A composite integrated circuit incorporating two LDMOSFETs of unlike designs, with the consequent creation of a parasitic transistor. A multipurpose resistor is integrally built into the composite integrated circuit in order to prevent the parasitic transistor from accidentally turning on. In an intended application of the composite integrated circuit to a startup circuit of a switching-mode power supply, the multipurpose resistor serves as startup resistor for limiting the flow of rush current during the startup period of the switching-mode power supply.