L6567 datasheet(9/15 Pages) STMICROELECTRONICS

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L6567

Other easy formulas fit rather well:

TDT ≅ 46.75 · 10

^-12 ·RREF

TPRE ≅ 224 · CP ·RREF

As far as df/dt is concerned, there are no easy formulas that fit the relation between CF,RF, and CI.CI is charged

and discharged by three different currents that are derived from different mirroring ratios by the current flowing

on RREF. The voltage variations on CI are proportional to the current that charges CF, that is to say they are

proportional to df/dt.

The values obtained in the testing conditions (CI = 100nF) are:

during preheating and working conditions the typical frequency increase is ~ 20KHz/ms, the typical decrease is

~-10Khz/ms;

During ignition the frequency variation is ~ -200Hz/ms.

If slower variations are needed, CI has to be increased.

Due to these tight relationships, it is recommended to follow a precise procedure: first RHV has to be chosen

looking at startup current needs and dissipation problems. Then the feed forward frequency range has to be

determined, and so CF is set.

Given a certain CF,RREF is set in order to fix FMIN. Now CP can be chosed to set the desired TPRE and TIGN.

The other external parameters (RSHUNT and CI) can be chosen at the end because they are just related to a

single circuit parameters.

L6567 Datasheet(PDF) 9 Page - STMicroelectronics