PM6686 datasheet(38/50 Pages) STMICROELECTRONICS

38 / 50 page

Application information

PM6686

38/50

Doc ID 15281 Rev 4

Application information

10.1

External components selection

10.1.1

Inductor

Once that switching frequency is defined, inductor selection depends on the desired

inductor ripple current and load transient performance.

Low inductance means great ripple current and could generate great output noise. On the

other hand, low inductor values involve fast load transient response.

A good compromise between the transient response time, the efficiency, the cost and the

size is to choose the inductor value in order to maintain the inductor ripple current

ΔI

between 20% and 50% of the maximum output current ILOAD (max). The maximum

ΔI

occurs at the maximum input voltage. With these considerations, the inductor value can be

calculated with the following relationship:

Equation 8

Where fSW is the switching frequency, VIN is the input voltage, VOUT is the output voltage

and

ΔI

L is the selected inductor ripple current.

In order to prevent overtemperature working conditions, inductor must be able to provide an

RMS current greater than the maximum RMS inductor current ILRMS:

Equation 9

Where

ΔI

L(max) is the maximum ripple current:

Equation 10

If hard saturation inductors are used, the inductor saturation current should be much greater

than the maximum inductor peak current Ipeak:

Equation 11

Using soft-saturation inductors it’s possible to choose inductors with saturation current limit

nearly to Ipeak.

Below there is a list of some inductor manufacturers.

OUT

−

(max))

(

(max))

(

LOAD

LRMS

max

OUT

max

(max)

−

(max)

Ipeak

LOAD

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