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OPA4374AIPWR Datasheet(PDF) 10 Page - Texas Instruments |
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OPA4374AIPWR Datasheet(HTML) 10 Page - Texas Instruments |
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10 / 37 page  OPA373, OPA2373 OPA374 OPA2374, OPA4374 SBOS279E − SEPTEMBER 2003 − REVISED MAY 2008 www.ti.com 10 R OPA373 V+ V IN V OUT 10mA max I OVERLOAD Figure 3. Input Current Protection for Voltages Exceeding the Supply Voltage RAIL-TO-RAIL OUTPUT A class AB output stage with common-source transistors is used to achieve rail-to-rail output. For light resistive loads ( > 100k Ω), the output voltage can typically swing to within 18mV from the supply rails. With moderate resistive loads (5k Ω to 50kΩ), the output can typically swing to within 100mV from the supply rails and maintain high open-loop gain. See the Typical Characteristic curve, Output Voltage Swing vs Output Current, for more information. CAPACITIVE LOAD AND STABILITY OPA373 series op amps can drive a wide range of capacitive loads. However, under certain conditions, all op amps may become unstable. Op amp configuration, gain, and load value are just a few of the factors to consider when determining stability. An op amp in unity-gain configuration is the most susceptible to the effects of capacitive load. The capacitive load reacts with the op amp output resistance, along with any additional load resistance, to create a pole in the small-signal response that degrades the phase margin. The OPA373 series op amps perform well in unity-gain configuration, with a pure capacitive load up to approximately 250pF. Increased gains allow the amplifier to drive more capacitance. See the Typical Characteristics curve, Small-Signal Overshoot vs Capacitive Load, for further details. One method of improving capacitive load drive in the unity-gain configuration is to insert a small (10 Ω to 20Ω) resistor, RS, in series with the output, as shown in Figure 4. This significantly reduces ringing while maintaining dc performance for purely capacitive loads. When there is a resistive load in parallel with the capacitive load, RS must be placed within the feedback loop as shown to allow the feedback loop to compensate for the voltage divider created by RS and RL. In unity-gain inverter configuration, phase margin can be reduced by the reaction between the capacitance at the op amp input and the gain setting resistors, thus degrading capacitive load drive. Best performance is achieved by using small valued resistors. However, when large valued resistors cannot be avoided, a small (4pF to 6pF) capacitor, CFB, can be inserted in the feedback, as shown in Figure 5. This significantly reduces overshoot by compensating the effect of capacitance, CIN, which includes the amplifier input capacitance and printed circuit board (PCP) parasitic capacitance. R S 10 Ωto 20Ω OPA373 C L R L V IN V OUT V+ Figure 4. Series Resistor in Unity-Gain Configuration Improves Capacitive Load Drive R I OPA373 V IN V OUT R F C FB C IN C L V+ Figure 5. Improving Capacitive Load Drive For example, when driving a 100pF load in unity-gain inverter configuration, adding a 6pF capacitor in parallel with the 10k Ω feedback resistor decreases overshoot from 57% to 12%, as shown in Figure 6. Load Capacitance (pF) 10 100 1k 10k 60 50 40 30 20 10 0 G= −1V/V R FB =10kΩ C FB =6pF Figure 6. Improving Capacitive Load Drive |
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