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During the primary stroke, the rectified ACinput voltage is measured by sensing the current
drawn from pin4 (DEM). This current is dependent on the voltage on pin 9 of transformer
5520and the value of R3522. The current informationrmation is used toadjust the peak drain
current, which is measured via pin I
SENSE
.
Short Winding Protection
If the ‘sense’ voltage onpin 5 exceeds the short winding protection voltage (0.75 V),
theconverter will stop switching. Once V
CC
drops below the UVLOlevel, capacitor C2521 will
be recharged and the supply will startagain. This cycle will be repeated until the short circuit is
removed(safe restart mode). The short winding protection will also protectin case of a
secondary diode short circuit.
This protection circuit is activated after the leadingedge blanking time (LEB).
LEB time
The LEB (Leading Edge Blanking) time is aninternally fixed delay, preventing false triggering of
the comparatordue to current spikes. This delay determines the minimum ‘on’ timeof the
controller.
Over Temperature protection
When the junction temperature exceeds the thermalshutdown temperature (typ. 140º C), the
IC will disablethe driver. When the V
CC
voltagedrops to UVLO, the V
CC
capacitorwill be
recharged to the V
(start)
level.If the temperature is still too high, the V
CC
voltage will drop
againto the UVLO level (Safe-Restart mode). This mode will persist untilthe junction
temperature drops 8 degrees typically below the shutdowntemperature.
Mains dependent operation enabling level
To prevent the supply from starting at a lowinput voltage, which could cause audible noise, a
mains detectionis implemented (Mlevel). This detection is provided via pin 8, thatdetects the
minimum start-up voltage between 60 and 100 V. As previous mentioned,the controller is
enabled between 60 and 100 V.
An additional advantage of this function is the protection againsta disconnected buffer
capacitor (C
IN
).In this case, the supply will not be able to start-up because theV
CC
capacitor
willnot be charged to the start-up voltage.
Control
Pa
g
e 24 of 32SPMS
7/8/2004