Datasheet

LM2599

SNVS123C –APRIL 1998–REVISED APRIL 2013

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Figure 35. (ΔI

IND

) 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, etc., 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 since 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. Also see section on OPEN CORE INDUCTORS.

When multiple switching regulators are located on the same PC board, 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 Schott, ferrite bobbin core for

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

Exceeding an inductor's maximum current rating may 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 and/or the LM2599. Different inductor types have different

saturation characteristics, and this should be kept in mind when selecting an inductor.

The inductor manufacturer's data sheets include current and energy limits to avoid inductor saturation.

DISCONTINUOUS MODE OPERATION

The selection guide chooses inductor values suitable for continuous mode operation, but for low current

applications and/or high input voltages, a discontinuous mode design may be a better choice. It would use an

inductor that would be physically smaller, and would need only one half to one third the inductance value needed

for a continuous mode design. The peak switch and inductor currents will be higher in a discontinuous design,

but at these low load currents (1A and below), the maximum switch current will still be less than the switch

current limit.

Product Folder Links: LM2599