Application details

L6725

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6

Application details

6.1

Inductor design

The inductance value is defined by a compromise between the transient response time, the

efficiency, the cost and the size. The inductor has to be calculated to sustain the output and the

maximum output current. The inductance value can be calculated with the following

relationship:

Figure 10 shows the ripple current vs. the output voltage for different values of the inductor,

Increasing the value of the inductance reduces the ripple current but, at the same time,

increases the converter response time to a load transient. If the compensation network is well

designed, during a load transient the device is able to set the duty cycle to 100% or to 0%.

When one of these conditions is reached, the response time is limited by the time required to

change the inductor current. During this time the output current is supplied by the output

capacitors. Minimizing the response time can minimize the output capacitor size.

Figure 10. Inductor current ripple.

Vin

Vout

I

Fsw

Vout

Vin

L

L

⋅

∆

⋅

−

≅

(2)

0

1

2

3

4

5

6

7

8

01

2

3

4

O UT P UT V O LT AG E (V )

V in = 5 V , L = 5 00n H

V in = 5 V , L = 1 .5 u H

V in = 1 2 V , L = 2 u H

V in = 1 2 V , L = 1 u H