DS12866 Rev 1

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HVLED007

Application information

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developed by the auxiliary winding, becomes high enough to arm the ZCD triggering block

and start self-triggered TM operation.

The Start Timer is invoked also whenever the IC stops during burst-mode operation (i.e. as

driving (pin COMP)") or after an overvoltage detection (see "Section 4.10: Overvoltage

protection (OVP function)") to restart switching activity.

4.9

Overload and short-circuit protection (OCP function)

The purpose of this function is to enable the HVLED007 to effectively control the converter

when the load exceeds its maximum specified value or in case of short-circuit at the output

so that no component in the power circuit is exposed to risk of failure.

This high-level goal is met by making the converter operate intermittently under these

conditions. To allow this, the low-level goal of the OCP function is to make the converter

operate at the frequency of the Start Timer, so that the supply voltage of the HVLED007

cannot be kept above the UVLO threshold at all times and the IC continuously shuts down

and restarts.

 Detection. Overload and short-circuit conditions (OL-SC) have a twofold detection:

a)

on pin COMP, when its voltage is over the upper end of the regulation range

b)

OL-SC conditions are then detected by the logic OR of the two detection circuits.

 Corrective actions. The corrective actions aim to prevent the ringing after leakage

inductance demagnetization from erroneously triggering a new switching cycle, which

would result in high frequency operation under OL-SC conditions.

Once this has been ensured, one can rely on the natural behavior of TM-operated

systems, where the higher the overload, the lower the operating frequency and, therefore

the power capability of the converter.

Eventually, under short-circuit, the output voltage being too low, the ZCD circuit can no

longer be armed and the converter works at the frequency of the internal Start Timer (≈3.6

kHz), with an extremely low power capability. Since under these conditions the supply

voltage of the IC cannot be kept above its UVLO threshold, the IC continuously shuts

down and restarts, as targeted.

To meet this goal, a time window is generated whenever the external power MOSFET is

switched off (signal MSK in the block diagram of Figure 10). The synch pulses coming

from the ZCD circuit are blanked out during this time window.

Normally, the duration of this time window is 1 μs but when an OL-SC is detected, it is

increased to 3 μs. In addition to blanking the synch pulses coming from the ZCD circuit,

during this 3 μs time window the pin ZCD is internally grounded to kill the initial spike that

occurs at MOSFET turn-off and prevent the ZCD circuit from being improperly armed.