ZL8800 - Overcurrent Fault Protection - Part 8

Added on 11月 04, 2013

Learn how Intersil's new ZL800's ADC monitors the entire output current waveform.



Video Transcript

In this video, we are going to talk about how the ZL8800 handles overcurrent fault protections.

Now if you looked at the video earlier regarding how output current telemetry sets up, you'll notice that we monitor with our ADC the entire continuous waveform of the output current through the inductor. So what you've got is you have a choice of two different slopes: upslope sampling or downslope sampling. Now the default is the downslope sampling, because for most applications if you're running from a 12V input, you're converting to low output voltages, so this is going to be the bulk of period of time.

The way the device works is it continuously samples the inductor current at all points. If you're using downslope, it will only look in this period of time after a certain blanking period, and you get to slack this time period to avoid any switch mode transitions which causes noise to come onto the signal. Right after the blanking time expires, the device will start sampling continuously.

Now, the first few readings are probably what is going to be tripping your peak overcurrent protection. This is a cycle-by-cycle protection. So every cycle it's constantly monitoring it and comparing it to the peak threshold that's set up. All the other points, including those, are continuously sampled and averaged together. That way you also get an average overfault response. So for peak current, or for an overcurrent event, you get two settings: peak and average. And the same thing applies for undercurrent. There's also a peak undercurrent and average undercurrent fault.

Now the question that occurs is, "Well if the device takes a peak fault or average, what occurs?" Well, that really comes down to how the device is setup, in terms of the fault response. This is another PMBus command in the device that allows you to configure exactly the currents and the next steps. There's a couple different options. Either the device will shut down instantly and remain off or you can have it to continuously retry. A third option is you could have it to retry a certain number of times with certain times intervals between the faults. And all of these changes are configurable through the GUI with the device using the PMBus interface.