L6727 datasheet(20/34 Pages) STMICROELECTRONICS

20 / 34 page

Application information

L6727

20/34

Doc ID 12933 Rev 4

Application information

10.1

Output inductor

Inductor value is defined by a compromise between dynamic response, ripple, efficiency,

cost and size. Usually, inductance is calculated to maintain inductor ripple current (

ΔI

between 20 % and 30 % of maximum output current. Given the switching frequency (FSW),

the input voltage (VIN), the output voltage (VOUT) and the desired ripple current (ΔIL),

inductance can be calculated as follows:

Figure 1 shows the ripple current vs. the output voltage for different inductance, with

VIN = 5 V and VIN = 12 V.

Increasing inductance reduces inductor ripple current (and output voltage ripple

accordingly) but, at the same time, increases the converter response time to load transients.

Higher inductance means that the inductor needs more time to change its current from initial

to final value. Until the inductor has not finished its charging, the additional output current is

supplied by output capacitors. Minimizing the response time lead to minimize the output

capacitance required. If the compensation network is designed with high bandwidth, during

an heavy load transient the device is able to saturate duty cycle (0 % or 80 %). When this

condition is reached, the response time is limited only by the time required to charge the

inductor.

Figure 10.

Inductor current ripple vs output voltage

OUT

–

ΔI

⋅

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

OUT

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

⋅

L6727 Datasheet(PDF) 20 Page - STMicroelectronics