C2472, C2473 and C2474
Datasheet
RDFC Controllers for Offline Applications
Product data
Page 12 of 18
DS-1423-0709C
© Cambridge Semiconductor Ltd 2007
26-Sep-2007
Protection Features
Collector De-saturation (Over Voltage) Protection (COVP)
To protect the primary switch from excessive power dissipation, the on-state voltage of the primary switching
transistor is limited by the controller. The controller will go to Foldback mode if the COL pin voltage is greater
than VCOVP at the end of the on-time for four consecutive cycles.
Over-temperature Protection (OTP)
Temperature sensing is integrated with the controller. If the temperature of the die rises above the shutdown
temperature, TSH, the BAS output is inhibited. It restarts once the temperature has fallen more than T SH (HYST)
below TSH. In typical applications “hiccup” operation will occur. While BAS is inhibited, the device is active
and draws IDDWAKE. This causes VDD to fall since auxiliary power is not provided by the transformer. Once VDD
reaches VUVDTHR, the controller enters the Sleep state and IDD falls to IDDSLEEP allowing VDD to rise again (via
the resistors Vht1 and Vht2). When VDD reaches VOVDTHR reset occurs and the controller re-starts. If the die
temperature is below TSH, BAS operation continues but if it is still above TSH, BAS operation ceases after a
short period and the hiccup cycle repeats.
Primary Switch Over-current Protection (OCP)
To protect the primary switch, the base drive is turned off if the primary switch current rises too high, sensed
via the CS input voltage falling below a preset negative threshold VOCPH. See also Power Control on page
10.
Output Overload/Short-circuit Protection
If the application circuit is overloaded beyond a certain limit the controller goes into Foldback mode with
reduced duty cycle, protecting the primary switch by reducing its power dissipation. Transition to Foldback
mode is triggered by the CS pin voltage crossing the VOCPH threshold within a time TFBTHR of the start of the
FON pulse. This typically happens when the load holds the output voltage low. See also Power Control on
page 10.
Under Voltage Protection
The controller is prevented from operating if the VDD supply is inadequate (VDD < VUVDTHR). Once the
controller has stopped operation it will not restart until the VDD supply voltage rises above VOVDTHR.
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