SNVS420D November   2008  – May 2018 LM7705


  1. Features
  2. Applications
  3. Description
    1.     Device Images
      1.      Typical Application
  4. Revision History
  5. Pin Configuration and Functions
    1.     Pin Functions
  6. Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 3.3-V Electrical Characteristics
    6. 6.6 5-V Electrical Characteristics
    7. 6.7 Typical Characteristics
  7. Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Supply Voltage
      2. 7.3.2 Output Voltage and Line Regulation
      3. 7.3.3 Output Current and Load Regulation
      4. 7.3.4 Quiescent Current
    4. 7.4 Device Functional Modes
      1. 7.4.1 General Amplifier Application
        1. One-Stage, Single-Supply True Zero Amplifier
        2. Two-Stage, Single-Supply True Zero Amplifier
        3. Dual-Supply, True Zero Amplifiers
  8. Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Functional Description
      2. 8.1.2 Technical Description
      3. 8.1.3 Charge Pump Theory
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. Basic Setup
      3. 8.2.3 Application Curves
  9. Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Examples
  11. 11Device and Documentation Support
    1. 11.1 Community Resources
    2. 11.2 Trademarks
    3. 11.3 Electrostatic Discharge Caution
    4. 11.4 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

General Amplifier Application

This section will discuss a general DC coupled amplifier application. First, one of the limitations of a DC coupled amplifier is discussed. This is illustrated with two application examples. A solution is a given for solving this limitation by using the LM7705.

Due to the architecture of the output stage of general amplifiers, the output transistors will saturate. As a result, the output of a general purpose op amp can only swing to a few 100 mV of the supply rails. Amplifiers using CMOS technology do have a lower output saturation voltage. This is illustrated in Figure 22. For example, Texas Instruments' LM7332 can swing to 200 mV to the negative rail, for a 10-kΩ load, over all temperatures.

LM7705 20173040.gifFigure 22. Limitation of the Output of an Amplifier

The introduction of operational amplifiers with output rail-to-rail drive capabilities is a strong improvement and the (output) performance of op amps is for many applications no longer a limiting factor. For example, Texas Instruments' LMP7701 (a typical rail-to-rail op amp), has an output drive capability of only 50 mV over all temperatures for a 10-kΩ load resistance. This is close to the lower supply voltage rail.

However, for true zero output applications with a single supply, the saturation voltage of the output stage is still a limiting factor. This limitation has a negative impact on the functionality of true zero output applications. This is illustrated in Figure 23.

LM7705 20173041.gifFigure 23. Output Limitation for Single-Supply True Zero Output Application

In the One-Stage, Single-Supply True Zero Amplifier section, two applications will be discussed, showing the limitations of the output stage of an op amp in a single supply configuration:

  • A single stage true zero amplifier, with a 12-bit ADC back end.
  • A dual stage true zero amplifier, with a 12-bit ADC back end.