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dependent operation level(Mlevel: between 60 and 100 V), the internal ‘Mlevel switch’ willbe
opened and the start-up current source is enabled to charge capacitorC2521 at the V
CC
pinas
shown below.
The ‘soft start’ switch is closed whenthe V
CC
reachesa level of 7 V and the ‘soft start’
capacitorC
SS
(C2522, betweenpin 5 and the sense resistor R3526), is charged to 0.5 V.
Once the V
CC
capacitoris charged to the start-up voltage V
CC-start
(11V), the IC starts driving
the MOSFET. Both internal current sourcesare switched ‘off’ after reaching this start-up
voltage.Resistor R
SS
(3524)will discharge the ‘soft start’ capacitor, suchthat the peak current
will slowly increase. This to prevent ‘transformerrattle’.
During start-up, the V
CC
capacitorwill be discharged until the moment that the primary
auxiliary windingtakes over this voltage.
Figure:
The moment that the voltage on pin 1 drops below the ‘under voltagelock out‘ level (UVLO = ±
9 V), the ICwill stop switching and will enter a safe restart from the rectifiedmains voltage.
Pa
g
e 18 of 32SPMS
7/8/2004