Understanding Smart Sensors - Nomads.usp

Mechatronics and Sensing Systems 251shall apply to devices with a power dissipation of 2W or more, capable of operatingat a case temperature of 100°C and with a continuous current of 1A ormore.” Smart-power devices also provide increased functionality as well assophisticated diagnostics and protection circuitry. Sensing of current levels andjunction temperature is a key aspect of the design for control during normaloperation and for detecting several fault modes.The power in smart-power ICs is typically a power MOSFET or a diffusedMOS device (DMOS). The size of this portion of the silicon area is determinedby the process technology, voltage rating, and desired on-resistance.Process technology determines the on-resistance per unit area and has providedimprovements based on reducing the cell size and improving cell geometry.The voltage rating inversely affects efficiency: a higher voltage rating means ahigher on-resistance for a given area for the power device and lower efficiency.For a given process and voltage rating, the number of cells is increased to meeton-resistance specification. Because increased silicon area directly affects thecost, the highest on-resistance to allow safe power dissipation in the system normallyis specified.The smart-power approach to system design means that a number of circuitelements can be consolidated into a single device. These devices previouslywould have been discrete components or a combination of a standard (or custom)IC and discrete output devices. As illustrated in Figure 11.1, input, frequentlyfrom an MCU, initiates the startup or turn-on procedure. The controlcan have circuitry for soft-start function or charge pump circuitry for increasingthe gate drive voltage. A high degree of functionality is achieved by smartpowerICs, because of the interaction among the control circuit, the powerdevice, the protection circuitry, and the load. Multiple power devices on a singlechip is the most cost-effective use of this technology. That provides spacesaving, component reduction, total system cost reduction, improved performance,and increased reliability from the reduced number of interconnections.Historically, the choice of process technology for smart-power ICs hasdepended on the type of control elements that were integrated. Some circuitelements, like op amps, comparators, and regulators are best implementedusing a bipolar IC process. MOS circuitry handles logic, active filters (timedelays), and current mirrors better than bipolar circuitry. Some circuits, such asADCs or power amplifiers, can be implemented equally well in either technology.Today, a process that has both MOS and bipolar for the control circuitrydoes not have to sacrifice performance or features; if it is combined with theappropriate output devices, the process can handle the power control functionsfor a number of system loads. However, just as a smart sensor needs to fullyutilize the capability of a more expensive process, a smart-power IC also mustprovide advantages beyond those that separate control IC and discrete power