How to Interpret Properties Graphs and Waveforms

In the section on [How to Read Power IC Data Sheets], we discussed [Data Sheet Cover Page], [Block Diagrams], [Absolute Maximum Ratings and Recommended Operating Conditions], and [Key to Electrical Characteristics]. In this section, as part of [How to Read Power Supply IC Data Sheets], we will explain [How to Interpret Properties Graphs and Waveforms].

In any case, in order to understand the properties of and evaluation methods on switching regulators, it is extremely important to correctly read power supply IC data sheets.

・How to view a graph

Data sheets provide properties graphs in addition to tables of specification values. These graphs provide information on continuous properties with respect to changes in range, tendency, and multiple conditions in order to complement the one-point values in a table of specifications. Also, data sheets include a number of graphs that depict properties that are unidentified as specification values or characteristics as an application circuit. What is important is that the design task should be performed by checking graphs and understanding the trends and continuous properties beyond that which is provided in the tables of specifications. It would not be an overstatement to say that graphs are essential tools in performing a design task. That said, we will provide a specific description of the design task by using actual graphs.

The table below represents excerpts from specification values provided in a data sheet. Let us look at the parameter I_CC, which is referred to as Operating Supply Current. For I_CC, the maximum value 500μA is guaranteed under a [Ta＝25℃] temperature condition and several voltage and current conditions. The typical value is 350 μA, without any minimum guaranteed value. What is apparent from the table is that at 25℃, the I_CC level is approximately 350μA, not exceeding 500μA.

Assuming that the device being designed has an operating temperature range of 0℃ to 60℃, for design purposes here it would be desirable to know the IC’s typical values at 0℃ or 60℃ and to grasp tendency that the property values will increase or decrease with the temperature.. The above table, however, does not tell us such a tendency.

Some temperature settings in the table of specifications provide specified values under a condition that has a certain full operating temperature range, such as from -40℃ to +85℃. In this case, in contrast to the numerical value at the single point at 25℃, it is evident that I_CC is within the guaranteed range if the temperature range for the above device is from 0℃ to 60℃; however, obviously it would be unreasonable to expect other information like the I_CC value at 0℃ beyond that latitude.

This graph, provided in the data sheet for the same IC, describes the relationship between the temperature and I_CC using curves. Furthermore, it describes two conditions on Vin which was not in the table of specifications. From the table, it is evident that in the 0℃ to 60℃ condition above, I_CC is approximately 400μA±20μA even with the 5V V_IN.

Also, the graph indicates that I_CC increases as the temperature rises, and the higher the Vin the higher is I_CC, which is an important piece of information for design purposes.

It should be borne in mind, however, that the values and trends that can be read from the graph are typical properties rather than guaranteed values.

Although I_CC is unlikely to exceed the maximum value 500μA for 25℃ even when the temperature rises above 80℃, in this example the correct interpretation would be, “probably, but not necessarily guaranteed.”

This graph, often seen in switching regulator IC data sheets, shows the relationship between efficiency and output current. In most cases, however, tables of specifications do not contain efficiency items, and no typical values are provided, not to mention maximum or minimum values. In other words, efficiency is a property devoid of any guaranteed levels.

That said, if no pointers, not necessarily guaranteed values, are given, it would be difficult to evaluate what would be the best design. To provide for these needs, properties (the best properties in most cases) are indicated as graphs by specifying conditions, circuits, and parts.

From this graph, it is possible to estimate the efficiency that can be achieved from the load current that is applicable to the device being designed. Therefore, the efficiency curve provides an important checkpoint in selecting a power supply IC.

・How to read a waveform

Some data sheets show not only graphs, but also operating waveforms. The purpose of the waveforms is to present properties that are difficult to provide in a table of specifications, as in the case of a graph.

This figure shows the waveform in an unedited form that would be displayed on an oscilloscope screen. In this figure, the vertical axis represents the voltage, and the horizontal axis is the time. The waveform represents ON and OFF for a switching node and ripple voltages that appear in the output.

Ripple voltages can be read from the output voltage waveform. A close look indicates that when the switching waveform is high, the ripple rises, and when it is low, it falls. Furthermore, the ripple frequency basically follows the switching frequency, that is, the two frequencies are in synch.

Obviously, this also represents a typical characteristic. In the sense of its being a typical characteristic, compared with the waveform for a node in your own circuit, it can be used as an object of comparison in the optimization process.

Here is a rundown of important points in reading graphs and waveforms:

• Switching regulator