An electronic device comprising: a system on chip, including: a plurality of internal subsystems (10, 12, 13); and a power management system comprising a plurality of internal voltage regulators (14, 16) arranged to supply power to the plurality of internal subsystems (10, 12, 13); and an external module comprising at least one external voltage regulator the electronic device characterised by: each of the internal voltage regulators (14, 16) having an associated current limiter (30, 32); and the external module being connected to the system on chip such that the external voltage regulator can provide power to at least one of said internal subsystems (10, 12, 13) on the system on chip, wherein the power management system is arranged during a start-up phase to enable said internal voltage regulators (14, 16) and said current limiters (30, 32) and in a subsequent phase to; determine an externally powered set of the internal subsystems (10, 12, 13) powered by the external voltage regulator(s); disable one or more of said internal voltage regulators (14, 16) corresponding to said externally powered set of internal subsystems; and disable one or more of said current limiters (30, 32) associated with the internal voltage regulators (14, 16) which correspond to the internal subsystems (10, 12, 13) not in the externally powered set of internal subsystems. a system on chip, including: a plurality of internal subsystems (10, 12, 13); and a power management system comprising a plurality of internal voltage regulators (14, 16) arranged to supply power to the plurality of internal subsystems (10, 12, 13); and a plurality of internal subsystems (10, 12, 13); and a power management system comprising a plurality of internal voltage regulators (14, 16) arranged to supply power to the plurality of internal subsystems (10, 12, 13); and an external module comprising at least one external voltage regulator the electronic device characterised by: each of the internal voltage regulators (14, 16) having an associated current limiter (30, 32); and the external module being connected to the system on chip such that the external voltage regulator can provide power to at least one of said internal subsystems (10, 12, 13) on the system on chip, wherein the power management system is arranged during a start-up phase to enable said internal voltage regulators (14, 16) and said current limiters (30, 32) and in a subsequent phase to; each of the internal voltage regulators (14, 16) having an associated current limiter (30, 32); and the external module being connected to the system on chip such that the external voltage regulator can provide power to at least one of said internal subsystems (10, 12, 13) on the system on chip, wherein the power management system is arranged during a start-up phase to enable said internal voltage regulators (14, 16) and said current limiters (30, 32) and in a subsequent phase to; determine an externally powered set of the internal subsystems (10, 12, 13) powered by the external voltage regulator(s); disable one or more of said internal voltage regulators (14, 16) corresponding to said externally powered set of internal subsystems; and disable one or more of said current limiters (30, 32) associated with the internal voltage regulators (14, 16) which correspond to the internal subsystems (10, 12, 13) not in the externally powered set of internal subsystems.