A form of sensorless current mode (SCM) control stabilizes sharing in multiphase input-series-output-parallel (ISOP) dc-dc converter topologies. Previously, ISOP converters have been proposed to reduce the voltage and current ratings of switching devices. Since the inputs are all connected in series, each phase need only be rated for a fraction of the total input voltage. Voltage and current sharing are key - if there is any phase-to-phase imbalance, the system benefits are substantially reduced. In the present work, a simple SCM controller is shown to guarantee stable sharing. Each phase acts independently on the same output reference and desired input voltage. The algorithm and the physics of the circuit lead to balanced input voltages and output currents, even during transients. The ISOP topology is a special case of an interleaved multiphase system. A reduced-order small-signal model is presented. The model is composed of two factors, a single-phase equivalent and a multidelay comb filter. The model fits a measured transfer function well and can be used in feedback design. Experimental results for a five-phase converter demonstrate fast response to a load step, line disturbance rejection, accurate static and dynamic sharing, and high efficiency.