Datasheet

LME49600
SNAS422E –JANUARY 2008–REVISED APRIL 2013 www.ti.com
OUTPUT CURRENT
The LME49600 can continuously source or sink 250mA. Internal circuitry limits the short circuit output current to
approximately ±450mA. For many applications that fully utilize the LME49600’s current source and sink
capabilities, thermal dissipation 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 heat sink when the metal tab of the LME49600’s surface mount TO–263
package is soldered directly to the circuit board reduces thermal impedance. This in turn reduces junction
temperature. The PCB copper area should be in the range of 3in
2
(12.9cm
2
) to 6in
2
(38.7cm
2
).
THERMAL PROTECTION
LME49600 power dissipated will cause the buffer’s junction temperature to rise. A thermal protection circuit in the
LME49600 will disable the output when the junction temperature exceeds 150°C. When the thermal protection is
activated, 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 paddle should be soldered to a circuit board copper area for good heat dissipation. Figure 30 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 LME49600 is stable under any capacitive load when driven 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 capacitance with a resistor.
OVERVOLTAGE PROTECTION
If the input-to-output differential voltage exceeds the LME49600’s Absolute Maximum Rating of 3V, the internal
diode clamps shown in Figure 2 and conduct, diverting current around the compound emitter followers of Q1/Q5
(D1 – D7 for positive input), or around Q2/Q6 (D8 – D14 for negative inputs). 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 current that flows during this situation can
damage the source that drives the LME49600’s input, add an external resistor in series with the input (see
Figure 29).
BANDWITH CONTROL PIN
The LME49600’s –3dB bandwidth is approximately 110MHz in the low quiescent-current mode (7.3mA typical).
Select this mode by leaving the BW pin unconnected.
Connect the BW pin to the V
EE
pin to extend the LME49600’s bandwidth to a nominal value of 180MHz. In this
mode, the quiescent current increases to approximately 13.2mA. Bandwidths between these two limits are easily
selected by connecting a series resistor between the BW pin and V
EE
.
Regardless of the connection to the LME49600’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 LME49600 will increase an operational amplifier’s output current. The
operational amplifier’s open loop gain will correct any LME49600 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 LME49600 must contribute less than 20° at the operational amplifier’s unity-gain
frequency. Various operating conditions may change or increase the total system phase shift. These phase shift
changes may affect the operational amplifier's stability.
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