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ICE2AS01 Datasheet(PDF) 8 Page - Infineon Technologies AG

Part No. ICE2AS01
Description  PWM-FF IC
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Maker  INFINEON [Infineon Technologies AG]
Homepage  http://www.infineon.com
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 8 page
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Version 2.0
8
1 Feb 2002
ICE2AS01/G
ICE2BS01/G
Functional Description
The primary current is sensed by the series resistor
R
Sense inserted in the source of the external Power
Switch. By means of Current Mode the regulation of the
secondary voltage is insensitive on line variations. Line
variation causes varition of the increasing current slope
which controls the duty cycle.
The external R
Sense allows an individual adjustment of
the maximum source current of the external Power
Switch.
Figure 6
Improved Current Mode
To improve the Current Mode during light load
conditions the amplified current ramp of the PWM-OP
is superimposed on a voltage ramp, which is built by
the switch T
2, the voltage source V1 and the 1st order
low pass filter composed of R
1 and C1 (see Figure 6,
Figure 7). Every time the oscillator shuts down for max.
duty cycle limitation the switch T2 is closed by V
OSC.
When the oscillator triggers the Gate Driver T2 is
opened so that the voltage ramp can start (see Figure
7).
In case of light load the amplified current ramp is to
small to ensure a stable regulation. In that case the
Voltage Ramp is a well defined signal for the
comparison with the FB-signal. The duty cycle is then
controlled by the slope of the Voltage Ramp.
By means of the C5 Comparator the Gate Driver is
switched-off until the voltage ramp exceeds 0.3V. It
allows the duty cycle to be reduced continously till 0%
by decreasing V
FB below that threshold.
Figure 7
Light Load Conditions
3.2.1
PWM-OP
The input of the PWM-OP is applied over the internal
leading edge blanking to the external sense resistor
R
Sense connected to pin ISense. RSense converts the
source current into a sense voltage. The sense voltage
is amplified with a gain of 3.65 by PWM OP. The output
of the PWM-OP is connected to the voltage source V1.
The voltage ramp with the superimposed amplified
current singal is fed into the positive inputs of the PWM-
Comparator, C5 and the Soft-Start-Comparator.
3.2.2
PWM-Comparator
The PWM-Comparator compares the sensed current
signal of the external Power Switch with the feedback
signal V
FB (see Figure 8). VFB is created by an external
optocoupler or external transistor in combination with
the internal pullup resistor R
FB and provides the load
information of the feedback circuitry. When the
amplified current signal of the external Power Switch
exceeds the signal V
FB the PWM-Comparator switches
off the Gate Driver.
x3 .6 5
PW M O P
0.8V
10k
Ω
O s cillator
P W M C om p arator
20 pF
T
2
R
1
C
1
FB
PW M -L a tch
V
1
C5
0.3V
G a te D rive r
V oltage Ram p
V
OS C
S o ft-S ta rt C o m p a ra to r
t
t
V
OS C
0.8 V
FB
G a te D rive r
V o ltag e R am p
t
ma x.
Du ty Cyc le
0.3 V




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