LED7707 datasheet(24/48 Pages) STMICROELECTRONICS

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Operation description

LED7707

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At the point marked as “1” in Figure 16, the row opens (row current drops to zero). From this

point on the output voltage is increased as long as the output voltage reaches the floating

row detection threshold (see Section 5.1.3 on page 14). Then (point marked as “2”) the

faulty row is disconnected and the device keeps on working only with the remaining rows.

Figure 16.

Open channel detection (MODE to AVCC)

5.3.4

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.

A 100 µs masking time is introduced to support ESD capacitors eventually connected

across the LEDs strings.

If the MODE pin is low, the fault threshold is VTH,FAULT = 4.0 V. When the voltage across a

row is higher than this threshold for more than 100

μs, 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 LED short-circuit protection is disabled.

The LED7707 simply keeps on regulating the set current without affecting the FAULT pin.

Despite the higher power dissipation, this option is useful to avoid undesired triggering of

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

Figure 17

shows an example of shorted LED detection in case MODE is connected to GND.

At the point marked as “1” in Figure 17 one LED fails becoming a short-circuit. The voltage

across the current generator of the channel where the failed LED is connected increases by

an amount equal to the forward voltage of the faulty LED. Since the voltage across the

current generator is above the threshold (4 V), the device is turned off and the fault pin is set

LED7707 数据表(PDF) 24 Page - STMicroelectronics