Old application note from ON Semiconductors on getting increased power capability by paralleling H-bridges and overcoming current limit imbalance between the two drivers by properly designing the board layout. Link here (PDF)
Two or more H-bridges can be operated in parallel to increase the current handling capacity of the circuit. In this application note, paralleling of H-bridges has been exemplified using a dual H-bridge model MC33932/MC34932. However, paralleling of H-bridges is not an easy task, as any offset or mismatch between the two MOSFETs can cause one of them to hit the over current/temperature limit before the other, forcing very high-current through one of the H-bridges in parallel configuration, which may initiate device shutdown.
The objective of this application note is to present a method to obtain twice the current from a dual H-bridge by paralleling the dual H-bridges located on the same die. This document also presents the various methods to calculate the junction temperature, to ensure the device operates within the thermal limits specified in the data sheet.
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