Datasheet

LM2592HV
SNVS075C –MAY 2001–REVISED APRIL 2013 www.ti.com
APPLICATION INFORMATION
INDUCTOR SELECTION PROCEDURE
Application Note AN-1197 titled "Selecting Inductors for Buck Converters" provides detailed information on this
topic. For a quick-start the designer may refer to the nomographs provided in Figure 25 to Figure 27. To widen
the choice of the Designer 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
current in the inductor will momentarily/repetitively hit the current limit I
CLIM
of the device, and this current
could be much higher than the rated load, I
LOAD
. This represents an overload situation, and can cause 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
device is usually able to protect itself by not allowing the current to ever exceed I
CLIM
. But if the DC input
voltage to the regulator is over 40V, 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, it is recommended, that if the DC Input
Voltage exceeds 40V, the inductor must ALWAYS be sized to handle an instantaneous current equal to I
CLIM
without saturating, irrespective of the type of core structure/material.
2. The Energy under steady operation is:
where
• L is in µH
• and I
PEAK
is the peak of the inductor current waveform with the regulator delivering I
LOAD
(1)
These are the energy values shown in the nomographs. See Example 1.
3. The Energy under overload is:
(2)
If V
IN
> 40V, the inductor should be sized to handle e
CLIM
instead of the steady energy values. The worst
case I
CLIM
for the LM2592HV is 4A. The Energy rating depends on the Inductance. See Example 2.
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 28). This was done to permit the use of smaller inductors
at light loads. Figure 28 however 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. It is a good idea to refer to AN-1197 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 27 shows the inductor selection curves for the Adjustable version. The y-axis is 'Et', in Vμsecs. It is
the applied volts across the inductor during the ON time of the switch (V
IN
-V
SAT
-V
OUT
) multiplied by the time
for which the switch is on in μsecs. See Example 3.
Example 1: (V
IN
≤ 40V) LM2592HV-5.0, V
IN
= 24V, Output 5V @ 1A
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 26) and a current rating equal to the
maximum load current. We therefore quick-select a 68μH/1A inductor (designed for 150 kHz operation) for this
application.
2. We should confirm that it is rated to handle 50 μJ (see Figure 26) by either estimating the peak current or by a
detailed calculation as shown in AN-1197, and also that the losses are acceptable.
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