L5953

Application note

27/31

6.3.3

Output capacitor selection

The criteria for the selection of the capacitor C7 is based on the output voltage ripple

requirements. The ripple on the output voltage is due to a capacitive contribute, often

negligible, equal to

and a resistive contribute given by the ESR of the capacitor and which is equal to

ΔVC fixes the value for C7 while ΔVESR limits the ESR of the capacitor.Usually the capacitor

is chosen so that the total ripple on the output regulated voltage Vo is equal to 1% of the

More often the minimum value of C7 is imposed by other considerations such as to get a

good dynamic behavior of the output voltage in case of large load variations.

6.4

Free-wheeling diode

6.5

Compensation network

In continuous mode, the voltage controlled buck converter shows two poles due to the

output LC filter and one zero due to the ESR of the output capacitor. The suggested

compensation network introduces two zeros and two poles:

–

the zeros compensate the double poles of the LC filter

–

one pole compensates the zero due to ESR of the output capacitor

the second pole is nominally located in the origin which means an infinite gain at frequency

null. In the reality the DC value of the closed loop gain can not be greater than the DC value

of the EA open loop gain and the pole is located at very low frequency.

The values for the components of the compensation network can be fixed when the inductor

L1 and the output capacitor C7 are chosen. The necessary steps are:

L

min

V

O

ΔI

Lmax

,

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

1

V

O

V

imax

,

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

–

1

f

sw min

,

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

⋅⋅

=

ΔV

c

ΔI

Lmax

,

8C7f

sw min

,

⋅⋅

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

=

ΔV

ESR

ESR

ΔI

Lmax

,

⋅

=

V

ripple

ΔV

c

2

ΔV

ESR

2

+

≥