L6598 datasheet(10/24 Pages) STMICROELECTRONICS

10 / 24 page

Block diagram description

L6598

10/24

Doc ID 6554 Rev 7

At the start-up (t = 0) the oscillator frequency is set by:

Equation 3

At the end of soft start (t = TSS) the second term of eq.1 decreases to zero and the switching

frequency is set only by Imin (i.e. Rfmin):

Equation 4

Since the second term of eq.1 is equal to zero, we have:

Equation 5

Note that there is not a fixed threshold of the voltage across CSS in which the soft start

finishes (i.e. the end of the frequency shifting), and TSS depends on CSS, Ifstart, gm, and ISS

(eq. 5). Making TSS independent of Ifstart, the ISS current has been designed to be a fraction

of Ifstart, so:

Equation 6

In this way the soft start time depends only on the capacitor CSS. The typical value of the kSS

constant (Soft start timing constant) is 0.15 s/

μF.

The current Iosc is fed to the oscillator as shown in fig. 7. It is twice mirrored (x4 and x8)

generating the triangular wave on the oscillator capacitor Cf. Referring to the internal

structure of the oscillator (fig.7), a good relationship to compute an approximate value of the

oscillator frequency in normal operation is:

Equation 7

OSC 0

() I

fmin

fstart

REF

fmin

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

fstart

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

⎝⎠

⎛⎞

OSC TSS

() I

fmin

REF

fmin

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

fstart

mIss

--------------T

–

ssIfstart

mIss

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

→

fstart

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

ssIfstart

mIfstartK

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

→

-----------

SSCSS

–

→

min

1.41

fminCf

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

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