SNVS124F November 1999 – April 2021 LM2596
All switching regulators have two basic modes of operation; continuous and discontinuous. The difference between the two types relates to the inductor current, whether it is flowing continuously, or if it drops to zero for a period of time in the normal switching cycle. Each mode has distinctively different operating characteristics, which can affect the regulators performance and requirements. Most switcher designs will operate in the discontinuous mode when the load current is low.
The LM2596 (or any of the SIMPLE SWITCHER family) can be used for both continuous or discontinuous modes of operation.
In many cases the preferred mode of operation is the continuous mode, which offers greater output power, lower peak switch, lower inductor and diode currents, and can have lower output ripple voltage. However, the continuous mode does require larger inductor values to keep the inductor current flowing continuously, especially at low output load currents or high input voltages.
To simplify the inductor selection process, an inductor selection guide (nomograph) was designed (see Figure 9-5 through Figure 9-8). This guide assumes that the regulator is operating in the continuous mode, and selects an inductor that will allow a peak-to-peak inductor ripple current to be a certain percentage of the maximum design load current. This peak-to-peak inductor ripple current percentage is not fixed, but is allowed to change as different design load currents are selected (see Figure 9-4.)
By allowing the percentage of inductor ripple current to increase for low load currents, the inductor value and size can be kept relatively low.
When operating in the continuous mode, the inductor current waveform ranges from a triangular to a sawtooth type of waveform (depending on the input voltage), with the average value of this current waveform equal to the DC output load current.
Inductors are available in different styles such as pot core, toroid, E-core, bobbin core, and so forth, as well as different core materials, such as ferrites and powdered iron. The least expensive, the bobbin, rod or stick core, consists of wire wound on a ferrite bobbin. This type of construction makes for an inexpensive inductor, but because the magnetic flux is not completely contained within the core, it generates more Electro-Magnetic Interference (EMl). This magnetic flux can induce voltages into nearby printed-circuit traces, thus causing problems with both the switching regulator operation and nearby sensitive circuitry, and can give incorrect scope readings because of induced voltages in the scope probe (see Section 9.1.7).
When multiple switching regulators are located on the same PCB, open-core magnetics can cause interference between two or more of the regulator circuits, especially at high currents. A torroid or E-core inductor (closed magnetic structure) should be used in these situations.
The inductors listed in the selection chart include ferrite E-core construction for Schottky, ferrite bobbin core for Renco and Coilcraft, and powdered iron toroid for Pulse Engineering.
Exceeding the maximum current rating of the inductor can cause the inductor to overheat because of the copper wire losses, or the core may saturate. If the inductor begins to saturate, the inductance decreases rapidly and the inductor begins to look mainly resistive (the DC resistance of the winding). This can cause the switch current to rise very rapidly and force the switch into a cycle-by-cycle current limit, thus reducing the DC output load current. This can also result in overheating of the inductor or the LM2596. Different inductor types have different saturation characteristics, so consider this when selecting an inductor.
The inductor manufacturer's data sheets include current and energy limits to avoid inductor saturation.