SBVA093 December   2022 LP2992 , TPS786 , TPS7A30 , TPS7A3001-EP , TPS7A33 , TPS7A39 , TPS7A4501-SP , TPS7A47 , TPS7A47-Q1 , TPS7A4701-EP , TPS7A49 , TPS7A52 , TPS7A52-Q1 , TPS7A53 , TPS7A53-Q1 , TPS7A53A-Q1 , TPS7A53B , TPS7A54 , TPS7A54-Q1 , TPS7A57 , TPS7A7100 , TPS7A7200 , TPS7A7300 , TPS7A80 , TPS7A8300 , TPS7A83A , TPS7A84 , TPS7A84A , TPS7A85 , TPS7A85A , TPS7A87 , TPS7A89 , TPS7A90 , TPS7A91 , TPS7A92 , TPS7A94 , TPS7A96 , TPS7B7702-Q1 , TPS7H1111-SEP , TPS7H1111-SP , TPS7H1210-SEP

 

  1.   Abstract
  2.   Trademarks
  3. 1Introduction
  4. 2Comprehensive Review of Error in LDO's
    1. 2.1 Commentary on Real World Error Voltage (VE) in Single LDO's
      1. 2.1.1 VREF Pin
      2. 2.1.2 VFB Pin
  5. 3Current Sharing and Load Voltage Analysis for n Parallel LDO's
    1. 3.1 Commentary on Parallel LDO's in Real World Applications
  6. 4Ballast Resistor Design and Analysis
    1. 4.1 Selecting the Ballast Resistor Value
    2. 4.2 PCB Ballast Resistor Design vs. Discrete Ballast Resistance
  7. 5Impacts and Opportunities of PCB Parasitic Impedance
  8. 6Design Examples
    1. 6.1 TPS7A57
    2. 6.2 TPS7A47xx
  9. 7Conclusion
  10. 8References

7 Conclusion

A complete current sharing and load voltage analysis has been presented for parallel LDO's using ballast resistors. The equations developed for this analysis have no theoretical limitations on how many LDO's can be placed in parallel, enabling the user to decide what is feasible based on system requirements and components selection. A detailed discussion is provided on sources of error and how to minimize this error in the current sharing design. The salient features of current sharing in LDO's are provided by equations Equation 4, Equation 5 and Equation 6. When VOUT and RB are the same for each parallel voltage regulator, and RB is large (typically at least 50 mΩ), the salient equations are modified to become Equation 6, Equation 9, and Equation 10. When VOUT and RB are the same for each parallel LDO, and when including the effects of parasitic PCB impedance, the analysis uses the modified equations Equation 14 through Equation 18.

An in-depth discussion on ballast resistor analysis and design was presented, including a methodical approach for PCB ballast resistor analysis and design. PCB ballast resistors are favored in applications with low cost, low current loading, or very high temperatures. Discrete ballast resistors are favored in applications requiring small form factor components for higher current loading. Discrete ballast resistors are also favored in applications with tight regulation bands or wide temperature operation.

System architecture development is addressed in reference [6] to provide the minimum number of parallel LDO's required to meet a list of system requirements. A down-loadable tool see reference [7] has been developed using worst case analysis to enable the user to quickly assess how many parallel LDO's are needed to meet their system requirements.