L6598D datasheet(7/16 Pages) STMICROELECTRONICS

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L6598

Figure 4. Soft Start and frequency shifting block

During the soft start time the current ISS charges the capacitor CSS, generating a voltage ramp which is delivered

to a transconductance amplifier, as shown in fig. 4. Thus this voltage signal is converted in a growing current

which is subtracted to Ifstart. Therefore the current which drives the oscillator to set the frequency during the soft

start is equal to:

[1]

where

[2]

At the start-up (t=0) the L6598 oscillates at fstart, set by:

[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):

[4]

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

[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:

[6]

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

Css

Ifmin

Ifstart

Iss

OSC

Iosc

osc

fmin

fstart

mVCss t

()

–

()

fmin

fstart

mIss

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

–





fmin

VREF

fmin

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

fsart

VREF

fstart

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

REF

Iosc 0

()

Ifmin Ifstart

VREF

fm in

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

fst art

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





osc Tss

()

fm in

REF

Rfmin

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

fst art

mIss

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

–

→

ssIfstart

m Iss

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

fstart

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

→

ssIfstart

mIfstartK

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

→

-----------

SSCSS

–

→

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