UCD7232

www.ti.com

SLUSAH3

– MAY 2011

For a synchronous buck regulator, the ripple current is highest at 50% duty cycle, or when Vout is one half of

Vin. At higher or lower duty cycles, the ripple current decreases.

For this design, the maximum output current is targeted to be 20A. If the 30% ripple current rule is applied, the

maximum allowable ripple current is 6APP. The previous equation can be rearranged to use this value to compute

a minimum inductance value that will meet our criteria.

LMIN = [(VIN – VOUT) × VOUT] / (VIN × FSW × ΔIMAX)

(12)

For this design, the maximum ripple occurs when Vin = 14V and Vout is at the highest targeted output voltage of

3.3V. This produces a value for LMIN of 0.84µH. This value is rounded up to 1µH, which is a popular value that is

available from inductor vendors.

Now that the inductance value has been determined, the current handling capacity of the inductor drives the next

step in the selection process. The inductor saturation limit and DCR heating limit are two key parameters. At full

load, the peak current in the inductor is equal to the load current plus one half of the

ΔIPP value. For this design,

the peak inductor current is approximately 23A. The inductor must have a saturation current rating, ISAT, of at

least 23A. The inductor saturation rating is the current level at which the inductance value falls by 20 or 30%

(depending on the vendor) from its no-load value. As current increases above this value, the inductance value

may fall sharply, depending on the core material and construction of the inductor. Operating an inductor in its

saturated region causes the current through it to increase rapidly, causing potentially damaging levels of current

to flow in the high-side FET. Good engineering practice dictates that there be should be 15% or more headroom

in the inductor saturation limit to allow for transient currents and surges that will be encountered in normal

operation. For this design, an ISAT rating of at least 1.15 × 23A = 26.5A would be required.

For highest efficiency, an inductor with the lowest DCR will always have the lowest I2R losses. However, low

resistance requires wire with a large cross section. This forces the inductor to be physically larger than a higher

DCR device. The DCR of the inductor will limit its current handling capacity due to the heating it will cause when

current flows through it. Inductor manufacturers typically give a maximum current rating for an inductor based on

the current that produces a 40

°C rise in the device temperature. Keep in mind that in an 85°C ambient

environment, a 40

°C rise will result in a device temperature of 125°C. Every inductor has two maximum current

ratings: one is the 40

°C rise rating, the other is the ISAT rating. The maximum usable current rating for the

inductor is the lower of the two values. In a well designed inductor, the 40

°C rise rating and ISAT are

approximately equal. The 40

°C rise rating should be at least equal to the maximum steady state load current of

the power stage. Headroom above the steady state 40

°C rise rating is not required. Momentary surge currents

above the rating value will not cause a significant temperature rise due to the thermal mass of the part.

The last key inductor consideration is the choice of core material. Core material affects cost, power dissipation

due to core loss, and saturation characteristics. There are three popular core materials used in power inductors:

powdered iron, ferrite, and powdered alloy. Powder iron is inexpensive and has a desirable soft saturation

characteristic that makes it tolerant of surge and transient currents. However, at high values of ripple current and

higher switching frequencies (500kHz and up), core losses become quite large. The heating due to core loss is in

addition to the I2R heating due to the winding DCR. Excessive core loss can cause the core temperature to rise

dramatically. In some cases, this can lead to permanent degradation of the core. Powdered iron cores are best

used at switching frequencies at or below 350kHz. Ferrite has the lowest core losses, making it ideal for higher

switching frequencies. Ferrite saturates easily, so ferrite based inductors are produced with some form of air gap

that lowers their effective permeability and extends their saturation limit. However, once the core reaches

saturation, the falloff in inductance is quite steep. This dictates the selection of a device that has some extra ISAT

headroom to allow for transient current surges. Ferrite is also the most costly core material. Powdered alloy

cores are an improved version of powdered iron cores. By using more exotic metal mixtures in the core, alloy

cores exhibit lower core loss at high frequencies and ripple currents compared to powered iron. In some cases,

they approach the performance of ferrite. The powdered alloy cores retain the desirable soft saturation

characteristic of powdered iron cores. Cost wise, powdered alloy usually falls between powdered iron and ferrite.

Now that the key inductor requirements are known, a device can be selected. In this design, a BI Technologies

HM00-08822LFTR device, for example, meets the requirements. This is a 0.95

µH device, with 1.2mΩ DCR. It

uses a ferrite core with an ISAT rating of 29A.

Copyright

© 2011, Texas Instruments Incorporated

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