ZL8800 - Protections for Input Voltage Rails - Part 7

Added on November 05, 2013

Learn how to set up voltage monitoring along with warning and fault levels for both overvoltage and undervoltage through PMBus.

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Now we're going to talk about some of the protection levels that are available for the input rails.

So for the device on the input voltage going to the VDP into the device, it will constantly monitor the input voltage and then you can set up warning levels and fault levels around the device. So with PMBus what you could set up is for overvoltage a warning limit and the fault limit. For the undervoltage, you'll set a warning limit and a fault limit. Now what these do is the warning limit is just if you happen to cross it in normal regulation, the device will issue a warning. The alert pin on PMBus will sit there and send a signal to your host micro control and say an event's happened, but it's not taking any action. Only when you cross the upper level or the lower level will the device shut down and take the appropriate action. And all of these levels are adjustable through PMBus commands.

A simple way also of setting the input voltage lock out or the lower level is to the use the UVLO pin. This is just using a simple resistor to ground. You can set a voltage based on the lookup table in a data sheet anywhere from about 4V all the way to 16V. So that's a great way of simply putting the lower level you want depending on the input voltage rail, whether it's 5V or a 12V input rail.

It can also be used from a programming standpoint. One trick is set the resistor size to set the UVLO level to 16V. That way when you apply 12V the device will not start switching. Then you can just program in all your settings through PMBus including the new UVLO level before you enable the device. Just a simple way of making sure you program the device before it starts switching.

There's also one other input bias rail. It's a VMON pin. Now this is a voltage monitoring pin. If you use a resistor ratio externally, at 16 to 1, you can monitor in the other bias rail. Kind of a common usage for this is the gate driver. So in this case, the ZL1505, you can measure the VCC or the input voltage for that driver and if it drops out, you can get the warning, as appropriately, to prevent a situation from occurring where you're trying to switch with too low of a gate driver. Conversely, it could also be used for any other rail in the system. There's quite a bit of flexibility with this.

That covers the different protection levels that are available for the input voltage rails.