LM2599 datasheet(21/47 Pages) TI2

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LM2599

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SNVS123D – APRIL 1998 – REVISED MAY 2016

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Application Information (continued)

9.1.7 Inductor Selection

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 operates in the

discontinuous mode when the load current is low.

The LM2599 (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. This mode offers greater output power,

lower peak switch, inductor and diode currents, and can have lower output ripple voltage. However, the

continuous mode requires larger inductor values to keep the inductor current flowing continuously, especially at

low output load currents and/or high input voltages.

To simplify the inductor selection process, an inductor selection guide (nomograph) was designed (see Figure 35

through Figure 38). This guide assumes that the regulator is operating in the continuous mode, and selects an

inductor that allows 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 34).

Figure 34. ΔIIND, Peak-to-Peak Inductor Ripple Current

(as a Percentage of the Load Current) vs Load Current

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 Open Core Inductors).

When multiple switching regulators are placed 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) must be used in these situations.

The inductors listed in the selection chart include ferrite E-core construction for Schott, ferrite bobbin core for

Renco and Coilcraft, and powdered iron toroid for Pulse Engineering.

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