L7986TA datasheet(17/43 Pages) STMICROELECTRONICS

17 / 43 page

L7986TA

Application information

Doc ID 022098 Rev 3

17/43

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 which is at minimum duty cycle (see Section 6.1 to

calculate minimum duty). So, by fixing

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

minimum inductance value can be calculated:

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

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 (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.

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.

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

--------

L7986TA датащи(PDF) 17 Page - STMicroelectronics

L7986TA датащи(HTML) 17 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

датащи скачать

ссылки URL