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LTC3617

3617fa

applications inForMation

Inductor Selection

For a given input and output voltage, the inductor value

and operating frequency determine the ripple current. The

ripple current

∆IL increases with higher VIN and decreases

with higher inductance:

∆IL =

VOUT

fSW •L









• 1–

VOUT

VIN









Having a lower ripple current reduces the core losses

in the inductor, the ESR losses in the output capacitors

and the output voltage ripple. A reasonable starting point

for selecting the ripple current is

∆IL = 0.3 • IOUT(MAX).

The largest ripple current occurs at the highest VIN. To

guarantee that the ripple current stays below a specified

maximum, the inductor value should be chosen according

to the following equation:

VOUT

fSW • ∆IL(MAX)











 • 1–

VOUT

VIN(MAX)













Inductor Core Selection

Once the value for L is known, the type of inductor must be

selected. Actual core loss is independent of core size for a

fixed inductor value, but it is very dependent on the induc-

tanceselected.Astheinductanceincreases,corelossesde-

crease.Unfortunately,increasedinductancerequiresmore

turns of wire and therefore, copper losses will increase.

Ferrite designs have very low core losses and are pre-

ferred at high switching frequencies, so design goals can

concentrate on copper loss and preventing saturation.

Ferrite core material saturates “hard,” meaning that induc-

tance collapses abruptly when the peak design current is

exceeded. This results in an abrupt increase in inductor

ripple current and consequently output voltage ripple. Do

not allow a ferrite core to saturate!

Different core materials and shapes will change the size/

currentandprice/currentrelationshipofaninductor.Toroid

or shielded pot cores in ferrite or permalloy materials are

small and don’t radiate much energy, but generally cost

more than powdered iron core inductors with similar

characteristics. The choice of which style inductor to use

mainly depends on the price versus size requirements

and any radiated field/EMI requirements. Table 1 shows

some typical surface mount inductors that work well in

LTC3617 applications.

Input Capacitor (CIN) Selection

In continuous mode, the source current of the top P-

channel MOSFET is a square wave of duty cycle VOUT/

VIN. To prevent large input voltage transients, a low ESR

capacitor sized for the maximum RMS current must be

used at VIN.

The maximum RMS capacitor current is given by:

IRMS =IOUT(MAX) •

VOUT

VIN

•

VIN

VOUT

– 1









ThisformulahasamaximumatVIN=2•VOUT,whereIRMS=

IOUT/2.Thissimpleworst-caseconditioniscommonlyused

for design because even significant deviations do not offer

much relief. Note that ripple current ratings from capacitor

manufacturers are often based on only 2000 hours of life

which makes it advisable to further derate the capacitor,

or choose a capacitor rated at a higher temperature than

required.Severalcapacitorsmayalsobeparalleledtomeet

size or height requirements in the design.

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