When using microcontrollers, we are frequently asked to turn an external load on and off. Often, this load draws a large current and also requires high-side switching, especially with automotive components that use the chassis for the current return path. If the wire to the load touches the chassis, the result can be disastrous: Tracks may end up being melted off the control PCB or, even worse, it could start a fire.
Several power switching devices actually embed overcurrent protection, but this is "open loop" -- the microcontroller has no idea that there is anything wrong. There are even some of these devices with a digital output that indicates that an overcurrent is occurring. In this circumstance, the controller can disable the output when the condition is detected and report on it as well. Examples of this are the Micrel MIC5020 and the Zetex ZXSM6002 (which has a three-level status output).
Digital indication is OK up to a point, but if you can measure the actual current flowing, it becomes possible to improve the unit's functionality. The analog signal, proportional to the current flowing, can be fed back to an analog-to-digital converter (ADC), and the controller can base its actions on this analog value. Using this kind of device, the controller is aware if there is a load present. The actual current can be reported, and decisions can be taken as to whether an overcurrent has occurred or is even approaching the limit. In some cases, as a load device ages its load current increases, and this information could be used for preventive maintenance.
It is possible to design your own current sense mechanism in the output path, but there are integrated "intelligent" solutions available that use an internal current mirror to provide a feedback proportional to the current being supplied.
Doubtless there are many more, but here are a few of this type of device to be getting on with:
- Manufacturer = NXP; part number = BUK7XXX; current sense ratio 500:1 (click here to see options and designs)
- Manufacturer = Infineon; part number = BTS 555; current sense ratio 30,000:1 (click here to see options)
- Manufacturer = On Semiconductor; part number = NILMS4501N; current sense ratio 250:1 (click here for more details)
- Manufacturer = ST; part numbers = VND5E050, VNQ5E050MK-E, and VN5E025ASO-E; single, dual, and quad (click here for more details)
- Manufacturer = International Rectifier; part number = IR3314SPBF; current sense ratio 5,300:1 (can use current sense to automatically shut down on overcurrent; click here for more details)
Of the above, the only one I have used is the IR3314, which I have found to be a device that is almost indestructible. It will survive and automatically shut down on a direct short to ground. I did find, however, that the current transfer ratio to the current mirror varied considerably (although still within spec), so calibration would definitely be required in production.
I know it may seem counter-intuitive to add an analog feedback to a digital output, but the benefits often outweigh the additional costs. The advantage is that your design is more robust since not only do you have the ability to prevent physical damage from occurring, you also can do something when an output has failed.
Are there any other devices that you recommend? Do you use any techniques to check that your outputs are working?