Measurements were captured of parallel TPS7A57 LDOs using ballast resistors [3]. Each LDO in parallel has small variations in PCB impedance which affect the total ballast resistance between the LDOs [1]. This variation in ballast resistance causes each LDO to provide slightly different current to the load [1], causing the output impedance of each LDO to vary slightly from one another (see Figure 3-1).

To reduce the influence of the unequal PCB impedances, the ballast resistor value was increased from 2.5mΩ to 50mΩ. The resulting noise spectra are shown in Figure 5-1.

If noise is a critical parameter that must be absolutely minimized, a better approach than increasing R_B is to match the impedance of the PCB planes after each R_B of the LDOs. If a voltage drop across the ballast resistors is not critical (such as when configuring the parallel LDOs as a constant current source [4]), then a potential simpler approach is to use a large enough R_B (such as 50mΩ) to mitigate the effects of parasitic PCB impedance.