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
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.