Understanding Smart Sensors - Nomads.usp

256 Understanding Smart Sensorsautomobile battery. The sensor’s output was applied to external circuitry thatprovided shutdown prior to device failure. The sensing capability also allowsthe output device to provide an indication (with additional external circuitry) ifthe heatsinking is not proper when the unit is installed in a module, or if achange occurs in the application that would ultimately cause a failure.11.3.2 Current Sensing in Power ICsCurrent sensing provides protection and control in power ICs. A device thatdemonstrates that approach is shown in Figure 11.4 [4]. The circuit, shown inFigure 11.4(a), includes a voltage regulator, an oscillator, and a charge pump toprovide a high-side switch with an N-channel power MOSFET. This customdevice has multiple current sensing levels for control and fault detection, asshown in Figure 11.4(b). The current-sensing technique uses a sample of thecurrent flowing through the total power MOSFET cells, the ratio of thenumber of sample cells to control cells, as an indication of the total current. Aresistor in series with the sense cells, which handles considerably lower current,provides a voltage proportional to the current in the power device. ThisSENSEFET technique avoids adding a series resistance to the total onresistanceand the subsequent power consumption, which could be excessive foran integrated circuit. 1 The isolated sensing cells for four different currentsensinglevels are inside the four “lassoed” areas (left side, lower section in thecell area) in Figure 11.4(b). The highest current level could trigger an overcurrentfault, and the lower levels can be used for system-specific control points.11.3.3 DiagnosticsThe eight-output driver with serial communications shown in Figure 11.2 representsa high level of complexity for diagnostics. Control and sensing of thefaults of individual output drivers could require over 24 connections for eightoutput drivers. By using the SPI protocol described in Chapter 6, 8-bit serialcontrol of the output and independent diagnostics are possible with only threeconnections. The SPI operates at a frequency of up to 2 MHz and utilizes thelow-voltage, high-speed CMOS capability in the SMARTMOS (Motorola’spower IC) process. 2 With both the input and the output tied directly to theMCU, the unit acts as a closed-loop subsystem between the load and the MCUfor each load. By using a daisy chain technique, several additional devices can1. SENSEFET is a trademark of Semiconductor Components Industries (d.b.a. ON Semiconductor).2. SMARTMOS is a trademark of Motorola, Inc.