THS3115CPWP Datasheet, PDF (14/35 Pages) Texas Instruments – LOW-NOISE, HIGH-SPEED, CURRENT FEEDBACK AMPLIFIERS

THS3112
THS3115
SLOS385C – SEPTEMBER 2001 – REVISED SEPTEMBER 2010
Figure 48 shows a configuration that uses two
amplifiers in parallel to double the output drive current
to larger capacitive loads. This technique is used
when more output current is needed to charge and
discharge the load faster as when driving large FET
transistors.
R_F
+V_S
R_G
5.11 W
24.9 W
-V_S
R_F
49.9 W
R_G
+V_S
+V_S
R_G
1 nF
5.11 W
24.9 W
-V_S
Figure 48. Parallel Amplifiers for Higher Output
Drive
Figure 49 shows a push-pull FET driver circuit typical
of ultrasound applications with isolation resistors to
isolate the gate capacitance from the amplifier.
+V_S
+V_S
5.11 W
-V_S
R_F
2R_G
R_F
+V_S
5.11 W
-V_S
-V_S
Figure 49. PowerFET Drive Circuit
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Saving Power with Shutdown Functionality
and Setting Threshold Levels with the
Reference Pin
The THS3115 features a shutdown pin
(SHUTDOWN) that lowers the quiescent current from
4.9 mA/amp down to 300 µA/amp, ideal for reducing
system power.
The shutdown pin of the amplifier defaults to the REF
pin voltage in the absence of an applied voltage,
putting the amplifier in the normal on mode of
operation. To turn off the amplifier in an effort to
conserve power, the shutdown pin can be driven
towards the positive rail. The threshold voltages for
power-on and power-down (or shutdown) are relative
to the supply rails and are given in the Shutdown
Characteristics table. Below the Enable threshold
voltage, the device is on. Above the Disable threshold
voltage, the device is off. Behavior between these
threshold voltages is not specified.
Note that this shutdown functionality is self-defining:
the amplifier consumes less power in shutdown
mode. The shutdown mode is not intended to provide
a high-impedance output. In other words, the
shutdown functionality is not intended to allow use as
a 3-state bus driver. When in shutdown mode, the
impedance looking back into the output of the
amplifier is dominated by the feedback and gain
setting resistors, but the output impedance of the
device itself varies depending on the voltage applied
to the outputs.
As with most current feedback amplifiers, the internal
architecture places some limitations on the system
when in shutdown mode. Most notably is the fact that
the amplifier actually turns on if there is a ±0.7 V or
greater difference between the two input nodes (IN+
and IN–) of the amplifier. If this difference exceeds
±0.7 V, the output of the amplifier creates an output
voltage equal to approximately [(IN+ – IN–) – 0.7V] ×
Gain. Also, if a voltage is applied to the output while
in shutdown mode, the IN– node voltage is equal to
V_O(applied) × R_G/(R_F + R_G) . For low gain configurations
and a large applied voltage at the output, the
amplifier may actually turn on because of the
behavior described here.
The time delays associated with turning the device on
and off are specified as the time it takes for the
amplifier to reach either 10% or 90% of the final
output voltage. The time delays are in the order of
microseconds because the amplifier moves in and out
of the linear mode of operation in these transitions.
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