22

LM2590HV

SNVS084C – DECEMBER 2001 – REVISED JULY 2016

www.ti.com

Product Folder Links: LM2590HV

Submit Documentation Feedback

Copyright © 2001–2016, Texas Instruments Incorporated

9.2.2 Detailed Design Procedure

9.2.2.1 Inductor Selection Procedure

For a quick-start, refer to the nomographs provided in Figure 33 to Figure 35. To widen the choices to a more

general selection of available inductors, the nomographs provide the required inductance and also the energy in

the core expressed in microjoules (µJ), as an alternative to just prescribing custom parts. The following points

need to be highlighted:

1. The energy values shown on the nomographs apply to steady operation at the corresponding x-coordinate

(rated maximum load current). However under start-up, without soft-start, or a short-circuit on the output, the

Inductor to saturate (if it has been designed only to handle the energy of steady operation). However most

types of core structures used for such applications have a large inherent air gap (for example powdered iron

types or ferrite rod inductors), and so the inductance does not fall off too sharply under an overload. The

voltage to the regulator is over 40 V, the current can slew up so fast under core saturation, that the device

may not be able to act fast enough to restrict the current. The current can then rise without limit till

destruction of the device takes place. Therefore to ensure reliability, TI recommends that if the DC input

without saturating, irrespective of the type of core structure or material.

2. Use Equation 2 to calculate the energy under steady operation.

where

•

L is in µH

•

(2)

These are the energy values shown in the nomographs. See Example 1.

3. The energy under overload is Equation 3.

(3)

4. The nomographs were generated by allowing a greater amount of percentage current ripple in the Inductor

as the maximum rated load decreases (see Figure 36). This was done to permit the use of smaller inductors

at light loads. However, Figure 36 shows only the median value of the current ripple. In reality there may be

a great spread around this because the nomographs approximate the exact calculated inductance to

standard available values. Refer to AN-1197 Selecting Inductors for Buck Converters (SNVA038) for detailed

calculations if a certain maximum inductor current ripple is required for various possible reasons. Also

consider the rather wide tolerance on the nominal inductance of commercial inductors.

5. Figure 35 shows the inductor selection curves for the Adjustable version. The y-axis is Et, in Vµsecs. It is the

which the switch is on in µsecs. See Example 3.

9.2.2.1.1

1. A first pass inductor selection is based upon Inductance and rated max load current. We choose an inductor

with the Inductance value indicated by the nomograph (see Figure 34) and a current rating equal to the

maximum load current. We therefore quick-select a 68-μH, 1-A inductor (designed for 150 kHz operation) for

this application

2. We must confirm that it is rated to handle 50 µJ (see Figure 34) by either estimating the peak current or by a

detailed calculation as shown in AN-1197 Selecting Inductors for Buck Converters (SNVA038), and also that

the losses are acceptable.

9.2.2.1.2

1. A first pass inductor selection is based upon Inductance and the switch current limit. We choose an inductor