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OPA2365AIDR Datasheet(PDF) 10 Page - Texas Instruments |
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OPA2365AIDR Datasheet(HTML) 10 Page - Texas Instruments |
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10 / 24 page  OPA365 OPA2365 SBOS365D − JUNE 2006 − REVISED JUNE 2009 www.ti.com 10 A simplified schematic illustrating the rail-to-rail input circuitry is shown in Figure 5. CAPACITIVE LOADS The OPA365 may be used in applications where driving a capacitive load is required. As with all op amps, there may be specific instances where the OPA365 can be- come unstable, leading to oscillation. The particular op amp circuit configuration, layout, gain and output load- ing are some of the factors to consider when establish- ing whether an amplifier will be stable in operation. An op amp in the unity-gain (+1V/V) buffer configuration and driving a capacitive load exhibits a greater tenden- cy to be unstable than an amplifier operated at a higher noise gain. The capacitive load, in conjunction with the op amp output resistance, creates a pole within the feedback loop that degrades the phase margin. The degradation of the phase margin increases as the ca- pacitive loading increases. When operating in the unity-gain configuration, the OPA365 remains stable with a pure capacitive load up to approximately 1nF. The equivalent series resistance (ESR) of some very large capacitors (CL > 1µF) is suffi- cient to alter the phase characteristics in the feedback loop such that the amplifier remains stable. Increasing the amplifier closed-loop gain allows the amplifier to drive increasingly larger capacitance. This increased capability is evident when observing the overshoot re- sponse of the amplifier at higher voltage gains. See the typical characteristic graph, Small-Signal Overshoot vs. Capacitive Load. One technique for increasing the capacitive load drive capability of the amplifier operating in unity gain is to in- sert a small resistor, typically 10 Ω to 20Ω, in series with the output; see Figure 6. This resistor significantly re- duces the overshoot and ringing associated with large capacitive loads. A possible problem with this technique is that a voltage divider is created with the added series resistor and any resistor connected in parallel with the capacitive load. The voltage divider introduces a gain error at the output that reduces the output swing. The error contributed by the voltage divider may be insignifi- cant. For instance, with a load resistance, RL = 10kΩ, and RS = 20Ω, the gain error is only about 0.2%. Howev- er, when RL is decreased to 600Ω, which the OPA365 is able to drive, the error increases to 7.5%. Regulated Charge Pump V OU T =VCC +1.8V Patent Pending Very Low Ripple Topology I BIAS V CC +1.8V I BI A S I BI A S I BIAS V S I BIAS V OU T V IN− V IN+ Figure 5. Simplified Schematic 10 Ω to 20 Ω V+ V IN V OUT R S R L C L OPA365 Figure 6. Improving Capacitive Load Drive |
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