SNVSA56B May   2015  – February 2017 LM2776

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Typical Application
      2.      Output Impedance vs Input Voltage IOUT = 100 mA
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
    1.     Pin Functions
  6. 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 Electrical Characteristics
    6. 6.6 Switching Characteristics
    7. 6.7 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Input Current Limit
      2. 7.3.2 PFM Operation
      3. 7.3.3 Output Discharge
      4. 7.3.4 Thermal Shutdown
      5. 7.3.5 Undervoltage Lockout
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Mode
      2. 7.4.2 Enable Mode
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application - Voltage Inverter
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Requirements
        1. 8.2.2.1 Efficiency
        2. 8.2.2.2 Power Dissipation
        3. 8.2.2.3 Capacitor Selection
        4. 8.2.2.4 Output Capacitor and Output Voltage Ripple
        5. 8.2.2.5 Input Capacitor
        6. 8.2.2.6 Flying Capacitor
      3. 8.2.3 Application Curve
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Community Resources
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

8.2.2.4 Output Capacitor and Output Voltage Ripple

The peak-to-peak output voltage ripple is determined by the oscillator frequency, the capacitance and ESR of the output capacitor COUT:

Equation 4. VRIPPLE = [(2 × ILOAD) / (ƒSW × COUT)] + (2 × ILOAD × ESRCOUT)

In typical applications, a 1-µF low-ESR ceramic output capacitor is recommended. Different output capacitance values can be used to reduce ripple shrink the solution size, and/or cut the cost of the solution. But changing the output capacitor may also require changing the flying capacitor and/or input capacitor to maintain good overall circuit performance.

NOTE

In high-current applications, TI recommends a 10-µF, 10-V low-ESR ceramic output capacitor. If a small output capacitor is used, the output ripple can become large during the transition between PFM mode and constant switching. To prevent toggling, a 2-µF capacitance is recommended. For example, a 10- µF, 10-V output capacitor in a 0402 case size typically only has 2-µF capacitance when biased to 5 V.

High ESR in the output capacitor increases output voltage ripple. If a ceramic capacitor is used at the output, this is usually not a concern because the ESR of a ceramic capacitor is typically very low and has only a minimal impact on ripple magnitudes. If a different capacitor type with higher ESR is used (tantalum, for example), the ESR could result in high ripple. To eliminate this effect, the net output ESR can be significantly reduced by placing a low-ESR ceramic capacitor in parallel with the primary output capacitor. The low ESR of the ceramic capacitor is in parallel with the higher ESR, resulting in a low net ESR based on the principles of parallel resistance reduction.