L6598 데이터 시트보기 (PDF) - STMicroelectronics
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L6598 Datasheet PDF : 17 Pages
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L6598
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. 7. 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:
Iosc
=
Ifmin + (Ifstart – gmVCss(t))
=
Ifmin
+
Ifs
t
a
r
t
–
g----m----I--s---s-
Css
t
[1]
where Ifmin = R-V----Rf--m-E---i-Fn- , Ifsart = R--V---f-R-s--t-E-a--F-r--t, VREF = 2V [2]
At the start-up (t=0) the oscillator frequency is set by:
Iosc(0)
=
Ifmin + Ifstart
=
VR
EF
------1-------
Rfmin
+
R-----f--s1--t--a---r--t
[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):
Iosc(Tss)
=
Ifmin
=
-V----R---E----F-
Rfmin
[4]
Since the second term of eq.1 is equal to zero, we have:
Ifstart
–
g----m----I--s---s-
Css
T
S
S
=
0 → TSS
=
-C----s---s--I--f--s---t-a---r--t
gmIss
[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:
ISS
=
I--f--s---t-a---r--t
K
→
TSS
=
--C----s---s---I-f--s---t--a--r--t-
gmIfstartK
→
TSS
=
--C----s---s--
gmK
→
TSS
–
kSSCSS
[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:
fmin
=
-----1---.--4---1-------
RfminCf
[7]
The degree of approximation depends on the frequency value, but it remains very good in the range from 30kHz
to 100kHz (figg.9-13)
7/17
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