Application information

HVLED007

24/33

DS12866 Rev 1

Figure 10. Functional schematic of the overload and short-circuit protection function

4.10

Overvoltage protection (OVP function)

The purpose of this function is to temporarily stop the operation of the converter in case the

output voltage overshoots at start-up or as a result of a sudden load drop, exceeding a

preset value.

 Detection. An overvoltage (OV) in the output voltage is sensed through the pin ZCD,

which is connected via a resistor divider to the auxiliary winding in the transformer that

serves both as zero current detection (to trigger a new switching cycle) and self-supply

voltage generator (to power the IC). In fact, during the OFF-time of the external power

MOSFET the voltage across the auxiliary winding tracks the output voltage. Tracking is

acceptably good for a protection except for a short period after turn-off because strongly

affected by the spike caused by the leakage inductance of the transformer and the ringing

that follows its demagnetization. To prevent this noise from falsely triggering the OVP

(= 5.5 V) on a filtered ZCD signal and the output of this comparator is blanked by the same

signal MSK (see Figure 10) used in the OCP function.

condition is assumed. In fact, to reduce sensitivity to noise and distinguish a real failure

from a disturbance (e.g. induced during ESD tests), the OVP comparator must be

triggered in two consecutive switching cycles in order for the corrective actions to take

place.

More in detail: the first time the OV comparator is triggered a warning flag is set; if in the

following switching cycle the OV comparator is triggered again, then the OV condition is

confirmed, the corrective action takes place and the warning flag is reset; otherwise no

action follows and the warning flag is reset.

 Corrective actions. When an OV condition is detected as specified above, an internal

MOSFET switch is turned on, thus connecting a resistor from the pin COMP to GND. The

that the COMP voltage is definitely taken below the burst-mode threshold. The MOSFET

switch is turned off as the COMP voltage falls below the burst-mode threshold.

The discharge switch is kept in the ON-state also when the IC is in UVLO (provided its