CoolSET™-F2
ICE2A165/265/365
ICE2A180/280
Functional Description
pullup resistor RSoft-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 VSoftS is less than
Feedback voltage VFB the Soft-Start-Comparator limits
the pulse width by reseting the PWM-Latch (see Figure
9). In addition to Start-Up, Soft-Start is also activated at
each restart attempt during Auto Restart. By means of
the above mentioned CSoft-Start the Soft-Start can be
defined by the user. The Soft-Start is finished when
6.5V
Soft-Start Com parator
RFB
FB
PW M -Latch
VSoftS 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 with
breaktrough voltage of 5.6V is to prevent the internal
circuit from saturation (see Figure 10).
PW M Com parator
0.8V
Optocoupler
PW M O P
Isense
x3.65
6.5V
Power-Up Reset
5.6V
RSoft-Start
Error-Latch
Improved
R
S
R
S
Q
Q
Q
Q
SoftS
6.5V
Current Mode
C4
G 2
5.3V
Figure 8
PWM Controlling
4.8V
RFB
C3
3.3
Soft-Start
Gate
Driver
FB
Clock
VSoftS
PW M -Latch
Figure 10 Activation of Protection Unit
5.6V
5.3V
The Start-Up time TStart-Up within the converter output
voltage VOUT is settled must be shorter than the Soft-
Start Phase TSoft-Start (see Figure 11).
TSoft-Start
TSoft − Start
CSoft − Start
=
G ate Driver
t
t
RSoft − 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.
Figure 9
Soft-Start Phase
The Soft-Start is realized by the internal pullup resistor
RSoft-Start and the external Capacitor CSoft-Start (see
Figure 2). The Soft-Start voltage VSoftS is generated by
charging the external capacitor CSoft-Start by the internal
Datasheet
9
September 2001
INFINEON [ Infineon ]
