L7981 datasheet(17/44 Pages) STMICROELECTRONICS

17 / 44 page

L7981

Application informations

Doc ID 15182 Rev 3

17/44

Equation 9

Where TON is the conduction time of the internal high side switch and TOFF is the conduction

time of the external diode (in CCM, FSW=1/(TON + TOFF)). The maximum current ripple, at

fixed Vout, is obtained at maximum TOFF that is at minimum duty cycle (see previous section

to calculate minimum duty). So fixing

ΔI

L=20% to 30% of the maximum output current, the

minimum inductance value can be calculated:

Equation 10

where FSW is the switching frequency, 1/(TON + TOFF).

For example for VOUT=5 V, VIN=24 V, IO=3 A and FSW=250 kHz the minimum inductance

value to have

ΔI

L = 30% of IO is about 18 μH.

The peak current through the inductor is given by:

Equation 11

So if the inductor value decreases, the peak current (that has to be lower than the current

limit of the device) increases. The higher is the inductor value, the higher is the average

output current that can be delivered, without reaching the current limit.

In the table below some inductor part numbers are listed.

Table 7.

Inductors

Manufacturer

Series

Inductor value (

μH)

Saturation current (A)

Coilcraft

MSS1038

3.8 to 10

3.9 to 6.5

MSS1048

12 to 22

3.84 to 5.34

Wurth

PD Type L

8.2 to 15

3.75 to 6.25

PD Type M

2.2 to 4.7

4 to 6

SUMIDA

CDRH6D226/HP

1.5 to 3.3

3.6 to 5.2

CDR10D48MN

6.6 to 12

4.1 to 5.7

ΔI

OUT

–

------------------------------ T

⋅

OUT

---------------------------- T

OFF

⋅

MIN

OUT

ΔI

MAX

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

MIN

–

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

⋅

LPK

ΔI

--------

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