Map Unequal Loads on a Dual-Output DC-DC Converter

A dual-output converter can remain below its total power rating while one output moves outside the range required by its load. The important operating point may be a heavily loaded positive rail and a lightly loaded negative rail, or the reverse. Review the pair of loads as a map rather than a single wattage total.

Read the cross-regulation condition

Murata’s legacy NDX datasheet defines cross regulation through a particular change in one output load while the other is held at a stated condition. It also identifies minimum loading for specified performance. This is an illustrative specification format, not a recommendation to select the obsolete series.

For the actual converter, record regulation limits, minimum-load conditions, individual output limits and total power constraints. Note which values are typical and which are specified bounds. A statement about balanced loads should not be applied automatically to a strongly asymmetric application.

Build a two-rail operating map

List standby, normal operation, maximum signal drive, disabled peripherals and startup. Calculate positive and negative rail current for each mode, including downstream regulators and bias networks. Preserve the sign convention so that currents returning between rails are not accidentally counted twice or omitted.

Identify the extreme combinations that can occur together. Adding both independent maximum currents may overstate one case while missing a light-load corner on the other rail. The table should describe physically possible operating states and explain any excluded combination.

Measure both outputs together

Apply representative load pairs and record both rail voltages relative to the intended common node. Include input-voltage variation and dynamic transitions relevant to the product. A test that measures only the rail whose load changed can miss the cross-regulation response on the opposite output.

Use the converter’s prescribed output network and document the measurement arrangement. Keep steady-state regulation separate from startup overshoot, ripple and transient recovery. Each may require its own criterion even though they appear on the same oscilloscope trace.

Evaluate the cost of a workaround

A preload may improve a specified operating condition while adding continuous dissipation. A post-regulator may require headroom that disappears at another corner. Independent converters can change size, noise and sequencing. Compare candidate solutions using the full load map instead of improving only the failing bench point.

Retain the accepted operating envelope and any required minimum loading in the design documentation. If firmware later disables a circuit to save power, revisit the map. Reducing one rail’s current can be a regulation change for the other rail even when the total converter power becomes smaller.