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

Functional Description

The LM7705, low-noise negative bias generator, can be used for many applications requiring a fixed negative voltage. A key application for the LM7705 is an amplifier with a true zero output voltage using the original parts, while not exceeding the maximum supply voltage ratings of the amplifier.

The voltage inversion in the LM7705 is achieved using a switched capacitor technique with two external capacitors (CFLY and CRES). An internal oscillator and a switching network transfers charge between the two storage capacitors. This switched capacitor technique is given in Figure 27.

LM7705 20173034.gifFigure 27. Voltage Inverter

The internal oscillator generates two anti-phase clock signals. Clock 1 controls switches S1 and S2. Clock 2 controls switches S3 and S4. When Switches S1 and S2 are closed, capacitor CFLY is charged to V+. When switches S3 and S4 are closed (S1 and S2 are open) charge from CFLY is transferred to CRES and the output voltage OUT is equal to –V+.

Due to the switched capacitor technique, a small ripple will be present at the output voltage with a frequency of the oscillator. The magnitude of this ripple will increase for increasing output currents. The magnitude of the ripple can be influenced by changing the values of the used capacitors.