L7985 datasheet(17/42 Pages) STMICROELECTRONICS

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L7985

Application information

Equation 13

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

For example, for VOUT = 5 V, VIN = 24 V, IO = 2 A and FSW = 250 kHz, the minimum

inductance value to have

IL = 30% of IO is about 28 H.

The peak current through the inductor is given by:

Equation 14

So, 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 (2 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.

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

Usually the resistive component of the ripple is much higher than the capacitive one, if the

output capacitor adopted is not a multi-layer ceramic capacitor (MLCC) with very low ESR

value.

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

LMIN

VOUT VF

I

MAX

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

1DMIN

–

FSW

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



ILPK



I

--------

V

OUT

ESR

I

MAX



I

MAX

8COUT fSW



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

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