CoolSET®-F3R
ICE3BR4765J
Functional Description
Within the soft start period, the duty cycle is increasing
from zero to maximum gradually (see Figure 12).
3.5
PWM Section
In addition to Start-Up, Soft-Start is also activated at
each restart attempt during Auto Restart.
0.75
PWM Section
Oscillator
VSoftS
Duty Cycle
max
tSoft-Start
VSOFTS32
Clock
Frequency
Jitter
VFB
t
t
Soft Start
Block
FF1
Q
4.0V
Gate Driver
&
S
R
Soft Start
Comparator
1
G8
PWM
G9
VOUT
Comparator
Current
Limiting
VOUT
CoolMOS®
Gate
tStart-Up
t
Figure 14
3.5.1
PWM Section Block
Figure 13 Start Up Phase
Oscillator
The Start-Up time tStart-Up before the converter output
voltage VOUT is settled, must be shorter than the Soft-
Start Phase tSoft-Start (see Figure 13).
The oscillator generates a fixed frequency of 65KHz
with frequency jittering of ±4% (which is ±2.6KHz) at a
jittering period of 4ms.
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 it helps to prevent saturation of the
transformer during Start-Up.
A capacitor, a current source and current sink which
determine the frequency are integrated. In order to
achieve
a very accurate switching frequency, the
charging and discharging current of the implemented
oscillator capacitor are internally trimmed. The ratio of
controlled charge to discharge current is adjusted to
reach a maximum duty cycle limitation of Dmax=0.75.
Once the Soft Start period is over and when the IC goes
into normal operating mode, the switching frequency of
the clock is varied by the control signal from the Soft
Start block. Then the switching frequency is varied in
range of 65KHz ± 2.6KHz at period of 4ms.
3.5.2
PWM-Latch FF1
The output of the oscillator block provides continuous
pulse to the PWM-Latch which turns on/off the
integrated CoolMOS®. After the PWM-Latch is set, it is
reset by the PWM comparator, the Soft Start
comparator or the Current -Limit comparator. When it is
in reset mode, the output of the driver is shut down
immediately.
Version 2.5
12
19 Nov 2012
INFINEON [ Infineon ]
