F3 PWM controller
ICE3BS03LJG
Functional Description
decrease gradually and the duty ratio of the gate drive In addition to Start-Up, Soft-Start is also activated at
increases gradually. The Soft Start will be finished in each restart attempt during Auto Restart.
20ms (TSoft-Start) after the IC is switched on. At the end
of the Soft Start period, the current sink is switched off.
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).
By means of Soft-Start there is an effective
minimization of current and voltage stresses on the
external power MOSFET, the clamp circuit and the
output overshoot and it helps to prevent saturation of
the transformer during Start-Up.
VSoftS
TSoft-Start
VSOFTS32
3.5
PWM Section
0.75
PWM Section
t
Oscillator
Gate
Driver
Duty Cycle
max
Clock
Frequency
Jitter
t
Figure 12 Gate drive signal under Soft-Start Phase
Within the soft start period, the duty cycle is increasing
from zero to maximum gradually (see Figure 12).
Soft Start
Block
FF1
Q
Gate Driver
&
S
R
Soft Start
1
VSoftS
Comparator
G8
PWM
G9
TSoft-Start
Comparator
VSOFTS32
Current
Limiting
Gate
VFB
t
t
Figure 14
PWM Section Block
4.0V
3.5.1
Oscillator
The oscillator generates a fixed frequency of 65KHz
with frequency jittering of ±4% (which is ±2.6KHz) at a
jittering period of 4ms.
A capacitor, a current source and a current sink which
determine the frequency are integrated. The charging
and discharging current of the implemented oscillator
capacitor are internally trimmed, in order to achieve a
very accurate switching frequency. The ratio of
controlled charge to discharge current is adjusted to
reach a maximum duty cycle limitation of Dmax=0.75.
VOUT
VOUT
TStart-Up
t
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
Figure 13
Start Up Phase
Version 2.0
11
6 Dec 2007
INFINEON [ Infineon ]
