AN2683 datasheet(8/38 Pages) STMICROELECTRONICS

8 / 38 page

Circuit description

AN2683

8/38

As a starting point for the value of the inductors we look at the full load current (Ifl) for each

output and let the inductor ripple (Ir) current equal 20 to 30 percent of it. For this design a

value of 30 percent is used.

Ir = Ifl * 0.3

for

●

Output 1: Ir = 0.75 A

●

Output 2: Ir = 3.15 A

Then the values of the inductors are calculated using the formula:

Equation 1:

where Vin is the nominal input voltage, Vout the output voltage and fsw the switching

frequency.

So for input 1:

Equation 2

and for output 2:

Equation 3

The output filter capacitors are roughly approximated so that the change in output voltage

(

∆Vout) during a positive load transient (load is reduced) is minimized. For this design an

output voltage change of two to three percent of the total output voltage is considered

acceptable. The formula used is:

Equation 4

out

–

⋅

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

out

-----------

⋅

1.8

–

300kHz

0.75

⋅

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

1.8

--------

6.8

µH

⋅

–

400kHz

3.15

⋅

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

------

0.7

µH

⋅

()2

⋅

out

–

() Λ V

out

⋅⋅

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

AN2683 Datasheet(PDF) 8 Page - STMicroelectronics

AN2683 Datasheet(HTML) 8 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

Datasheet Download

Link URL