Basic functions of VI chip

These two boards can be joined together to form a complete DC-DC converter . On the other hand, the VTM can be viewed as a point-of-load device, which places the current multiplication and voltage division functions at the point-of-load and is isolated, while the upper-level PRM acts as a non-isolated regulator, controlling the ratioed bus to supply power to the VTM to achieve output voltage regulation. The voltage of the ratioed bus is relatively high, so the PRM can be placed in an unimportant place far away from the point-of-load, which means that valuable point-of-load space can be vacated for other devices with more important functions.

Before testing, you should prepare the data sheets of each device and the instructions for use of the evaluation board so that you can refer to them whenever you need them during the test.

We will mainly demonstrate the operation of the adaptive mode. We will see that this PRM-VTM combination can provide tight regulation without the need for actual remote sense wiring.

In this mode, the PRM can control the factored bus Vf to achieve the required voltage regulation accuracy at the load point Vo.

This is roughly the case with the test setup. Here we try to separate the two boards quite far apart and use only light-diameter twisted-pair wires to transmit the power flowing on the split bus.

Input changes, ratio bus voltage regulation (Vf)

The PRM provides a lot of flexibility, allowing us to set its operating mode and other parameters, using only simple peripheral resistors. Here we can also see the PRM's open loop operating mode. In this open loop mode, the PRM will tighten the factored bus to ground for regulation.

Now suppose that Vf is set to 36V. This is done with an appropriate Ros. The bus voltage will be regulated all the time, and will remain so throughout the input range. The input range of our PRM is 16V to 50V. Regardless of the input operating at any point between 16V and 50V, the factorized bus will remain regulated at 36V.

Use Ros to set the factored bus voltage (Vf)

Just now we set the ratio bus to 36V, but it can be set to any operating point between 26V and 50V, just select Ros. The evaluation board has a potentiometer that can easily set the ratio bus voltage you want, which actually means the load point voltage you want to set, because the input and output of the VTM are in a linear ratio, that is, a fixed input and output ratio, called the K factor.

Increasing Ros will increase Vf (26-50V)

Downregulation of Vf using Rsc

Once the factored bus voltage, or equivalently the output point-of-load voltage, is set, the output voltage can be varied using the SC terminal, which is another useful flexibility. On the evaluation board, there is a potentiometer that can be used to fine-tune the factored bus or load voltage. The transfer function from the SC terminal to Vf has a fairly high bandwidth, which can be used to formulate many useful applications.

The PRM preset current limit point can be adjusted down through the IL terminal

Another flexibility of the PRM is that its preset current limit can be adjusted down, which can be done with a resistor of appropriate value, and fine-tuned at the IL port. It is worth noting that the downstream VTM may also have its own current limit setting. It is usually more ideal to let the PRM replace the VTM's current limit function, so it may be necessary to move the PRM's current limit setting down if its preset point is higher than that of the VTM.

Open loop mode - Vf regulation

Now let's assume that the factored bus is set to 36V, and then change the load current to see how it operates. The factored bus is regulated, and the load voltage is also regulated. For many applications, this is enough, and its regulation rate is about 5%. Can it be improved further? Of course it can. In fact, the PRM is currently operating in open-loop mode, only stabilizing the factored bus. The PRM is more commonly used in another mode called adaptive loop mode, which we will soon see what it is like.

Now it is still in open loop mode, the ratio bus is tightly to ground for voltage regulation, and the load voltage will drop at high current, because the VTM output has internal resistance. But in fact, the internal resistance is very small, only in the order of a few mΩ. The VTM used in our test has an internal resistance of about 5.1 mΩ.

Vo will be stabilized under adaptive loop

All that needs to be done when starting the adaptive loop is to add a compensation device, which is just to add a resistor at the CD terminal; the amount of resistance must be determined according to the VTM used, which can be found in the data sheet table, or calculated using the formula, which can also be found in the data sheet.

Now we start testing, the 28V input is ready, but there is still no output, because we haven't started the PRM yet.

We first set the load current to 0A, then release the PC terminal of the PRM to start it.

Enable PRM via PC

Startup; the factored bus Vf is rising, and the next thing that happens is that the PRM sends a start VC pulse to start the following VTM. This is seen in the oscilloscope diagram.

The output meter, because it is a slow-reacting measuring tool, cannot determine the output value during the output establishment change.

The output is now settled and is 4.5V, which is the expected number.

We will increase the load current and see how the voltage regulation is at the load point of the VTM output.

Increase load current

The load current is now 5A. Vo is tight to regulation.

Higher load current, 10A now. We appear to be getting very good load regulation, why?

Note that the factored bus voltage is being controlled by the PRM.

Increase load current (25A)

Now it is heavily loaded, and the load voltage regulation is excellent. The reason is that the PRM can control the ratio bus, which is controlled according to the feedback current information. In this way, the voltage drop of the output internal resistance is compensated, and the output voltage is thus stabilized. It should also be noted that the voltage at the load point is controlled and stabilized without remote voltage sampling. This working mode of the PRM-VTM combination is called adaptive loop mode.