Component selection

As parts of [How to Read Power Supply IC Datasheets], thus far we have discussed [Datasheet Cover Page], [Block Diagram], [Absolute Maximum Ratings and Recommended Operating Conditions], [Key to Electric Characteristics], [How to Interpret Property Graphs and Waveforms] and [Application Circuit Examples]. In this section we explain how to interpret information on the [Component Selection] necessary for design purposes, taking as examples [Output LC Filter] and [Phase Compensation] that are important in creating a switching regulator.

・Component selection

In the section [How to Read Power Supply IC Datasheets: Application Circuit Examples], we explained that some manufacturers, depending on specific ICs involved, provide information such as a list of component values based on output voltages and other settings, in addition to showing circuit examples. Undoubtedly, such information is highly beneficial. However, when subtle adjustments are needed or an IC needs to be used under conditions that are not specifically defined, suitable components must be selected, and the component values must be calculated, which can be a daunting task. For this reason, datasheets include functions that are provided, a description on how to set them, a method for selecting required external components, and formulas for calculating the components values.

The following are excerpts from a description on [Output LC Filter Constant] in the [Selection of Components] section, in terms of what equations are to be employed to calculate values for the determination of output LC filter constants. Additional explanations are also provided so that constants that satisfy the conditions to be set can be calculated.

Another example, taken from the datasheet, shows a method for calculating the components and constants necessary for the design of a switching regulator phase compensation circuit.

Viewing it from another angle, it can be interpreted that items with detailed equations, starting with an explanation of operation are essential to design, as in the case of this datasheet. For the optimization of operation and properties, design and debugging cannot be performed without an adequate understanding of those items.

For example, the description of resistor value and equations for the determination of an output voltage does not provide a great deal of details so that, basically, such items can be determined based on equations. The reason may be that it is not crucial to know the “why”. By contrast, we can assume that in the case of output filters and phase compensation mentioned above, issues cannot be addressed unless we know “what it is” and “why is it so.” Furthermore, in conducting design and debugging tasks, we must assume that those items represent issues that must be addressed squarely.

Although it may not be easy to understand it at a first glance, construction of a power supply cannot be completed without understanding the content of it. Fortunately, in the case of the output filter and phase compensation mentioned above, even though circuit configurations, constants, and equations may differ slightly from one manufacture or IC to another, the questions of “what is it?” and “why is it so?” are all based on the same set of concepts. Although this may be a digression from the essence of this section, it will be extremely helpful to understand the underlying principles on these two points in undertaking the design of a power supply device.

Details on [Component Selection] are provided in the form of application notes, design manuals, and other separate volumes under different names, not necessarily “datasheets”, by manufacturer or IC. These items, based on the assumption that you will be engaging in a design task, can basically be made available. Even when explanations and separate volumes are provided, there may be cases where not everything you would like to see is covered. If that is the case, the manufacturer involved needs to be contacted.

• Switching regulator