ZHCSUW0D March   1998  – February 2024 LMC660 , LMC662

PRODUCTION DATA  

  1.   1
  2. 1特性
  3. 2应用
  4. 3说明
  5. 4Pin Configuration and Functions
  6. 5Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 ESD Ratings
    3. 5.3 Recommended Operating Conditions
    4. 5.4 Thermal Information LMC662
    5. 5.5 Thermal Information LMC660
    6. 5.6 Electrical Characteristics
    7. 5.7 Typical Characteristics
  7. 6Application and Implementation
    1. 6.1 Application Information
      1. 6.1.1 Amplifier Topology
      2. 6.1.2 Compensating Input Capacitance
      3. 6.1.3 Capacitive Load Tolerance
      4. 6.1.4 Bias Current Testing
    2. 6.2 Typical Applications
    3. 6.3 Layout
      1. 6.3.1 Layout Guidelines
        1. 6.3.1.1 Printed Circuit Board Layout for High-Impedance Work
  8. 7Device and Documentation Support
    1. 7.1 接收文档更新通知
    2. 7.2 支持资源
    3.     Trademarks
    4. 7.3 静电放电警告
    5. 7.4 术语表
  9. 8Revision History
  10. 9Mechanical, Packaging, and Orderable Information

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Capacitive Load Tolerance

Like many other op amps, the LMC66x can oscillate when the applied load appears capacitive. The threshold of oscillation varies both with load and circuit gain. The configuration most sensitive to oscillation is a unity-gain follower. See also Section 5.7.

The load capacitance interacts with the op amp output resistance to create an additional pole. If this pole frequency is sufficiently low, the pole can degrade the op amp phase margin so that the amplifier is no longer stable at low gains. Figure 6-3 shows that the addition of a small resistor (50Ω to 100Ω) in series with the op amp output, and a capacitor (5pF to 10pF) from inverting input to output pins, returns the phase margin to a safe value without interfering with lower-frequency circuit operation. Thus, larger values of capacitance can be tolerated without oscillation. In all cases, the output can ring heavily when the load capacitance is near the threshold for oscillation.

GUID-3A86BBF0-5711-4F1E-93FB-462BCFDBD4A8-low.pngFigure 6-3 Rx, Cx Improve Capacitive Load Tolerance

Capacitive load driving capability is enhanced by using a pullup resistor to V+, as in Figure 6-4. Typically, a pullup resistor conducting 500μA or more significantly improves capacitive load responses. The value of the pullup resistor must be determined based on the current sinking capability of the amplifier with respect to the desired output swing. The open-loop gain of the amplifier can also be affected by the pullup resistor (see the Electrical Characteristics).

GUID-61B90093-C71B-4237-8AF7-2ED4B216BD2A-low.pngFigure 6-4 Compensating for Large Capacitive Loads With a Pullup Resistor