AN4501 datasheet(18/48 Pages) STMICROELECTRONICS

18 / 48 page

Selection of the inductor

AN4501

18/48

DocID026394 Rev 1

Equation 2:

where

η is the converter efficiency.

The duty cycle is a decreasing function of VIN. In particular, the maximal duty cycle, noted

DMAX, is reached for the minimum input voltage, VIN,MIN.

The duty cycle range must be compliant with two device parameters with the device

minimum duty cycle (parameter TON_MIN, maximum specification: 14%) and maximum duty

cycle (parameter Duty Max, minimum specification: 88%)

4.2

Inductor average current

In steady state, the average current of the output capacitor over one period must be equal to

zero. Since the inductor delivers current to the load only during the converter’s off-phase

(see Figure 7), inductor current averaged during tOFF is equal to the output current:

Extracting IL from this equation gives:

Equation 3:

We can see that IL is independent from the inductor value. Moreover, the worst case

average inductor DC current is maximal for the maximal duty cycle. As a consequence, a

worst case calculation of the inductor DC current must consider VIN,MIN.

Equation 4:

with

ef fect ive

OUT

-------------------

–

OFF

------------

–

()

OU T

–

-------------

LMAX

OU T

MAX

–

-------------------------

MAX

IN MIN

OUT,MAX

--------------------------------

–

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