ZL8800 - Input Current Measurement Capability - Part 3

Added on November 09, 2013

Understanding how the ZL8800 measures input current and how it can monitor efficiency of a two-phase device.

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In this video we're going to talk about the input current measurement capability of the ZL8800. Now the ZL8800 is a two-phase or a dual output device. But you do get one input current measurement per chip. So what this means is, if you're using a two-channel output, you're just going to get a single measurement. But in a two-phase application, now you can actually use that current measurement to monitor the efficiency of the device.

The way it works and is implemented is, it's just a shrunt resistor that's used. This is going to be a small, couple milliohm resistor. Now the thing you have to be careful of is the maximum voltage, you can see across the input for the differential sensing, is 20mV. So you need to make sure that you calculate your input current and the resistor appropriately such that you're always below 20mV if you want to get the reading.

Quick way of doing this, just calculate what your output power is and use a worst case efficiency for max load and then from that you can size approximately the input resistor. If you don't wish to use this function, you can just tie these two pins, the IIN positive (IINP) and the IIN negative (IINN) to the VIN. One practical consideration you may want to take in mind is, placing this resistor before you have any of those high-frequency ceramic capacitors for bypassing. Because when this Hi-Fi switch turns on, you're getting a lot of AC current circling through those CADs. This resistor, what you really want by placing before, is you see the average current coming in.

Now we can go to the bench and take a look at the GUI on how this input current measurement is taken and what results we see.

So let's do a demonstration showcasing the ZL8800 with an input current measurement. This is the dual phase board for the ZL8800. So it's using both of the channels tied together at a single output, to create a two-phase solution. You'll notice on the board, we have a resistor that's placed here near the input. This is the current sense resistor that is used to measure the input current. It's a 5mOhm resistor and we're just measuring the voltage drop across it with internal ADC.

So right now as you can see with the GUI, we're using a 12V input, a 1.2V output and we don't have any load currents. Now as you start loading it down, you'll see in the GUI, the load current will change and also the input current will change. So let's put a 5A load on. And right away in the scope, we have a current probe connected to the output device and you can see that load current starts increasing and the input current will increase by a small amount also. It's fairly linear so as we keep increasing the load, we'll accurately be able to track that and the input current should follow it roughly the same.

So let's move up to 10A. You can see the output current is about 5A per division on the scope, so we're pulling about 10A load right now and the input current is following it commensurately. I can keep increasing it. I can go way up to 20A. And once again we get accurate tracking with the GUI and the input current is accurately tracking also.

Now you notice the input current doesn't appear to be quite linear, that's because as we increase the load current, we're actually moving up the efficiency curve. So we're moving from maybe 82% to 85% to 90% as we start moving across the load range, because this part's really optimized for high efficiency between 20% to about 80% of full load.

This evaluation board is capable of doing up to 60A. It's designed to do 30A per channel, and once again as they're combined, it means a total output capability of 60A. For more information, please continue watching the remaining videos or you can evaluate your own ZL8800 by getting an evaluation board from renesas.com.