Version 2.0 10 1 Feb 2002
ICE2AS01/G
ICE2BS01/G
Functional Description
Figure 11
Start Up Phase
3.4
Oscillator and Frequency
Reduction
3.4.1
Oscillator
The oscillator generates a frequency f
switch
= 100kHz. A
resistor, a capacitor and a current source and 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 max. duty cycle limitation of D
max
=0.72.
3.4.2
Frequency Reduction
The frequency of the oscillator is depending on the
voltage at pin FB. The dependence is shown in Figure
12. This feature allows a power supply to operate at
lower frequency at light loads thus lowering the
switching losses while maintaining good cross
regulation performance and low output ripple. In case
of low power the power consumption of the whole
SMPS can now be reduced very effective. The minimal
reachable frequency is limited to 20kHz / 21.5 kHz to
avoid audible noise in any case.
Figure 12
Frequency Dependence
3.5
Current Limiting
There is a cycle by cycle current limiting realised by the
Current-Limit Comparator to provide an overcurrent
detection. The source current of the external Power
Switch is sensed via an external sense resistor R
.
By means of R
the source current is transformed to
a sense voltage V
Sense
. When the voltage V
Sense
exceeds the internal threshold voltage V
csth
the
Current-Limit-Comparator immediately turns off the
gate drive. To prevent the Current Limiting from
distortions caused by leading edge spikes a Leading
Edge Blanking is integrated at the Current Sense.
Furthermore a Propagation Delay Compensation is
added to support the immedeate shut down of the
Power Switch in case of overcurrent.
3.5.1
Leading Edge Blanking
Figure 13
Leading Edge Blanking
Each time when the external Power Switch is switched
on a leading spike is generated due to the primary-side
capacitances and secondary-side rectifier reverse
t
t
V
SoftS
t
5.3V
4.8V
T
Soft-Start
V
OUT
V
FB
V
OUT
T
Start-Up
f
standby
f
norm
1,0
1,1
1,2
1,3
1,4
1,5
1,6
1,7
1,8
1,9
2
FB
V
V
kHz
O
f
f
norm
:
f
standby
:
ICE2BS01
67kHz
ICE2AS01
100kHz
20kHz
21.5kHz
t
V
Sense
V
csth
t
LEB
= 220ns
