Application information

HVLED001

22/27

DocID027333 Rev 1

7.5

Autorestart timer (ART)

The autorestart timer unit is responsible for the generation of the protection's intervals and

of the restart times after the low consumption mode. A summary of all possible combination

of times is described in each protection paragraph.

7.6

Protections

A comprehensive set of protections is embedded to ensure a high level of reliability of the

final application using a limited number of components.

7.6.1

Overcurrent protection (OCP)

To prevent any damage to active components in case of inductor's saturation the MOSFET

is immediately turned off by the fast OCP protection. At this occurrence the IC temporarily

enters the stop state for a time equal to TOCP.

7.6.2

Input overvoltage protection (I-OVP)

Disturbances of the input voltage like surges or bursts may increase the voltage applied to

the transformer primary side. Worst, an excessive input voltage could be applied to the

application. These occurrences may result into MOSFET damage during the off-state when

the drain voltage rises to Vin plus reflected voltage, eventually above the maximum absolute

rating of the MOSFET itself.

An input voltage higher than VSurge, measured by HVSU structure, immediately stops the

IC. If the extra voltage diminishes before Tsurge the device restarts immediately without

activating the start-up procedure, otherwise the low consumption mode is entered until the

input voltage returns below the safety threshold. An internal hysteresis improves the noise

rejection of this feature. In the latter case the ART activates the start-up procedure.

7.6.3

Brownout protection (BO)

The current sourced by the ZCD pin's negative clamp during on time is compared to

a minimum value to determine whether the input voltage is lower than the input range

specification (brownout protection). If a value lower than IBO for a time longer than TBO,

managed by the ART, is detected, the IC is stopped for Trec and then restarted.

When the protection is triggered, the ART performs the autorelaoding procedure after Trec.

The brownout protection is active during the active mode, but blanked during the soft-start.

7.6.4

Optocoupler failure protection (OFP)

This protection detects either the absence of the optocoupler control (no pull-down) or the

overload condition for more than a time equal to TOFP and switches off the application

putting the device in the low consumption mode. This prevents the output power from rising

above excessive values due to the loss of control.

The ART manages the TOFP interval and performs the auto-reloading procedure after Trec.

The OFP is active during the active mode, but blanked during the soft-start.