LED7706TR datasheet(26/31 Pages) STMICROELECTRONICS

26 / 31 page

Fault management

LED7706

26/31

8.3

Open LED fault

In case a row is not connected or a LED fails open, the device has two different behaviors

according to the MODE pin status.

Connecting the MODE pin to SGND, the LED7706 behaves in a different manner: as soon

as an open row is detected, the FAULT pin is tied low and the device is turned off. The

internal logic latches this status: to restore the normal operation, the device must be

restarted by toggling the EN pin or performing a Power-On Reset (POR occurs when the

voltage at the LDO5 pin falls below the lower UVLO threshold and subsequently rises above

the upper one).

If the MODE pin is high (i.e. connected to AVCC), the open row is excluded from the control

loop and the device continues to work properly with the remaining rows. The FAULT pin is

not affected. Thus, if less than six rows are used in the application, the MODE pin must be

set high.

8.4

Shorted LED fault

When a LED is shorted, the voltage across the related current generator increases of an

amount equal to the missing voltage drop of the faulty LED. Since the feedback voltage on

each active generator is constantly compared with a fault threshold VTH,FAULT, the device

detects the faulty condition and acts according to the MODE pin status.

If the MODE pin is low, the fault threshold is VTH,FAULT = 3.4 V. When the voltage across a

row is higher than this threshold, the FAULT pin is set low and the device is turned off. The

internal logic latches this status until the EN pin is toggled or a POR is performed.

In case the MODE pin is connected to AVCC, the fault threshold is set to 6 V. The LED7706

simply disconnects the rows whose voltage is higher than the threshold and the FAULT pin

is forced low. This option is also useful to avoid undesired triggering of the shorted-LED

protection simply due to the high voltage drop spread across the LEDs.

LED7706TR Arkusz danych(PDF) 26 Page - STMicroelectronics