SBVA100 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. 2System Architecture Design – How Many Parallel LDO's are Required?
    1. 2.1 Noise
    2. 2.2 PSRR
    3. 2.3 Specifications That Limit the Maximum LDO Current
      1. 2.3.1 Dropout (or Available Headroom Voltage)
      2. 2.3.2 Temperature
    4. 2.4 Load Current (ILOAD) and Load Voltage (VLOAD)
  5. 3System VLOAD and Current Sharing Simulation of Parallel LDOs using Ballast Resistors
  6. 4Examples
    1. 4.1 TPS7A94
    2. 4.2 TPS7A47xx
    3. 4.3 TPS7A57
  7. 5References

2.4 Load Current (ILOAD) and Load Voltage (VLOAD)

The load voltage and load current analysis for parallel LDOs using ballast resistors is discussed in detail in [see reference 4]. To recap, the number of parallel LDO's required to provide the system load current ILOAD is a function of the parameters in Equation 5 and Equation 6, or when VOUT and RB are identical for each LDO, Equation 7. Additional limitations exist on the available current per LDO as described in Section 2.3.

Equation 5. I O U T _ n = V O U T n - V L O A D R B n + V E n R B n
Equation 6. I L O A D = ∑ n = 1 n V O U T n - V L O A D + V E n R B n
Equation 7. I O U T _ n = I L O A D - ∑ n = 1 n V E n R B n + V E n R B

The number of parallel LDO's required to provide the system load voltage is a function of the parameters in Equation 8.

Equation 8. 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

Through some additional mathematics and worse case analysis we can derive the optimum ballast resistance needed to minimize the number of parallel LDO's required while simultaneously meeting the load current and load voltage requirements. The resulting formulas have been placed into an easy to use software tool available for download, see reference 5.