When usingmicrocontrollers, we are frequently asked toturn an external load on and off. Often,this load draws a large current and alsorequires high-side switching, especiallywith automotive components that use thechassis for the current return path. If thewire to the load touches the chassis, theresult can be disastrous: Tracks may end upbeing melted off the control PCB or, evenworse, it could start a fire.
Several power switching devices actuallyembed overcurrent protection, but this is”open loop” — the microcontroller has noidea that there is anything wrong. There areeven some of these devices with a digitaloutput that indicates that an overcurrent isoccurring. In this circumstance, thecontroller can disable the output when thecondition is detected and report on it aswell. Examples of this are the Micrel MIC5020 and the Zetex ZXSM6002 (which hasa three-level status output).
Digital indication is OK up to a point, butif you can measure the actual currentflowing, it becomes possible to improve theunit's functionality. The analog signal,proportional to the current flowing, can befed back to an analog-to-digital converter(ADC), and the controller can base itsactions on this analog value.
Using thiskind of device, the controller is aware ifthere is a load present. The actual currentcan be reported, and decisions can be takenas to whether an overcurrent has occurred oris even approaching the limit. In somecases, as a load device ages its loadcurrent increases, and this informationcould be used for preventive maintenance.
It is possible to design your own currentsense mechanism in the output path, butthere are integrated “intelligent” solutionsavailable that use an internal currentmirror to provide a feedback proportional tothe current being supplied.
Doubtless there are many more, but here area few of this type of device to be gettingon with:
- Manufacturer = NXP; part number =BUK7XXX; current sense ratio 500:1 (click here to seeoptions and designs)
- Manufacturer = Infineon; part number =BTS 555; current sense ratio 30,000:1 (click here to seeoptions)
- Manufacturer = On Semiconductor; partnumber = NILMS4501N; current sense ratio250:1 (click here for moredetails)
- Manufacturer = ST; part numbers =VND5E050, VNQ5E050MK-E, and VN5E025ASO-E;single, dual, and quad (click here for moredetails)
- Manufacturer = International Rectifier;part number = IR3314SPBF; current senseratio 5,300:1 (can use current sense toautomatically shut down on overcurrent; click here for moredetails)
Of the above, the only one I have used isthe IR3314, which I have found to be adevice that is almost indestructible. Itwill survive and automatically shut down ona direct short to ground. I did find,however, that the current transfer ratio tothe current mirror varied considerably(although still within spec), so calibrationwould definitely be required in production.
I know it may seem counter-intuitive to addan analog feedback to a digital output, butthe benefits often outweigh the additionalcosts. The advantage is that your design ismore robust since not only do you have theability to prevent physical damage fromoccurring, you also can do something when anoutput has failed.
Are there any other devices that yourecommend? Do you use any techniques tocheck that your outputs are working?
Aubrey Kagan is Engineering Manager at Emphatec
This article was previously published on EETimes Designlines.