As part of [How to Read Power Supply IC Datasheets] and in succession to the [Data Sheet Cover Page], [Block Diagram], and [Absolute Maximum Ratings and Recommended Operating Conditions] sections, in this section we discuss [Key to Electrical Characteristics].
For understanding switching regulator properties and evaluation methods, reading and deciphering the power supply IC data sheets are essential requirements.
・Key to Electrical Characteristics
Data sheets always contain a table of specification values that provide the properties of the IC being designed and their guaranteed values. While the design process involves the use of the property values, unfortunately it is not possible to determine property values for all conditions. Consequently, property values are set forth under fixed conditions, such as supply voltage and temperature. In most cases, a property value is established under a given condition, such as a value for Ta＝25℃, but not Ta＝25.1℃. In actual operating conditions, however, obviously a condition that is exactly Ta＝25℃ can be expected to occur: It is not uncommon that the temperature, which is at room temperature when the device is started, rises to 50℃ in the case as time passes.
In other words, to use the values in the table of specifications, one needs to understand under what conditions those values hold, and whether they are guaranteed values, such a maximum or minimum, or specification values (typ), and to consider these factors. The table below shows examples of values in the table of specification values. The red arrow indicates conditions under which a given value is set forth. Although under these conditions, the indicated value is guaranteed, a slight variation in conditions puts the value out of guarantee. In this example, the temperature condition is Ta=25℃. In terms of temperatures, there may be cases where a guaranteed value holds over the relatively full temperature range. In such a case, it goes without saying that one needs to reference the guaranteed value over the full temperature range.
We now discuss the relationship between IC characteristic values and application circuit characteristic values that must be considered. It is important to understand that the specifications for a power supply IC do not directly translate to characteristic values for the power supply unit in which the IC is deployed.
As an example, for the FB terminal voltage (some manufacturers refer to it as a reference voltage) that represents the criteria by which an output voltage is established, maximum and minimum values are set forth and the voltage is guaranteed, which is an obvious point, given that it is a critical characteristic value that determines the output voltage.
Although ICs for newer switching supplies are high-precision ICs with an FB terminal voltage accuracy of ±1%, if an attempt is made to set the output voltage by connecting a voltage-dividing resistor (based on specifications), the voltage accuracy of ±1% cannot be guaranteed. In simpler terms, the output voltage calculated by using the resistor value and nominal value of the FB pin voltage, except for coincidence, never matches the calculated value as long as there is an allowable difference between the resistor and FB pin voltage. The example given below shows an error in output voltage in situations where a ±1% accuracy resistor is used and where the accuracy of the FB pin voltage falls within ±1%:
Example of specification value never being equal to power supply characteristic
The accuracy of the output voltage includes the following factors:
・FB voltage error: ±1%
・Nominal resistance: An approximate value selected
from accuracy (example: ±1%) + series (a resistance value equal to a calculated value may never occur)
In other words, if a power supply unit with an output voltage accuracy of ±1% is to be created and if a power supply IC with an FB pin voltage accuracy of ±1% is selected, an output voltage accuracy level of ±1% cannot be assured. Although it may be commented that “that would be a worst-scenario calculation, and at the typical value the attainable accuracy level should fall within ±1%,” in situations where rigorous guarantee is required, the conclusion should be deemed “not attainable”.
Thus, a specification value only means an IC’s characteristic value where if the power supply IC is built as a circuit, the value is reflected in the circuit; however, it should be borne in mind that the specification value or characteristic cannot be attained as is.
・ For the design of a power supply unit, one needs to read and decipher the data sheet for the power supply IC.
・ IC specification values and characteristic values as a power supply circuit do not directly match.
・ Not all properties are standardized.
・ Graphs and waveforms, which may provide supplementary values not included in the specification, can provide valuable information.