LM2591HV datasheet(13/20 Pages) NSC

Part #	LM2591HV

Description	SIMPLE SWITCHER Power Converter 150 kHz 1A Step-Down Voltage Regulator

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Manufacturer	NSC [National Semiconductor (TI)]

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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 pro-

vided in Figure 2 to Figure 4. 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 cur-

rent 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.

These are the energy values shown in the nomographs.

See Example 1 below.

3.

The Energy under overload is

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 LM2591HV is 3A. The Energy rating

depends on the Inductance. See Example 2 below.

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 5). This

was done to permit the use of smaller inductors at light

loads. Figure 5 however shows only the ’median’ value

of the current ripple. In reality there may be a great

spread around this because the nomographs approxi-

mate the exact calculated inductance to standard avail-

able values. It is a good idea to refer to AN-1197 for

detailed calculations if a certain maximum inductor cur-

rent ripple is required for various possible reasons. Also

consider the rather wide tolerance on the nominal induc-

tance of commercial inductors.

5.

Figure 4 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-VSAT-VOUT) multiplied by the time for

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

Example 1: (V

IN

≤ 40V) LM2591HV-5.0, V

IN = 24V, Output

5V @ 0.8A

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 (Figure 3) and

a current rating equal to the maximum load current. We

therefore quick-select a 100µH/0.8A inductor (designed for

150 kHz operation) for this application.

2. We should confirm that it is rated to handle 50 µJ (see

Figure 3) by either estimating the peak current or by a

detailed calculation as shown in AN-1197, and also that the

losses are acceptable.

Example 2: (V

IN > 40V) LM2591HV-5.0, VIN = 48V, Output

5V @ 1A

1. A first pass inductor selection is based upon Inductance

and the switch currrent limit. We choose an inductor with the

Inductance value indicated by the nomograph (Figure 3) and

a current rating equal to I

CLIM. We therefore quick-select a

100µH/3A inductor (designed for 150 kHz operation) for this

application.

2. We should confirm that it is rated to handle e

CLIM by the

procedure shown in AN-1197 and that the losses are accept-

able. Here e

CLIM is:

Example 3: (V

IN

≤ 40V) LM2591HV-ADJ, V

IN = 20V, Output

10V @ 1A

1. Since input voltage is less than 40V, 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 Figure 4 and a current rating

equal to the maximum load. But we first need to calculate Et

for the given application. The Duty cycle is

where V

D is the drop across the Catch Diode (

) 0.5V for a

Schottky) and V

SAT the drop across the switch (

)1.5V). So

And the switch ON time is

where f is the switching frequency in Hz. So

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