We’ve already seen how to use an H-bridge chip to switch relatively large voltages (and the corresponding big currents) needed to drive electric motors. In many other cases, you will want to turn large voltages on or off, and in this section I will show you an easy way of doing just that.
The Motorola MC33298 is a chip that is controlled by a microprocessor using SPI and can switch eight power sources on or off. This chip can handle voltages between 5V and 26.5V, with currents as large as 6 Amps. If you need to turn electrical systems on or off, this chip is for you. Its primary use is for industrial and automotive applications, controlling power to subsystems such as heaters, small air-conditioning units, moderate voltage lightbulbs, small pumps, and so on. Obviously, it won’t handle the high AC voltages that come out of your wall socket, so don’t use it for switching power to your home appliances!
The basic schematic for the circuit is shown in Figure 12-35.
The MC33298 has two power-supply pins. VDD is a 5V supply and powers the chip’s internal digital logic. It’s decoupled to ground using a 100nF capacitor. V PWR is the supply voltage for the external subsystems (represented in the figure by each LOAD rectangle) and can range from 5V to 26.5V. There are eight switch outputs, labeled OUT0 through OUT7. When a given switch is activated, the corresponding output is connected through to the V PWR supply, thereby turning that subsystem on. The MC33298 has short-circuit detection and shutdown (with automatic retry), overvoltage detection and shutdown, current limiting on the outputs, output clamping during inductive switching, and thermal shutdown if the device is dissipating too much power. Higher currents may be switched by tying two or more outputs together so that the current is shared by more than one pin. By tying all outputs together, currents as high as 48A may be switched, limited only by the total power dissipation and corresponding thermal shutdown limit.
The chip has a standard SPI port, allowing it to be interfaced to, and therefore controlled by, most microprocessors. The SPI signals MOSI, MISO, and SCLK are connected directly to a processor’s SPI pins. The chip’s select input, CSB , is controlled by a digital output of the processor and is used to select the device during a SPI transfer. The device may be reset and all outputs turned off by asserting its RESET input. Again, this too can be driven by a digital output of the processor so that the chip may be turned off under software control. The MC33298 supports SPI daisy-chaining, so multiple devices may be coupled together.
The SPFD pin is Short Fault Protect Disable. Sending this pin high allows the internal over-current detection circuitry to be disabled. When switching some loads, such as lightbulbs, there is a very high current for a short period of time. This would normally cause the MC33298 to register an overcurrent fault and shut that output off. The SPFD pin allows this protection to be overridden so that such loads may be controlled. Even though the overcurrent protection is bypassed, the MC33298 is still protected. If the high current lasts long enough, the chip’s thermal shutdown circuit will kick in, thereby preventing damage. SPFD may be driven by a processor digital output and should be used with caution! For normal operation (with overcurrent protection on), this pin should be low.
And with that, we’ve reached the end of Designing Embedded Hardware. In this book, I have tried to introduce you to the basics of creating small computer systems, without getting bogged down in complicated detail. As a result, this book isn’t the final word on computer electronics. It is merely the beginning, providing you with sufficient knowledge to read and explore further. Building you own computer hardware is rewarding and fun. I wish you the best of luck as you join the ranks of computer designers around the world.