LM2596 datasheet(23/31 Pages) NSC

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Application Information (Continued)

In a switching regulator design, knowing the value of the

peak-to-peak inductor ripple current (

∆I

IND) can be useful for

determining a number of other circuit parameters. Param-

eters such as, peak inductor or peak switch current, mini-

mum load current before the circuit becomes discontinuous,

output ripple voltage and output capacitor ESR can all be

calculated from the peak-to-peak

∆I

IND. When the inductor

nomographs shown in

Figure 4 through 8 are used to select

an inductor value, the peak-to-peak inductor ripple current

can immediately be determined. The curve shown in

Figure

18 shows the range of (

∆I

IND) that can be expected for

different load currents. The curve also shows how the

peak-to-peak inductor ripple current (

∆I

IND) changes as you

go from the lower border to the upper border (for a given load

current) within an inductance region. The upper border rep-

resents a higher input voltage, while the lower border repre-

sents a lower input voltage (see Inductor Selection Guides).

These curves are only correct for continuous mode opera-

tion, and only if the inductor selection guides are used to

select the inductor value

Consider the following example:

OUT = 5V, maximum load current of 2.5A

IN = 12V, nominal, varying between 10V and 16V.

The selection guide in

Figure 5 shows that the vertical line

for a 2.5A load current, and the horizontal line for the 12V

input voltage intersect approximately midway between the

upper and lower borders of the 33 µH inductance region.

A 33 µH inductor will allow a peak-to-peak inductor current

(

∆I

IND) to flow that will be a percentage of the maximum load

current. Referring to

Figure 18, follow the 2.5A line approxi-

mately midway into the inductance region, and read the

peak-to-peak inductor ripple current (

∆I

IND) on the left hand

axis (approximately 620 mA p-p).

As the input voltage increases to 16V, it approaches the

upper border of the inductance region, and the inductor

ripple current increases. Referring to the curve in

Figure 18,

it can be seen that for a load current of 2.5A, the

peak-to-peak inductor ripple current (

∆I

IND) is 620 mA with

12V in, and can range from 740 mA at the upper border (16V

in) to 500 mA at the lower border (10V in).

Once the

∆I

IND value is known, the following formulas can be

used to calculate additional information about the switching

regulator circuit.

Peak Inductor or peak switch current

Minimum load current before the circuit becomes dis-

continuous

Output Ripple Voltage = (

∆I

IND)x(ESR of COUT)

= 0.62Ax0.1

Ω=62mVp-p

OPEN CORE INDUCTORS

Another possible source of increased output ripple voltage or

unstable operation is from an open core inductor. Ferrite

bobbin or stick inductors have magnetic lines of flux flowing

through the air from one end of the bobbin to the other end.

These magnetic lines of flux will induce a voltage into any

wire or PC board copper trace that comes within the induc-

tor’s magnetic field. The strength of the magnetic field, the

orientation and location of the PC copper trace to the mag-

netic field, and the distance between the copper trace and

the inductor, determine the amount of voltage generated in

the copper trace. Another way of looking at this inductive

coupling is to consider the PC board copper trace as one

turn of a transformer (secondary) with the inductor winding

as the primary. Many millivolts can be generated in a copper

trace located near an open core inductor which can cause

stability problems or high output ripple voltage problems.

If unstable operation is seen, and an open core inductor is

used, it’s possible that the location of the inductor with

respect to other PC traces may be the problem. To deter-

mine if this is the problem, temporarily raise the inductor

away from the board by several inches and then check

circuit operation. If the circuit now operates correctly, then

the magnetic flux from the open core inductor is causing the

problem. Substituting a closed core inductor such as a tor-

roid or E-core will correct the problem, or re-arranging the

PC layout may be necessary. Magnetic flux cutting the IC

device ground trace, feedback trace, or the positive or nega-

tive traces of the output capacitor should be minimized.

Sometimes, locating a trace directly beneath a bobbin in-

ductor will provide good results, provided it is exactly in the

center of the inductor (because the induced voltages cancel

themselves out), but if it is off center one direction or the

other, then problems could arise. If flux problems are

present, even the direction of the inductor winding can make

a difference in some circuits.

This discussion on open core inductors is not to frighten the

user, but to alert the user on what kind of problems to watch

out for when using them. Open core bobbin or “stick” induc-

tors are an inexpensive, simple way of making a compact

efficient inductor, and they are used by the millions in many

different applications.

01258333

FIGURE 18. Peak-to-Peak Inductor

Ripple Current vs Load Current

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