CoolSET™-F2
Functional Description
pull-up resistor R
Soft-Start
. The Soft-Start-Comparator
compares the voltage at pin SoftS at the negative input
with the ramp signal of the PWM-OP at the positive
input. When Soft-Start voltage V
SoftS
is less than
Feedback voltage V
FB
the Soft-Start-Comparator limits
the pulse width by resetting the PWM-Latch (see
Figure 12). In addition to Start-Up, Soft-Start is also
activated at each restart attempt during Auto Restart.
By means of the above mentioned C
Soft-Start
the Soft-
Start can be defined by the user. The Soft-Start is
finished when V
SoftS
exceeds 5.3V. At that time the
Protection Unit is activated by Comparator C4 and
senses the FB by Comparator C3 wether the voltage is
below 4.8V which means that the voltage on the
secondary side of the SMPS is settled. The internal
Zener Diode at SoftS has a clamp voltage of 5.6V to
prevent the internal circuit from saturation (see Figure
13).
6.5V
5.6V
6.5V
R
FB
FB
Soft-Start Comparator
PWM-Latch
PWM Comparator
0.8V
Optocoupler
PWM OP
Isense
x3.65
Improved
Current Mode
Power-Up Reset
Error-Latch
R
Q
R
Soft-Start
SoftS
6.5V
5.3V
4.8V
R
FB
FB
C4
G2
Figure 11
PWM Controlling
S
Q
3.3
Soft-Start
C3
Clock
R
Q
Gate
Driver
S
Q
V
SoftS
5.6V
5.3V
PWM-Latch
Figure 13
Activation of Protection Unit
T
Soft-Start
Gate Driver
t
The Start-Up time T
Start-Up
within the converter output
voltage V
OUT
is settled must be shorter than the Soft-
Start Phase T
Soft-Start
(see Figure 14).
T
Soft
–
Start
C
Soft
–
Start
= ------------------------------------
-
R
Soft
–
Start
´
1.69
By means of Soft-Start there is an effective
minimization of current and voltage stresses on the
integrated CoolMOS™, the clamp circuit and the output
overshoot and prevents saturation of the transformer
during Start-Up.
t
Figure 12
Soft-Start Phase
The Soft-Start is realized by the internal pull-up resistor
R
Soft-Start
and the external Capacitor C
Soft-Start
(see
Figure 5). The Soft-Start voltage V
SoftS
is generated by
charging the external capacitor C
Soft-Start
by the internal
Version 2.8
12
30 Aug 2011
INFINEON [ INFINEON TECHNOLOGIES AG ]
