AN2683 datasheet(9/38 Pages) STMICROELECTRONICS

9 / 38 page

AN2683

Circuit description

9/38

For output 1 a

∆Vout of 2.5% of 1.8 VDC or 45 mV is used, thus:

Equation 5

This is a nonstandard value so a 47

µF is used.

For output 2 a

∆Vout of 2% of 1.0 VDC or 20 mV is used:

Equation 6

As the formula indicates the capacitor value should be greater than that calculated. Even

though the board area is small, this section allows the use of ceramic capacitors that are

comprised of two 100

µF and one 47 µF all in parallel and which still fit in the required

footprint.

With these values of capacitors the ripple voltage can be checked. This is dominated by the

equivalent series resistance (ESR) of the capacitors. The ESR must be equal or less than

the value calculated by:

Equation 7:

where Vr is the output ripple voltage and Ir is the inductor ripple current. The ESR for the

capacitors is given in their datasheets at the frequency they are used at as shown in the

graphs. The value is basically the same at both 300 and 400 kHz. For the 47

µF the ESR is

2 m

Ω and for the 100 µF it is 1.5 mΩ. With these values we can calculate the ripple voltage

Vr by:

Equation 8:

46.2

µF

6.8

µH2.5

()2

⋅

212

1.8

–

() 0.045

⋅⋅

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

175

µF

0.7

µH

10.5

()2

⋅

212

1.0

–

() 0.020

⋅⋅

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

ESR

-----

≤

ESR

⋅

AN2683 Datasheet(PDF) 9 Page - STMicroelectronics

AN2683 Datasheet(HTML) 9 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