LED7707TR datasheet(23/48 Pages) STMICROELECTRONICS

23 / 48 page

LED7707

Operation description

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5.3.1

FAULT pin

The FAULT pin is an open-collector output, (with 4 mA current capability) active low, which

gives information regarding faulty conditions eventually detected. This pin can be used

either to drive a status LED or to warn the host system.

The FAULT pin status is strictly related to the MODE pin setting (see Table 7 for details).

5.3.2

MODE pin

The MODE pin is a digital input and can be connected to AVCC or SGND in order to choose

the desired fault detection and management. The LED7707 can manage a faulty condition

in two different ways, according to the application needs. Table 7 summarizes how the

device detects and handles the internal protections related to the boost section (over-

current, over-temperature and over-voltage) and to the current generators section (open and

shorted LEDs).

5.3.3

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 (i.e. connected to AVCC), the

FAULT pin is set high as soon as the device recognizes the event; the open row is excluded

from the control loop and the device continues to work properly with the remaining rows.

Thus, if less than six rows are used in the application, the MODE pin must be set high.

Connecting the MODE pin to SGND, the LED7707 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).

Figure 16

shows an example of open channel detection in case of MODE connected to

AVCC.

Table 7.

Faults management summary

FAULT

MODE to GND

MODE to VCC

Internal MOSFET

over-current

FAULT pin HIGH

Power MOS turned OFF

Output over-voltage

FAULT pin LOW

Device turned OFF, latched condition

Thermal shutdown

FAULT pin LOW. Device turned OFF.

Automatic restart after 30 C temperature drop.

LED short circuit

“FAULT pin LOW, Device turned

OFF (100s masking time),

latched condition (Vth = 4.0 V)”

Open row(s)

FAULT pin LOW

Device turned OFF at first

occurrence, latched condition

FAULT pin HIGH faulty row(s)

disconnected.

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