Functional Description
3 Functional Description
3.1 Power Management
Figure 6 Power Management
The Undervoltage Lockout monitors the external
supply voltage V
VCC
. In case the IC is inactive the
current consumption is max. 55μA. When the SMPS is
plugged to the main line the current through R
Start-up
charges the external Capacitor C
VCC
. When V
VCC
exceeds the on-threshold V
CCon
=13.5V the internal bias
circuit and the voltage reference are switched on. After
that the internal bandgap generates a reference
voltage V
REF
=6.5V to supply the internal circuits. To
avoid uncontrolled ringing at switch-on a hysteresis is
implemented which means that switch-off is only after
active mode when Vcc falls below 8.5V.
In case of switch-on a Power Up Reset is done by
resetting the internal error-latch in the protection unit.
When V
VCC
falls below the off-threshold V
CCoff
=8.5V the
internal reference is switched off and the Power Down
reset let T1 discharging the soft-start capacitor C
Soft-Start
at pin SoftS. Thus it is ensured that at every switch-on
the voltage ramp at pin SoftS starts at zero.
3.2 Improved Current Mode
Figure 7 Current Mode
Current Mode means that the duty cycle is controlled
by the slope of the primary current. This is done by
comparison the FB signal with the amplified current
sense signal.
Figure 8 Pulse Width Modulation
In case the amplified current sense signal exceeds the
FB signal the on-time T
on
of the driver is finished by
resetting the PWM-Latch (see Figure 8).
The primary current is sensed by the external series
resistor R
Sense
inserted in the source of the integrated
CoolMOS™. By means of Current Mode regulation, the
Internal
Bias
Voltage
Reference
6.5V
4.8V
Undervoltage
Lockout
13.5V
8.5V
Power-Down
Reset
Power-Up
Reset
Power Management
5.3V
4.0V
T1
PWM-Latch
R
S
Q
Q
Error-Latch
SoftS
6.5V
Error-Detection
VCC
Main Line (100V-380V)
Primary Winding
Soft-Start Comparator
C
VCC
R
Soft-Start
R
Start-Up
C
Soft-Start
x3.65
PWM OP
Improved
Current Mode
0.8V
PWM Comparator
PWM-Latch
Isense
FB
R
S
Q
Q
Driver
Soft-Start Comparator
t
FB
Amplified Current Signal
T
on
t
0.8V
Driver
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