AN938 datasheet(19/33 Pages) STMICROELECTRONICS

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AN938

Typical application

Doc ID 5410 Rev 10

19/33

3.3

Output voltage selection

The two available devices can directly regulate the output voltage of 3.3 V for the L4973V3.3

and 5.1 V for the L4973V5.1. Each of the two devices can regulate an output voltage higher

than the nominal value, up to 40 V, by adding an external voltage divider.

In case of requested output voltage lower than 3.3 V, it is possible to use the L4973V3.3 with

the external connections as shown in Figure 24 with the function below plotted in Figure 25.

Equation 10

Only two resistors are used and no curve intercepts 3.3 V, but all curves have an asymptotic

trend to this value.

3.4

Inductor selection

The criteria used in fixing the inductor value have been dictated by the desired output ripple

voltage, 50 mV max., performance obtained of course in combination with output capacitors

too.

The inductor ripple current, fixed at 30% of Iomax, i.e., 1.05 A, requires an inductor value of:

Equation 11

It is possible to plot Equation 11 as a function of Vo and Vccmax at 150 kHz and 200 kHz (see

Figure 26, 27).

ref

–

() R3

--------

⋅

–

Figure 24.

Example of output regulated

voltage lower than 3.3 V

Figure 25.

Output voltage vs. R5 using R3 as

parameter

min

–

()

ΔI

⋅

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

⋅

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