, diverting current
around the compound emitter followers of Q1/Q5 (D1 – D7 for
positive input), or around Q2/Q6 (D8 – D14 for negative in-
puts). Without this clamp, the input transistors Q1/Q2 and Q5/
Q6 will zener and damage the buffer.
To ensure that the current flow through the diodes is held to
a save level, the internal 200Ω resistor in series with the input
limits the current through these clamps. If the additional cur-
rent that flows during this situation can damage the source
that drives the LME49610’s input, add an external resistor in
series with the input see Figure 5.
BANDWITH CONTROL PIN
The LME49610’s –3dB bandwidth is approximately 110MHz
in the low quiescent-current mode (13mA typical). Select this
mode by leaving the BW pin unconnected.
Connect the BW pin to the V_EE pin to extend the LME49610’s
bandwidth to a nominal value of 180MHz. In this mode, the
quiescent current increases to approximately 19mA. Band-
widths between these two limits are easily selected by con-
necting a series resistor between the BW pin and V_EE .
Regardless of the connection to the LME49610’s BW pin, the
rated output current and slew rate remain constant. With the
power supply voltage held constant, the wide-bandwidth
mode’s increased quiescent current causes a corresponding
increase in quiescent power dissipation. For all values of the
BW pin voltage, the quiescent power dissipation is equal to
the total supply voltage times the quiescent current (I_Q *
(V_CC + |V_EE |)).
BOOSTING OP AMP OUTPUT CURRENT
When placed in the feedback loop, the LME49610 will in-
crease an operational amplifier’s output current. The opera-
tional amplifier’s open loop gain will correct any LME49610
errors while operating inside the feedback loop.
To ensure that the operational amplifier and buffer system are
closed loop stable, the phase shift must be low. For a system
gain of one, the LME49610 must contribute less than 20° at
the operational amplifier’s unity-gain frequency. Various op-
erating conditions may change or increase the total system
phase shift. These phase shift changes may affect the oper-
ational amplifier's stability.
Unity gain stability is preserved when the LME49610 is placed
in the feedback loop of most general-purpose or precision op
amps. When the LME46900 is driving high value capacitive
loads, the BW pin should be connected to the V_EE pin for wide
bandwidth and stable operation. The wide bandwidth mode is
also suggested for high speed or fast-settling operational am-
plifiers. This preserves their stability and the ability to faithfully
amplify high frequency, fast-changing signals. Stability is en-
sured when pulsed signals exhibit no oscillations and ringing
is minimized while driving the intended load and operating in
the worst-case conditions that perturb the LME49610’s phase
response.
HIGH FREQUENCY APPLICATIONS
The LME49610’s wide bandwidth and very high slew rate
make it ideal for a variety of high-frequency open-loop appli-
cations such as an ADC input driver, 75Ω stepped volume
attenuator driver, and other low impedance loads. Circuit
board layout and bypassing techniques affect high frequency,
fast signal dynamic performance when the LME49610 oper-
ates open-loop.
A ground plane type circuit board layout is best for very high
frequency performance results. Bypass the power supply pins
(V_CC and V_EE) with 0.1μF ceramic chip capacitors in parallel
with solid tantalum 10μF capacitors placed as close as pos-
sible to the respective pins.
Source resistance can affect high-frequency peaking and
step response overshoot and ringing. Depending on the sig-
nal source, source impedance and layout, best nominal re-
sponse may require an additional resistance of 25Ω to
200Ω in series with the input. Response with some loads (es-
pecially capacitive) can be improved with an output series
resistor in the range of 10Ω to 150Ω.
THERMAL MANAGEMENT
Heat Sinking
For some applications, the LME49610 may require a heat
sink. The use of a heat sink is dependent on the maximum
LME49610 power dissipation and a given application’s max-
imum ambient temperature. In the TO–263 package, heat
sinking the LME49610 is easily accomplished by soldering
the package’s tab to a copper plane on the PCB. (Note: The
tab on the LME49610’s TO–263 package is electrically con-
nected to V_EE.)
Through the mechanisms of convection, heat conducts from
the LME49610 in all directions. A large percentage moves to
the surrounding air, some is absorbed by the circuit board
material and some is absorbed by the copper traces connect-
ed to the package’s pins. From the PCB material and the
copper, it then moves to the air. Natural convection depends
on the amount of surface area that contacts the air.
If a heat conductive copper plane has perfect thermal con-
duction (heat spreading) through the plane’s total area, the
temperature rise is inversely proportional to the total exposed
area. PCB copper planes are, in that sense, an aid to con-
vection. These planes, however, are not thick enough to
ensure perfect heat conduction. Therefore, eventually a point
of diminishing returns is reached where increasing copper
area offers no additional heat conduction to the surrounding
air. This is apparent in Figure 6. 2 oz copper boards will have
decrease thermal resistance providing a better heat sink com-
pared to 1oz. copper. Beyond 1oz or 2oz copper plane areas,
external heatsinks are required. Ultimately, the 1oz copper
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