L6730C datasheet(30/50 Pages) STMICROELECTRONICS

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Application details

L6730C - L6730D

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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

(∆IL) 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 25. shows the ripple current vs. the output voltage for different values of the inductor,

with Vin = 5V and Vin = 12V at a switching frequency of 400KHz.

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 25. Inductor current ripple.

Vin

Vout

Fsw

Vout

Vin

⋅

∆

⋅

−

≅

(6)

OUT P UT V O LT AG E (V )

V in = 5 V , L= 5 0 0 n 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

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