Functional Description
But the Soft-Start Phase must be finished within the
Start Up Phase to force the voltage at pin FB below the
failure detection threshold of 4.8V.
3.8.2 Overvoltage due to Open Loop with
No Load
Figure 23 Auto Restart Mode
Figure 23 shows the Auto Restart Mode for open loop
and no load condition. In case of this failure mode the
converter output voltage increases and also VCC. An
additional protection by the comparators C1, C2 and
the AND-gate G1 is implemented to consider this
failure mode (see Figure 24).The overvoltage detection
is provided by Comparator C1 only in the first time
during the Soft-Start Phase till the Soft-Start voltage
exceeds the threshold of the Comparator C2 at 4.0V
and the voltage at pin FB is above 4.8V. When VCC
exceeds 16.5V during the overvoltage detection phase
C1 can set the Error-Latch and the Burst Phase during
Auto Restart Mode is finished earlier. In that case
T
Burst2
is shorter than T
Soft-Start
. By means of C2 the
normal operation mode is prevented from overvoltage
detection due to varying of VCC concerning the
regulation of the converter output. When the voltage
V
SoftS
is above 4.0V the overvoltage detection by C1 is
deactivated.
Figure 24 Overvoltage Detection
3.8.3 Thermal Shut Down
Thermal Shut Down is latched by the Error-Latch when
junction temperature T
j
of the pwm controller is
exceeding an internal threshold of 140°C. In that case
the IC switches in Auto Restart Mode.
Note: All the values which are mentioned in the
functional description are typical. Please refer
to Electrical Characteristics for min/max limit
values.
Open loop & no load condition
t
Driver
13.5V
16.5V
FB
4.8V
5μs Blanking
Failure
Detection
5.3V
SoftS
4.0V
Overvoltage
Detection Phase
Soft-Start Phase
t
t
T
Restart
T
Burst2
VCC
8.5V
Overvoltage Detection
t
6.5V
C
Soft-Start
VCC
R
Soft-Start
C1
16.5V
C2
4.0V
T1
SoftS
G1
Error Latch
Power Up Reset
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