Checking Current Sharing Between Parallel LDO Regulators

Connecting two regulator outputs together does not guarantee that each supplies half the load. Small differences in their regulated voltages can create a large current imbalance. A parallel arrangement needs an explicit sharing method and verification that each device remains within its individual limits.

Establish the approved architecture

TI’s parallel-LDO white paper discusses using ballast resistors to improve sharing and the associated effect on load voltage. Its analysis also considers regulator accuracy and resistor selection rather than assuming identical outputs.

Confirm that the selected regulators and their protection behavior are suitable for the proposed connection. Record the sharing resistors, feedback pickup points, output capacitors and any required isolation. Do not copy a parallel arrangement from another regulator family without checking the relevant operating requirements.

Estimate mismatch sensitivity

For an idealized pair of voltage sources with equal series resistors, the difference between branch currents is approximately the source-voltage difference divided by one branch resistance. Thus, a 10 mV mismatch with 0.1 ohm in each branch produces approximately 100 mA of current imbalance.

This example shows sensitivity only. Real regulators have dynamic control loops, limits and temperature dependence. Include resistor tolerance, connection resistance and the voltage drop allowed at the shared load. Increasing ballast resistance improves one aspect of sharing while consuming voltage and power budget.

Measure each branch

Provide a method to measure individual regulator current without introducing a different resistance into one branch. Capture startup, steady operation and load steps. Record each device’s temperature after the system reaches a representative thermal condition.

Repeat across input voltage and load, including conditions near dropout. Check whether one regulator reaches current limit or thermal protection first and what that does to the remaining branch. A stable total output current can conceal one device carrying a disproportionate share.

Review abnormal and transitional states

Examine one regulator disabled, one input absent and a delayed startup of one branch. Determine whether the powered output can drive current into the inactive regulator and whether that state is permitted. Include the documented fault and recovery behavior in the review.

Keep the sharing calculation, component tolerances and branch-current measurements with the schematic. State the verified combined load and thermal conditions. If a substitute regulator changes voltage accuracy, current limiting or reverse-current behavior, repeat the affected checks. A combined current rating should follow demonstrated sharing and individual operating margins, rather than being the arithmetic sum of two package labels. Include connector and routing resistance in the assembled-board comparison.