SLVSAF7C September   2010  – April 2019 TPS61251

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Simplified Schematic
  4. 4 Revision History
  5. 5 Description (Continued)
  6. 6 Device Options
  7. 7 Pin Configuration and Functions
    1.     Pin Functions
  8. 8 Specifications
    1. 8.1 Absolute Maximum Ratings
    2. 8.2 ESD Ratings
    3. 8.3 Recommended Operating Conditions
    4. 8.4 Thermal Information
    5. 8.5 Electrical Characteristics
    6. 8.6 Typical Characteristics
  9. 9 Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
      1. 9.3.1 Current Limit Operation
      2. 9.3.2 Soft Start
      3. 9.3.3 Enable
      4. 9.3.4 Undervoltage Lockout (UVLO)
      5. 9.3.5 Power Good
      6. 9.3.6 Input Overvoltage Protection
      7. 9.3.7 Load Disconnect and Reverse Current Protection
      8. 9.3.8 Thermal Regulation
      9. 9.3.9 Thermal Shutdown
    4. 9.4 Device Functional Modes
      1. 9.4.1 Power-Save Mode
      2. 9.4.2 Snooze Mode
      3. 9.4.3 100% Duty-Cycle Mode
  10. 10Application and Implementation
    1. 10.1 Application Information
    2. 10.2 Typical Application
      1. 10.2.1 Design Requirements
      2. 10.2.2 Detailed Design Procedure
        1. 10.2.2.1 Custom Design With WEBENCH® Tools
        2. 10.2.2.2 Output Voltage Setting
        3. 10.2.2.3 Average Input Current Limit
        4. 10.2.2.4 Maximum Output Current
        5. 10.2.2.5 Inductor Selection
        6. 10.2.2.6 Output Capacitor
        7. 10.2.2.7 Input Capacitor
        8. 10.2.2.8 Checking Loop Stability
      3. 10.2.3 Application Curves
  11. 11Power Supply Recommendations
  12. 12Layout
    1. 12.1 Layout Guidelines
    2. 12.2 Layout Example
    3. 12.3 Thermal Consideration
  13. 13Device and Documentation Support
    1. 13.1 Device Support
      1. 13.1.1 Third-Party Products Disclaimer
    2. 13.2 Development Support
      1. 13.2.1 Custom Design With WEBENCH® Tools
    3. 13.3 Receiving Notification of Documentation Updates
    4. 13.4 Community Resource
    5. 13.5 Trademarks
    6. 13.6 Electrostatic Discharge Caution
    7. 13.7 Glossary
  14. 14Mechanical, Packaging, and Orderable Information

10.2.2.6 Output Capacitor

The second energy storing device is the output capacitor. When selecting output capacitors for large pulsed loads, the magnitude and duration of the pulsing current, together with the ripple voltage specification, determine the choice of the output capacitor. Both the ESR of the capacitor and the charge stored in the capacitor each cycle contribute to the output voltage ripple. The ripple due to the charge is approximately what results from Equation 8.

Equation 8. TPS61251 eq9_Vripple_lvsaf7.gif

where

  • IPULSE and tON are the peak current and on time during transmission burst.
  • ISTANDBY is the current in standby mode.

The above is a worst-case approximation assuming all the pulsing energy comes from the output capacitor.

The ripple due to the capacitor ESR is defined by Equation 9.

Equation 9. TPS61251 eq10_Vesr_lvsaf7.gif

High capacitance values and low ESR can lead to instability in some internally compensated boost converters. The internal loop compensation of the TPS61251 device is optimized to be stable with output capacitor values greater than 150 μF with very low ESR.

Because big bulk capacitors can not be placed very close to the IC, it is required to put a small ceramic capacitor of about 4.7 µF as close as possible to the output terminals. This will reduce parasitic effects that can influence the functionality of the converter.

Table 3. List Of Bulk Capacitors

VENDOR (alphabetical order) CAPACITANCE PART NUMBER
Kemet 470 µF, 6.3 V, 55 mΩ T520W477M006ATE055
Sanyo 470 µF, 6.3 V, 35 mΩ 6TPE470MAZU