PM6600 Datasheet, PDF (37/43 Pages) ON Semiconductor – 6-rows 30 mA LEDs driver with boost regulator for LCD panels backlight

PM6600
Fault management
9.3
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.
If the MODE pin is high (connected to AVCC), the open ROW is excluded from the control
loop and the device continues to work properly with the remaining ROWs, without asserting
the FAULT pin.
Connecting the MODE pin to SGND, the PM6600 behaves in a different manner: as soon as
one open ROW is detected, the FAULT pin is tied low. In case a second open ROW is
detected, 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).
As a consequence, If less than six ROWs are used in the application, the MODE pin must be
set high.
9.4
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 fixed fault threshold V_TH,FAULT = 8.2 V,
the device detects the faulty condition and acts according to the MODE pin status.
In case the MODE pin is connected to AVCC, the PM6600 disconnects the ROWs whose
voltage is higher than the threshold and the FAULT pin is tied 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.
If the MODE pin is low, when the voltage across one ROW is higher than V_TH,FAULT
threshold, the FAULT pin is set low and that ROW is disabled. If the voltage of a second
ROW becomes higher than V_TH,FAULT threshold, the device is turned off. The internal logic
latches this status until the EN pin is toggled or a POR is performed.
9.5
Intermittent connection
For intermittent connection it is intended the condition where the flat cable connector from
the leds backlight driver to the leds can have some issues on moving the panel of the
notebook. This kind of issue is represented as an intermittent connection, that means the
physical electrical connection between the ROWx pins of the PM6600 device and the White
LEDs can be open for a while.
The device will detect an open row fault.
There is one possible solution to determine whether the fault is due to the intermittent
connection or to a broken persistent electrical connection (open circuit). Since the device
disables the open rows during the intermittent connection, one possible solution is, on the
customer side, to toggle the EN pin and verify if the fault condition is still present.
In fact, once you disconnect one row, it will result as a off-row (Fault -> open row, latched).
When you connect it again, it is as a shorted led (Vrow higher than the threshold).
This is because the short led detection is still active.
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