AL5814

Document number: DS40390 Rev. 3 - 2

10 of 14

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August 2018

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AL5814

The table below shows some approximate supply voltages that the LED-open detection becomes active by different resistor values.

R7 (kΩ)

5.01

9.1

5.50

10

6.60

12

8.25

15

9.90

18

11.55

21

If SFAULT is shorted to GND then by default LED-open fault detection is automatically entered. If SFAULT is left open then LED-open detection is

inhibited.

VFAULT

The AL5814 detects open conditions on the collector/drain of the external transistor driving the LEDs using the VFAULT pin, see Figure 6. The

VFAULT pin has a 14µA current source generated out of this pin.

Figure 6. LED-Open Detection Using VFAULT

If the LED string becomes open, VFAULT pin will be pulled down by the power bipolar or MOSFET below its internal 2.5V threshold. This condition

triggers an output disable condition causing OUT to go low, turning off the external MOSFET/BJT. A resistor (R3, 100K) is needed to keep the

VFAULT signal low during a fault condition. A 100k will keep this node low. When the fault condition is fixed and VFAULT pin rises above 2.63V

the device will operate normally.

Feedback Loop

The device has internal compensation and therefore it is not recommended that any components are added to the feedback loop.

LED Thermal Compensation

The LED current is set by the reference voltage at the emitter/drain voltage of the LED driving transistor. This reference voltage is determined by

the resistor divider between REF and VSET. The LED driver

’s LED current behavior under different ambient or LED temperatures can be

configured by using NTC for R6 shown in Figure 7.