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

4.1 Selecting the Ballast Resistor Value

A ballast resistor is an electrical component used to prevent current faults in a system. For parallel LDO designs it is placed between separate LDO outputs, and also between the LDO output and the load. This limits the current that a single LDO can provide to the load.

Equation 4 and Equation 5 have too many unknowns to be uniquely solvable. For simplicity, engineers will typically set VOUT of each converter to be the same. Similarly all ballast resistors will be set to the same value. As shown in Equation 4 the remaining unique contributor to IOUTn is VEn.

Traditionally the ballast resistance was chosen using Equation 9 to set the current imbalance IMAX of the parallel LDO's. This formula ensures that the LDOs do not go into current limit but does not account for the required load voltage, VLOAD, which is also a requirement for most modern power supplies designed with parallel LDO's. A down-loadable software tool has been developed to design RB for commonly used LDO's given a set of system requirements (see references 6 and 7).

Use either Equation 9 or the software tool to calculate the ballast resistor. Then using Equation 4, Equation 5, and Equation 9 we can solve for IOUTn in Equation 10. Calculate IOUTn in accordance with Section 3 to verify that no LDO enters current limit after accounting for component tolerance. Use Equation 10 in designs with identical VOUT and RB values for each LDO, otherwise use Equation 4.

Equation 9. RB =max1<x<n⁡VEn-min1<x<n⁡VEn∆IMAX
Equation 10. I O U T n = I L O A D - ∑ n = 1 n V E n R B n + V E n R B