What is claimed is:1. A switched inductor DC-DC power converter, comprising:a first powertrain phase having a first maximum power conversion efficiency at a first load current and comprising:a first power switch; anda first thin-film inductor electrically coupled to an output of the first power switch;second powertrain phases in parallel electrically with one another and in parallel electrically with the first powertrain phase, each second powertrain phase having a second maximum power conversion efficiency at a second load current and comprising:a second power switch; anda second thin-film inductor electrically coupled to an output of the second power switch; andcontrol circuitry in electrical communication with the first and second powertrain phases, the control circuitry varying a number of the first and second powertrain phases that are electrically coupled to a power delivery channel, the number varied to maximize a power conversion efficiency for delivering an output current to a load,wherein:the first load current is different than the second load current,only the first powertrain phase is electrically coupled to the power delivery channel when a target output current is less than or equal to a threshold output current,at least one second powertrain phase is electrically coupled to the power delivery channel when the target output current is greater than the threshold output current,two second powertrain phases are electrically coupled to the power delivery channel when the target output current is twice the second load current or higher.2. The switched inductor DC-DC power converter of claim 1, wherein M second powertrain phases are electrically coupled to the power delivery channel when the target output current is at least M multiplied by the second load current, wherein M is an integer greater than or equal to 2.3. The switched inductor DC-DC power converter of claim 1, wherein the threshold output current is 100 mA.4. The switched inductor DC-DC power converter of claim 1, wherein the first load current is 50 mA.5. The switched inductor DC-DC power converter of claim 1, wherein the second load current is 500 mA.6. The switched inductor DC-DC power converter of claim 1, further comprising an output capacitor in parallel electrically with the power delivery channel.7. The switched inductor DC-DC power converter of claim 6, wherein the first powertrain includes a first LC filter that comprises the first thin-film inductor and the output capacitor, and each second powertrain includes a second LC filter that comprises the second thin-film inductor and the output capacitor.8. The switched inductor DC-DC power converter of claim 1, wherein the control circuitry regulates a switching frequency and duty cycle of the first and second power switches to adjust the output current such that is equal to a target output current.9. The switched inductor DC-DC power converter of claim 1, wherein the switched inductor DC-DC power converter is integrated on a common substrate.10. The switched inductor DC-DC power converter of claim 1, wherein the switched inductor DC-DC power converter is integrated on a common substrate that comprises a multilevel wiring network, and the first thin-film inductor and each second thin-film inductor are integrated in the multilevel wiring network.11. The switched inductor DC-DC power converter of claim 10, wherein:the multilevel wiring network includes first and second integration layers, andthe first thin-film inductor and each second thin-film inductor are integrated in the first and second integration layers.12. The switched inductor DC-DC power converter of claim 10, wherein:the multilevel wiring network includes first, second, third, and fourth integration layers,each second thin-film inductor is integrated in the first and second integration layers, andthe first thin-film inductor is integrated in the third and fourth integration layers.13. The switched inductor DC-DC power converter of claim 12, wherein the first and second integration layers are disposed closer to the substrate than the third and fourth integration layers.14. The switched inductor DC-DC power converter of claim 10 wherein:the multilevel wiring network includes first, second, and third integration layers,the second thin-film inductors comprise first and second thin-film solenoid inductors that are integrated in the first and second integration layers, the first and second thin-film solenoid inductors including respective first and second ferromagnetic cores,the first thin-film inductor comprises a thin-film spiral inductor that is integrated in a third integration layer, andthe thin-film spiral inductor includes at least a portion of the first and second ferromagnetic cores.15. A switched inductor DC-DC power converter, comprising:a first powertrain phase having a first maximum power conversion efficiency at a first load current and comprising:a first power switch; anda first thin-film inductor electrically coupled to an output of the first power switch;second powertrain phases in parallel electrically with one another and in parallel electrically with the first powertrain phase, each second powertrain phase having a second maximum power conversion efficiency at a second load current and comprising:a second power switch; anda second thin-film inductor electrically coupled to an output of the second power switch; andcontrol circuitry in electrical communication with the first and second powertrain phases, the control circuitry varying a number of the first and second powertrain phases that are electrically coupled to a power delivery channel, the number varied to maximize a power conversion efficiency for delivering an output current to a load,wherein:the first load current is different than the second load current, andthe switched inductor DC-DC power converter is integrated on a common substrate that comprises a multilevel wiring network, and the first thin-film inductor and each second thin-film inductor are integrated in the multilevel wiring network.16. The switched inductor DC-DC power converter of claim 15, wherein:the multilevel wiring network includes first and second integration layers, andthe first thin-film inductor and each second thin-film inductor are integrated in the first and second integration layers.17. The switched inductor DC-DC power converter of claim 15, wherein:the multilevel wiring network includes first, second, third, and fourth integration layers,each second thin-film inductor is integrated in the first and second integration layers, andthe first thin-film inductor is integrated in the third and fourth integration layers.18. The switched inductor DC-DC power converter of claim 17, wherein the first and second integration layers are disposed closer to the substrate than the third and fourth integration layers.19. The switched inductor DC-DC power converter of claim 15 wherein:the multilevel wiring network includes first, second, and third integration layers,the second thin-film inductors comprise first and second thin-film solenoid inductors that are integrated in the first and second integration layers, the first and second thin-film solenoid inductors including respective first and second ferromagnetic cores,the first thin-film inductor comprises a thin-film spiral inductor that is integrated in a third integration layer, andthe thin-film spiral inductor includes at least a portion of the first and second ferromagnetic cores.