R7986ATR datasheet(17/37 Pages) STMICROELECTRONICS

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R7986A

Application information

6.2

Inductor selection

The inductance value fixes the current ripple flowing through the output capacitor. So the

minimum inductance value, in order to have the expected current ripple, must be selected.

The rule to fix the current ripple value is to have a ripple at 20% - 40% of the output current.

In continuous current mode (CCM), the inductance value can be calculated by the following

equation:

Equation 12

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 Section 6.1 to

calculate minimum duty). So, fixing

IL = 20% to 30% of the maximum output current, the

minimum inductance value can be calculated as:

Equation 13

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

IL= 30% of IO is about 18 H.

The peak current through the inductor is given by:

Equation 14

Therefore, if the inductor value decreases, then the peak current (that must be lower than

the minimum current limit of the device) increases. According to the maximum DC output

current for this product family (3 A), the higher the inductor value, the higher the average

output current that can be delivered, without triggering the overcurrent protection.

In Table 7 some inductor part numbers are listed.

Table 7. Inductors

Manufacturer

Series

Inductor value (

H)

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

I

VIN VOUT

–

------------------------------ TON



VOUT VF

---------------------------- TOFF



LMIN

VOUT VF

I

MAX

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

1DMIN

–

FSW

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



ILPK



I

--------

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