ZL8800 - Compensation-Free Control Loop Stability - Part 2

Added on November 09, 2013
Learn how Intersil's proprietary ChargeMode Digital Control Loop delivers superior stability, eliminating the need for any compensation.

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Video Transcript

In this section, we're going to take a look at how to set the output voltage and the corresponding compensation settings on the ZL8800. Now we're just showing one output channel here for the device, but it holds true for both channels. As you can see on the output voltage, we have a differential line sensing directly at the point of load. Either at the output capacitance or the final load.

Now to set the output voltage, there are a couple of different methods. You can set the PMBus command to set precisely the output voltage at the target level you want or initially you can also use a resistor setting.

Now the ZL8800 supports what we call pin strapping and registor set up. Pin strapping just means that you could take the two pins, v set 0 and 1, and you can tie them high, tie them to ground, or leave them floating to get a combination of the common output voltages, such as 1.8 volts, 3.3. If you need a more precise setting, you can just tie a resistor to ground to the pin and that sets up a precise output voltage based on the look-up table that's shown in the data sheet. Now that we have the output voltage set up, the next stage is to determine the compensation settings and apply it to the device. The nice thing about the ZL8800 is with a digital controller, there is no compensation. It's essentially compensation-free. The way this works is it give us our feedback loop. There's no compensation components. No Rs, No Cs. If you look at the voltage remote controller, you typically have to set polls and 0s in the feedback loop.

The ZL8800 doesn't require this. There are no polls and 0s. It's all internally calculated with a nonlinear control loop, that we call ChargeMode remote control. It's built-in with default settings so for a common application, you should be able to power up and it will work just fine. If you want to make optimizations in turn of a transient response, it is possible to adjust the gain and residual and settings in the device through the PMBus commands.

In the next video sections, we'll go through exactly how to change these and what it appears to be when we look at the waveforms of the load transient on the bench.

One of the main benefits of the ZL8800 and the ChargeMode control loop is the fact that it doesn't require compensation. Therefore, no matter what output stage you use, the device will always come up in regulation and be stable. To showcase this, what he have is a Dual Channel board running, and we removed all the bolt capacitance except for two capacitors, one POSCAP, and one aluminum electrolytic.

So what we're going to test is, once the device is running in load transient, and we have a 5 amp to 15 amp load step, we're going to disconnect one of those capacitors. We're just going to cut the aluminum electrolytic cap out of the circuit, thereby effectively reducing the output capacitance by half during run time. The benefit of ZL8800 is it will automatically adapt to the new capacitance, keeping the device in regulation, and keeping the device stable.

What we have set up here is the Dual Channel ZL8800 evaluation board. So we're just going to use this lower channel, which is set with a 12 volt Vin, and a 1.2 volt Vout. As you can see here at the bottom we have a small POSCAP, which is at full capacitance, in parallel with aluminum electrolytic. So we're going to run a continuous load transient with this load box, so we turn this on. As you can see on the scope, we're running a continuous 5 amp to 15 amp load transient.

Now our output voltage deviation during this time looks to be about 20 millivolts. So as we cut this capacitor out, we should have the total capacitance. We would expect a slight change in the output voltage of deviation but we should see the device will remain in regulation. So let's go ahead and pop this cap.

So as we just cut the capacitor, you saw the output voltage deviation on the scope did change as a result of less output capacitance, but at all times, the ZL8800 remained in regulation with a stable waveform. Power good remained active. The device was always tightly regulated. Therefore, the benefit of the ZL8800 is it can always adapt to any change on the output filter stage. Different inductor sizes. Different capacitance values. Even as those change over time, the device will work continuously to adapt to make sure you have a stable output waveform.