A7986ATR datasheet(17/42 Pages) STMICROELECTRONICS

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A7986A

Application information

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

ΔIL= 30% of IO is about 18 μH.

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 below, some inductor part numbers are listed.

6.3

Output capacitor selection

The current in the capacitor has a triangular waveform which generates a voltage ripple

across it. This ripple is due to the capacitive component (charge or discharge of the output

capacitor) and the resistive component (due to the voltage drop across its ESR). So the

output capacitor must be selected in order to have a voltage ripple compliant with the

application requirements.

The amount of the voltage ripple can be calculated starting from the current ripple obtained

by the inductor selection.

Equation 15

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

MIN

OUT

ΔI

MAX

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

MIN

–

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

⋅

LPK

ΔI

--------

ΔV

OUT

ESR

ΔI

MA X

⋅

ΔI

MA X

OUT

⋅⋅

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

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