PM6600 datasheet(37/43 Pages) STMICROELECTRONICS

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PM6600

Fault management

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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 VTH,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 VTH,FAULT

threshold, the FAULT pin is set low and that ROW is disabled. If the voltage of a second

ROW becomes higher than VTH,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.

PM6600 Datasheet(PDF) 37 Page - STMicroelectronics

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