AN1673 datasheet(4/24 Pages) STMICROELECTRONICS

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AN1673 APPLICATION NOTE

4/24

Where:

[2]

At the start-up (t=0) the L6598 oscillates (see also fig3 and relation [7]) at fstart:

[3]

set by:

[3b]

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. Rƒmin):

[4]

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

[5]

Note: there is not a fixed threshold of the voltage on CSS in which the soft start finishes (i.e. the end of the

frequency shifting), and TSS depends on CSS, Iƒstart, gm, and ISS (eq. 5). Making TSS independent of

Iƒstart, the ISS current has been designed to be a fraction of Iƒstart, so:

[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. 3. 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.3):

[7]

Where: IOSC = 4 · IRƒmin ; ∆VC is the peak to valley oscillator value (≈2.84V).

A good relationship to compute an approximate value of the oscillator frequency in normal operation is:

[7b]

The degree of approximation depends on the frequency value, but it remains much good in the range from

30kHz to 100kHz (fig.4).

Iƒ

min

VREF

Rƒ

min

----------------- ;

Iƒ

start

VREF

Rƒ

start

------------------- ;

VREF = 2V

start

---

OSC

2Cƒ

∆

⋅

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

Iosc0

Iƒ

min

Iƒ

start

Vref

Rƒ

min

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

Rƒ

start

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





OS C TSS

()

Iƒ

min

VREF

Rƒ

min

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

Iƒ

start

⋅

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

⋅

–

⇒

Iƒ

start

⋅

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

Iƒ

start

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

⇒

Iƒ

start

⋅

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

⇒

⋅

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

⇒

⋅

min

---

OSC

Cƒ

∆

⋅

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

min

1.41

Rƒ

min

Cƒ

⋅

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

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