LM2597, LM2597HV
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SNVS119C –MARCH 1998–REVISED APRIL 2013
Once the ΔIIND value is known, the following formulas can be used to calculate additional information about the
switching regulator circuit.
1. Peak Inductor or peak switch current
(1)
2. Minimum load current before the circuit becomes discontinuous
(2)
3. Output Ripple Voltage
= (ΔIIND)×(ESR of COUT
)
= 0.150A×0.240Ω=36 mV p-p
4. Placeholder to force break
(3)
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 inductor's magnetic field. The strength of the magnetic field, the orientation and location of the
PC copper trace to the magnetic 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 determine 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 torroid 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 negative 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” inductors are an inexpensive, simple way of
making a compact efficient inductor, and they are used by the millions in many different applications.
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