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A5974D

Application information



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 20

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

VOUT = 3.3 V, VIN = 12 V and IO = 0.9 A, the inductor value is about 12 µH. The peak

current through the inductor is given by:

Equation 21

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 8, some

inductor manufacturers are listed.

8.2

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

high switching current loop areas should be kept as small as possible and lead lengths as

short as possible.

High impedance paths (in particular the feedback connections) are susceptible to

interference, so they should be as far as possible from the high current paths. A layout

example is provided in Figure 13.

The input and output loops are minimized to avoid radiation and high frequency resonance

problems. The feedback pin connections to the external divider are very close to the device

to avoid pickup noise. Another important issue is the ground plane of the board. Since the

package has an exposed pad, it is very important to connect it to an extended ground plane

in order to reduce the thermal resistance junction to ambient.

Table 8. Inductor selection

Manufacturer

Series

Inductor value (µH)

Saturation current (A)

Coilcraft

DO3316T

5.6 to 12

3.5 to 4.7

Coilcraft

MSS1260T

5.6 to 15

3.5 to 8

Wurth Elektronik

WE-PD L

4.7 to 27

3.55 to 6

VIN VOUT

–



I

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



IPK

I

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