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LED7706TR Datasheet(PDF) 26 Page - STMicroelectronics |
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LED7706TR Datasheet(HTML) 26 Page - 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. |
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