L6725

Application details

17/27

6.2

Output capacitors

The output capacitors are basic components for the fast transient response of the power

supply. For example, during a positive load transient, they supply the current to the load until

the converter reacts. The controller recognizes immediately the load transient and sets the duty

cycle at 100%, but the current slope is limited by the inductor value. The output voltage has a

first drop due to the current variation inside the capacitor (neglecting the effect of the ESL):

Moreover, there is an additional drop due to the effective capacitor discharge that is given by:

drop due to the ESR is the biggest one while the drop due to the capacitor discharge is almost

negligible. Moreover the ESR value also affects the voltage static ripple, that is:

6.3

Input capacitors

The input capacitors have to sustain the RMS current flowing through them, that is:

losses in worst case are:

ESR

Iout

Vout

ESR

⋅

∆

=

∆

(3)

)

max

min

,

(

2

2

Vout

D

Vin

Cout

L

Iout

Vout

COUT

−

⋅

⋅

⋅

⋅

∆

=

∆

(4)

L

I

ESR

Vout

∆

⋅

=

∆

(5)

)

1

(

D

D

Iout

Irms

−

⋅

⋅

=

(6)

2

)

5

.

0

(

Iout

ESR

P

⋅

⋅

=

(7)