L6598_04 STMICROELECTRONICS [STMicroelectronics], L6598_04 Datasheet - Page 7

• Manufacturer Part Number: L6598_04
• Description: HIGH VOLTAGE RESONANT CONTROLLER
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

During the soft start time the current I
to a transconductance amplifier, as shown in fig. 7. Thus this voltage signal is converted in a growing current
which is subtracted to I
start is equal to:
At the start-up (t=0) the oscillator frequency is set by:
At the end of soft start (t = T
only by I
Since the second term of eq.1 is equal to zero, we have:
Note that there is not a fixed threshold of the voltage across C
the frequency shifting), and T
the I
In this way the soft start time depends only on the capacitor C
Start Timing Constant) is 0.15 s/ F.
The current I
wave on the oscillator capacitor C
to compute an approximate value of the oscillator frequency in normal operation is:
The degree of approximation depends on the frequency value, but it remains very good in the range from 30kHz
to 100kHz (figg.9-13)
SS
current has been designed to be a fraction of I
min
(i.e. R
osc
I
SS
is fed to the oscillator as shown in fig. 7. It is twice mirrored (x4 and x8) generating the triangular
I
osc
fmin
=
):
I
------------- -
=
fstart
fstart
where
I
K
osc
I
fmin
. Therefore the current which drives the oscillator to set the frequency during the soft
I
SS
fstart
SS
0
) the second term of eq.1 decreases to zero and the switching frequency is set
I
+
fmin
T
depends on C
f
=
. Referring to the internal structure of the oscillator (fig.7), a good relationship
SS
I
–
fstart
SS
I
g
------------- - T
I
fmin
=
C
osc
=
m
charges the capacitor C
ss
I
------------- - I
R
V
ss
------------------------- -
g
–
C
REF
+
fmin
m
T
g
ss
f
ss
I
I
min
m
SS
fstart
fstart
I
SS
V
fstart
Css
, I
fsart
=
=
=
fstart
K
I
=
0
------------------- -
R
fmin
fstart
t
1.41
fmin
, g
V
=
T
REF
m
, so:
T
=
--------------- - V
R
V
=
SS
, and I
C
SS
fstart
REF
I
f


V
------------- -
R
fmin
SS
SS
------------- -
R
=
SS
[7]
REF
fmin
=
, generating a voltage ramp which is delivered
in which the soft start finishes (i.e. the end of
fmin
SS
. The typical value of the k
1
---------- -
g
C
+
C
----------------------- -
m
(eq. 5). Making T
ss
ss
g
REF


K
[4]
+
I
m
fstart
I
fstart
I
--------------- -
R
ss
fstart
=
T
1
SS
–
2V
g
------------- - t
[5]


C
–
m
ss
k
I
[2]
[3]
ss
SS
SS


C
independent of I
SS
[1]
SS
[6]
constant (Soft
L6598
fstart
7/17
,