ICE2AS01_06 Datasheet, PDF (8/24 Pages) Infineon Technologies AG – Off-Line SMPS Current Mode Controller

ICE2AS01/S01G
ICE2BS01/S01G
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 variation 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.
V_OSC
max.
Duty Cycle
Soft-Start Comparator
Voltage Ramp
t
PWM Comparator
FB
0.8V
FB
PWM-Latch
0.3V
Oscillator ^0.3V
C5
Gate Driver
t
V_OSC
Gate Driver
T₂
C₁
0.8V
10kΩ
R₁
V₁
20pF
x3.65
PWM OP
Voltage Ramp
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 T₂, the voltage source V₁ and the 1st order
low pass filter composed of R₁ 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 continuously till 0%
by decreasing V_FB below that threshold.
t
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. R_Sense 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 signal 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). V_FB 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.
Datasheet
8
Preliminary Data
30 Jun 2006