Datasheet

Block diagram description L6598
10/23 DocID6554 Rev 8
At the startup (t = 0) the oscillator frequency is set by:
Equation 3
At the end of the soft-start (t = T
SS
) the second term of eq.1 decreases to zero and the
switching frequency is set only by Imin (i.e. R
fmin
):
Equation 4
Since the second term of Equation 1 is equal to zero, we have:
Equation 5
Note that there is not a fixed threshold of the voltage across C
SS
in which the soft-start
finishes (i.e. the end of the frequency shifting), and T
SS depends on C
SS
, Ifstart, g
m
, and I
SS
(Equation 5). Making T
SS
independent of Ifstart, the ISS current has been designed to be
a fraction of I
fstart
, so:
Equation 6
In this way the soft-start time depends only on the capacitor C
SS
. The typical value of the k
SS
constant (Soft-start timing constant) is 0.15 s/F.
The current I
osc
is fed to the oscillator as shown in Figure 7. It is twice mirrored (x4 and x8)
generating the triangular wave on the oscillator capacitor C
f
. Referring to the internal
structure of the oscillator (Figure 7), a good relationship to compute an approximate value of
the oscillator frequency in normal operation is:
Equation 7
I
OSC
0 I
fmin
I
fstart
+ V
REF
1
R
fmin
--------------
1
R
fstart
----------------+


==
I
OSC
T
SS
I
fmin
V
REF
R
fmin
--------------==
I
fstart
g
m
I
ss
C
ss
-------------- T
SS
– 0T
SS
C
ss
I
fstart
g
m
I
ss
----------------------- -==
I
SS
I
fstart
K
------------- - T
SS
C
ss
I
fstart
g
m
I
fstart
K
-------------------------- T
SS
=
C
ss
g
m
K
----------- T
SS
k
SS
C
SS
–==
f
min
1.41
R
fmin
C
f
--------------------=