DocID6554 Rev 8 9/23
L6598 Block diagram description
23
5 Block diagram description
5.1 High/low side driving section
A high and low side driving section provide the proper driving to the external power MOS or
IGBT. A high sink/source driving current (450/250 mA typ.) ensure fast switching times also
when size for power MOS are used. The internal logic ensures a minimum deadtime to
avoid cross conduction of the power devices.
5.2 Timing and oscillator section
The device is provided of a soft-start function. It consists in a period of time, T
SS
, in which
the switching frequency shifts from fstart to fmin. This feature is explained in the following
description (refer to Figure 6 and Figure 7).
Figure 6. Soft-start and frequency shifting block
During the soft-start time the current I
SS
charges the capacitor C
SS,
generating a voltage
ramp which is delivered to a transconductance amplifier, as shown in Figure 6. 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:
Equation 1
Equation 2
&VV
,IPLQ,IVWDUW
,VV
JP 26&
,RVF
$0Y
I
osc
I
fmin
I
fstart
g
m
V
Css
t–+ I
fmin
I
fstart
g
m
I
ss
C
ss
--------------–
+==
where I
fmin
V
REF
R
fmin
-------------- I
fstart
V
REF
R
fstart
---------------- V
REF
2V=,==
