L6730 datasheet(33/52 Pages) STMICROELECTRONICS

33 / 52 page

L6730 - L6730B

Application details

Doc ID 11938 Rev 3

33/52

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 maintain the ripple

current (

ΔI

L) between 20% and 30% of the maximum output current. The inductance value

can be calculated with the following relationship

Where FSW is the switching frequency, VIN is the input voltage and VOUT is the output

voltage. Figure 28 shows the ripple current vs. the output voltage for different values of the

inductor, with VIN = 5 V and VIN = 12 V at a switching frequency of 400 kHz.

Increasing the value of the inductance reduces the current ripple 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 28.

Inductor current ripple

Vin

Vout

Fsw

Vout

Vin

⋅

−

≅

(6)

OUT P UT V O LT AG E (V )

Vin=5V, L=500nH

Vin=5V, L=1.5uH

Vin=12V, L=2uH

Vin=12V, L=1uH

L6730 Datasheet(PDF) 33 Page - STMicroelectronics