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

3 Current Sharing and Load Voltage Analysis for n Parallel LDO's

A model of the parallel LDO's can be developed, and used to provide a universal equation for current sharing in n parallel converters (Figure 3-1). Using mesh current analysis, we can derive the output current for any n number of parallel LDO's:

Equation 4. I O U T n = V O U T n - V L O A D R B n + V E n R B n
GUID-20221017-SS0I-HDM4-WTJX-TQWZ0XFZ7SK9-low.png Figure 3-1 Equivalent Model for n Parallel LDO's Using Ballast Resistors.

The total load current is equal to the summation of current provided by each individual LDO. Thus:

Equation 5. ILOAD=∑n=1nVOUTn-VLOAD+VEnRBn

In addition to deriving the current sharing of each LDO, we wish to compute the voltage on the load. Equation 5 can be rearranged to calculate VLOAD.

Equation 6. V L O A D = ∑ n = 1 n V O U T n + V E n R B n   -   I L O A D ∑ n = 1 n 1 R B n

The universal equations for the current sharing and load voltage analysis are summarized in Equation 4, Equation 5 and Equation 6 for any n number of parallel LDO's using ballast resistors.

Equation 5 can be used to determine the maximum current provided by any LDO in a parallel system. This occurs when the error term for one LDO is the most positive while the error terms for the other LDO's are the most negative. In this worst-case scenario, IOUTn raises to its maximum possible value using Equation 4. This analysis provides the maximum current that each LDO must be designed to, and should be used in power dissipation analysis and thermal analysis for the parallel LDO's.