As switching regulator basics, we need to verify the important properties of the switching regulator. In this article, we explain the properties from an “IC specifications” point of view. In the next articles, we will discuss them from a “power supply” perspective.
Currently, the design of a switching regulator depends in a large measure on the power IC used in it. Consequently, it is critical to understand power IC specifications and what they signify.
Tables of specifications in switching regulator IC data sheets list a number of parameters along with specification values. Such tables describe input voltage ranges, output voltage ranges, output currents, switching frequencies, and operating temperature ranges as basic important parameters.
Input voltage range
“Input voltage range” refers to the range of input voltages in which the IC can operate. Because different maximum values such as 5V and 40V are provided depending on IC specifications, we need to select an input voltage range that matches the input voltage source specifications for the IC to be used. Beyond maximum voltages, for many ICs minimum voltages at which an IC can operate are also specified. In the case of a step-down power supply, a voltage somewhat higher than a set output voltage represents the operable minimum voltage. Below that level, the IC ceases to operate altogether or operates erratically in some cases. To prevent such eventualities, there are ICs that are endowed with a protection function which is referred to as Under-Voltage Lock Out (UVLO).
In addition to the steady-state input voltage, the possibility of occurrence of surges and other transient voltages requires scrutiny. Some ICs are subject to separate specifications in terms of steady-state and transient voltages.
The difference between those voltages and the maximum rating input voltage is that the maximum rating refers to the voltage that can safely be applied to a given IC, irrespective of the resulting operating behavior of the IC.
Conversion from a high voltage to a low voltage may be subject to a restriction on step-down ratio and it may not be applicable to the entire voltage range.
Output voltage range
This is a range over which output voltage levels may be set, not applicable to fixed output voltage types of ICs. In a step-down converter, as a general rule the minimum output voltage cannot be set to a voltage level lower than the internal reference voltage. The maximum output voltage is equal to the input voltage minus some loss voltage.
Output voltage levels are influenced by the accuracy of both the IC reference voltage and the resistor used to set the output voltage.
The term refers to the current level that can be output. For some ICs only the minimum output current is guaranteed, while for other ICs both minimum and maximum current levels are guaranteed. A current level is dependent on the ability of the switching transistor at the output stage and the amount of heat generated. In general, in situations where large currents are needed, it is advantageous to externally provide the switching transistor at the output stage.
To match the power necessary for output, it is important to provide some margin of safety (derating). Without an adequate amount of margin, the IC could be ruined by heat. In addition, in situations where transient load currents flow, you need to make sure that the output voltage is stable or it becomes stable within the required length of time.
“Output current” (continuous current) and “switching current” are terms used to represent the limit values. “Output current” (continuous current) means that the current can be supplied continuously. In the case of a switching current, which is a current that can be supplied on a switching on/off basis, is not a continuously supplied current. In this case, the amount of current that can be supplied continuously is equal to the switching current minus some percentage points.
The term refers to a frequency that can be switched on and off. In the case of PWM, the frequency is fixed at a set frequency. In PFM, the frequency fluctuates according to the prevailing conditions. High frequencies permit the use of lower capacitance output capacitors and inductors, resulting in a reduced component size at the expense of a lower efficiency. Consequently, in choosing a switching frequency, trade-offs between efficiency and size must be considered.
Given that they perform switching operations, switching regulators necessarily generate harmonics and switching noise. Although noise can be reduced by means of a filter, in radio and audio circuits that are sensitive to noise, noise can unwittingly reduce the S/N ratio.
Operating temperature range
The term refers to the temperature range in which the switching regulators can operate; it is defined in terms of an ambient temperature Ta or a junction temperature Tj. An operating temperature range needs to be selected by considering the environment in which the device in question is used or an operating guarantee that must be provided.
In the case of a guarantee provided in terms of Ta, the guarantee does not mean that the switching regulator can be used over the entire Ta range. Because of heat generation, the ambient temperature and principally the load current must be taken into account so that the Tj max level will not be exceeded. The issue of heat, affecting reliability and the possibility of causing accidents, is a critical item.
The above discussion may be summarized in a table format as follows:
|Specifications and their meanings for switching regulator IC||Points|
|Input voltage range
The input voltage range over which the switching regulator can operate. The regulator may be subject not only to maximum voltage, but also to operable minimum voltage requirements.
|Output voltage range
The voltage range that can be established for output by the switching regulator.
The amount of current that can be output, and it depends on the ability of the switching transistor at output state and the amount of heat generated.
The frequency at which the switching regulator may be turned on and off. In the case of PWM, this is fixed to a set frequency. In PFM, the frequency fluctuates depending on the prevailing conditions.
|Operating temperature range
The temperature range over which the switching regulator can operate, in terms of ambient and/or junction temperatures.
・ Because the design of a switching regulator is constrained by the manner in which the power supply IC is employed, a critical requirement is to understand and study the applicable IC specifications.
・ The reader needs to understand the relationship between power supply specifications and IC specifications.