TLC5917: Every time error on the same led

Part Number: TLC5917

Hi,

i have designed the led driver circuit using TLC5917 driving 4 ir leds, one per output from OUT0 to OUT3. The IC is powered by a 3.3V, and the Vled (stated in the ds) has the same supply rail.

I have an issue with the error detection on the least significative led; the error detection on the leds (open or short circuit) is a necessary feature for mine application.

What i have noticed is:

if i light on all the leds simultaneously and then i read the mask error, in absence of errors (short or open) i should read number 15 in the error register.

Since an error is found in the least significative led, the mask error is 14.

if i light on one led per time and then i read the mask error, i do not get any error. Why?

The design example on the ds shows a Vled rail different from the IC supply and the rail should have a voltage greater than the sum of the voltage drop on each led: 4 leds*2.7V=10.8V -> Vled at least = 11V

Why? 

My leds are not cascaded and does not share just one driver output, but each one have its own output.

Thanks

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