OPA4364-Q1 Datasheet, PDF (12/21 Pages) Texas Instruments – 1.8-V, 7-MHz, 90-dB CMRR, SINGLE-SUPPLY, RAIL-TO-RAIL I/O OPERATIONAL AMPLIFIER

OPA2364-Q1, OPA4364-Q1
SGLS363A – JUNE 2006 – REVISED OCTOBER 2006
www.ti.com
APPLICATION INFORMATION (continued)
Capacitive Load
The OPAx364 series operational amplifiers can drive a wide range of capacitive loads. However, all operational
amplifiers under certain conditions may become unstable. Operational amplifier configuration, gain, and load
value are just a few of the factors to consider when determining stability. An operational amplifier in unity-gain
configuration is the most susceptible to the effects of capacitive load. The capacitive load reacts with the output
resistance of the operational amplifier to create a pole in the small-signal response, which degrades the phase
margin.
In unity gain, the OPAx364 series operational amplifiers perform well with a pure capacitive load up to
approximately 1000 pF. The equivalent series resistance (ESR) of the loading capacitor may be sufficient to
allow the OPAx364 to directly drive large capacitive loads (>1 µF). Increasing gain enhances the amplifier’s
ability to drive more capacitance as shown in Figure 13.
One method of improving capacitive load drive in the unity gain configuration is to insert a 10-Ω to 20-Ω resistor
in series with the output, as shown in Figure 22. This significantly reduces ringing with large capacitive loads.
However, if there is a resistive load in parallel with the capacitive load, it creates a voltage divider introducing a
dc error at the output and slightly reduces output swing. This error may be insignificant. For instance, with
R_L = 10 kΩ and R_S = 20 Ω, there is only about a 0.2% error at the output.
V+
R
S
OPAx364
V
OUT
V
IN
10 W to
20 W
R
L
C
L
Figure 22. Improving Capacitive Load Drive
Input and ESD Protection
All OPAx364 pins are static protected with internal ESD protection diodes tied to the supplies. These diodes
provide overdrive protection if the current is externally limited to 10 mA, as stated in the absolute maximum
ratings and shown in Figure 23.
I
OVERLOAD
10 mA max
V
IN
5 kW
V+
–
OPAx364
+
V
OUT
Figure 23. Input Current Protection
Achieving Output Swing to the Operational Amplifier's Negative Rail
Some applications require an accurate output voltage swing from 0 V to a positive full-scale voltage. A good
single-supply operational amplifier may be able to swing within a few mV of single supply ground, but as the
output is driven toward 0 V, the output stage of the amplifier prevents the output from reaching the negative
supply rail of the amplifier.
The output of the OPAx364 can be made to swing to ground, or slightly below, on a single-supply power source.
To do so requires use of another resistor and an additional, more-negative power supply than the operational
amplifier's negative supply. A pulldown resistor may be connected between the output and the additional
negative supply to pull the output down below the value that the output would otherwise achieve as shown in
Figure 24.
12
Submit Documentation Feedback