ST1S14 datasheet(24/46 Pages) STMICROELECTRONICS

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Application information

ST1S14

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DocID17977 Rev 2

A list of some tantalum capacitor manufacturers is provided in Table 9.

7.1.3

Inductor

The inductor value is very important as it fixes the ripple current flowing through the output

capacitor. The ripple current is usually fixed at 20 - 40 % of Iomax, which is 0.6 - 1.2 A with

IOmax = 3 A. The approximate inductor value is obtained using the following formula:

Equation 22

where TON is the ON time of the internal switch, given by D · T. For example, with

VOUT = 3.3 V, VIN = 24 V, and ΔIO = 0.8 A, the inductor value is about 4.7 µH. The peak

current through the inductor is given by:

Equation 23

and it can be observed that if the inductor value decreases, the peak current (which must be

lower than the current limit of the device) increases. So, when the peak current is fixed, a

higher inductor value allows a higher value for the output current. In Table 10, some inductor

manufacturers are listed.

7.2

Layout considerations

The layout of switching DC-DC converters is very important to minimize noise and

interference. Power-generating portions of the layout are the main cause of noise and so

Table 9. Output capacitor selection

Manufacturer

Series

Rated voltage (V)

Cap value (µF)(1)

1. see Section 6: Closing the loop for the selection of the output capacitor

ESR (m

Ω)(1)

Nippon Chemicon

KZE

6.3 to 50

Sanyo POSCAP(2)

2. POSCAP capacitors have some characteristics which are very similar to tantalum.

TAE

4 to 16

THB/C/E

4 to 16

AVX

TPS

4 to 35

π ESR COUT

⋅⋅

⋅

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

Table 10. Inductor selection

Manufacturer

Series

Inductor value (µH)

Saturation current (A)

Wurth Elektronik

WE-HCI 7040

1 to 4.7

20 to 7

WE-HCI 7050

4.9 to 10

20 to 4.0

Coilcraft

XPL 7030

2.2 to 10

29 to 7.2

OUT

–

()

ΔI

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

⋅

ΔI

-----

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