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

2.2 PSRR

If the input voltage is held constant, then the PSRR of an LDO goes down as load current goes up. By paralleling LDOs the load current is shared among each LDO which raises the system PSRR when using the same input voltage. When LDOs are paralleled using ballast resistors, the only increase in power dissipation is the small loss contributed by the ballast resistance. To assess how many parallel LDOs are needed to meet the system requirements, first determine the PSRR required in the system. Then review the LDO data sheet to estimate how much current a single LDO can provide with your system's headroom while still meeting the system PSRR specification (ILDO_PSRR). Finally, divide the total system load current, ILOAD, by ILDO_PSRRto obtain the number of parallel LDOs, n, required to meet the system PSRR specification.

Equation 2. n=ILOADILDO_PSRR
GUID-20221108-SS0I-QNPZ-ZCFF-MSDZB5RHVNTP-low.png
VIN= 1.05 V, VOUT = 0.75 V
Figure 2-2 PSRR vs. Frequency and IOUT for 3x Parallel TPS7A57 and 1x Single TPS7A57