SCY99194 Datasheet, PDF (33/40 Pages) ON Semiconductor – Fixed Frequency Current Mode Controller

SCY99194
A 3 bit A/D converter with the peak detector senses the ac
input, and its output is periodically sampled and reset, in
order to follow closely the input voltage variations. The
sample and reset events are given by the output from the ac
line unplug detector. The sensed HV pin voltage peak value
is validated when no HV edges from comparator are present
after last falling edge during two sample clocks. See
Figure 63 for details.
Overcurrent Protection with Fault timer
The overload protection depends only on the current
sensing signal, making it able to work with any transformer,
even with very poor coupling or high leakage inductance.
When an overcurrent occurs on the output of the power
supply, the FB loop asks for more power than the controller
can deliver, and the CS setpoint reaches V_ILIM. When this
event occurs, an internal t_fault timer is started: once the timer
times out, DRV pulses are stopped and the controller is either
latched off (latched protection, option A) or this latch can be
released in autorecovery mode (option B), the controller
tries to restart after t_autorec. Another possibility of the latch
release is the V_CC power on reset or the ac line unplug event
detected via ac detector. Therefore the latch can be released
by the end of the 1^st X2 discharge event. The timer is reset
when the CS setpoint goes back below V_ILIM before the
timer elapses. The fault timer is also started if the driver
signal is reset by the max duty−ratio. The controller also
enters the same protection mode if the voltage on the CS pin
reaches 1.5 times the maximum internal setpoint V_CS(stop)
(allows to detect winding short−circuits) or there appears
low V_CC supply. See Figures 64 and 65 for the timing
diagram.
In autorecovery mode if the fault has gone, the supply
resumes operation; if not, the system starts a new burst cycle.
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