How to interpret power supply IC specifications: How to read characteristic graphs and waveforms

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How to interpret power supply IC specifications: How to read characteristic graphs and waveforms [Copy link]

When formulating a power supply solution, it is important to understand the characteristics and evaluation methods of switching regulators, including the technical specifications of power supply ICs.

How to read graphs

In addition to the specification value table, the technical specifications also include characteristic graphs. The reason for this is that the values in the specification value table are the values at a certain point in the specified conditions, and continuous characteristics such as amplitude or trend, and changes in multiple conditions cannot be known, so they are supplemented. In addition, in terms of specification values, there are many graphs that show unspecified characteristics or circuit characteristics. Here, we should not only mention the values in the specification value table, but also remind you to check the graph and understand the trend or continuous characteristics before designing. The key point is that graphs can be said to be indispensable for design. Then, let's explain it in detail with actual graphs.

The following table is a partial table of specification values of technical specifications. Please observe the parameters of I _CC and circuit current during operation. I _CC has a guaranteed maximum value of 500μA under the temperature condition of "Ta＝25℃" and several voltage and current conditions. The standard value is 350μA, and there is no guaranteed minimum value. From this table, it can be seen that "I _CC is around 350μA at 25℃, not exceeding 500μA.

Here, if the operating temperature range of the device to be designed is 0℃ to 60℃, it is best to suppress the tendency of increasing or decreasing the standard value or temperature fluctuation at 0℃ or 60℃ in advance for design. However, this cannot be known from this table.

Sometimes the temperature condition setting of the specification table shows all operating temperature range conditions, such as the specification value of the range condition of -40℃～+85℃. In this case, it is different from the value of 25℃. If the temperature range of the above equipment is 0℃～60℃, it can be known that it is within the guaranteed value displayed, but there is no need to explain it if it cannot be further.

This graph is also recorded in the IC's technical specifications, and shows the relationship between temperature and ICC in a curve _. In addition, it further shows two VIN-related conditions that are not in the specification value table. If it is the 0℃ to 60℃ condition just now, even when VIN is high, it can read about 400μA±20μV.

_{In addition, it is possible to read the tendency that ICC} increases as the temperature rises and _{that ICC} increases as _VIN increases. This will be important information in design.

It should be noted that the values or trends that can be read from the graphs are not guaranteed values but standard characteristics.

Although _ICC will not exceed the 25℃ maximum value of 500μA even if it exceeds 80℃, the correct understanding in this example is "maybe, but it is not guaranteed to be stable."

The following graph shows the relationship between efficiency and output current. This graph is often seen in the technical specifications of switching regulator ICs. However, in almost all cases, there is no efficiency item in the specification table, and of course, not only the maximum/minimum values, but even the standard values are not shown. In other words, efficiency is a characteristic that is not guaranteed.

However, although there is no guarantee, there is no way to explore without a goal. In another sense, it is an example of specifying conditions or circuits, components, etc. and showing (roughly optimal) characteristics in a graph.

From this graph, you can imagine the efficiency that can be achieved by the load current of the circuit itself. The efficiency curve is also an important check point when selecting a power supply IC.

How to read waveforms

In addition to graphs, technical specifications may also display operating waveforms, etc. The purpose is basically the same as that of graphs, and the purpose is to indicate characteristics that are difficult to indicate in the specification value table.

This figure shows the waveform displayed on the oscilloscope screen. The vertical axis is voltage and the horizontal axis is time. It shows the ON/OFF waveform of the switching node and the ripple voltage presented at the output.

The ripple voltage can be read from the output voltage waveform. If you look closely, you can see that the ripple increases when the switching waveform is H and decreases when it is L. In addition, you can also see that the ripple frequency is basically the same as the switching frequency, that is, synchronized.

Of course, these will also become standard characteristics. The standard means that they are compared with the waveforms of the same nodes in the circuit itself, and can also be used as comparison objects for optimization.

The key points of how to read charts and waveforms are summarized below.

◆Graphs or waveforms may sometimes provide characteristics that are not included in the specification values, so be sure to check

• The continuous characteristics of the supplementary point value in the specification value table can be confirmed
• Changes in specification values due to changing conditions such as temperature can be checked
• It can display characteristics that are not in the specification values (output ripple and efficiency in the above example) and can also be used as a comparison target for optimization degree.
• The values in the chart are standard values, not guaranteed values.

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