L6732 datasheet(22/37 Pages) STMICROELECTRONICS

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Application details

L6732

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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 sustain the output and

the input voltage variation to maintain the ripple current (

∆I

L) between 20 % and 30 % of the

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

relationship:

Equation 6

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

voltage. Figure 17. 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 500 kHz.

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 17.

Inductor current ripple

Vin

Vout

Fsw

Vout

Vin

⋅

∆

⋅

−

≅

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

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