L6728 datasheet(19/32 Pages) STMICROELECTRONICS

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L6728

Application information

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Application information

11.1

Inductor design

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

efficiency, the cost and the size. The inductor has to be calculated to maintain the ripple

current (

∆I

L) between 20% and 30% of the maximum output current (typ). 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 9 shows the ripple current vs. the output voltage for different values of the

inductor, with VIN = 5V and VIN = 12V.

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

increases the converter response time to a dynamic load change. The response time is the

time required by the inductor to change its current from initial to final value. Until the inductor

has not finished its charging time, the output current is supplied by the output capacitors.

Minimizing the response time can minimize the output capacitance required. If the

compensation network is well designed, during a load variation the device is able to set a

duty cycle value very different (0% or 80%) from steady state one. When this condition is

reached, the response time is limited by the time required to change the inductor current.

Figure 9.

Inductor current ripple vs. output voltage

OUT

–

∆I

⋅

------------------------------

OUT

--------------

⋅

012

Output voltage [V]

Vin=12V, L=1uH

Vin=12V, L=2uH

Vin=5V, L=500nH

Vin=5V, L=1.5uH

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