Which Driver Chip to be used for L.E.D.

[koolguy]

One of the semiconductor manufacturers makes the IC that detects a burned out open circuit LED and shorts it so that remaining LEDs in series with it continue to light.
All the traffic lights in my city use LEDs and I can see burned out LEDs in some of the lights. Each traffic light color has many LEDs so they are probably all in series. They must use that IC that shorts the open LEDs.

Hello again,
Which chip is that?
I am using 120ohm Resistance with every string to control the Current. but i notice some Led has little bit difference in Vf, so in some string one LED is remain OFF but rest remain light. but that LED glow when tested separately.
why does it happen?
and how to overcome these low bright and burnout problem(indirectly increasing life of LED when working)

 

[koolguy]

The LM3464/64A provides four individual current regulator channels and works in
conjunction with external N-channel MOSFETs and sense resistors to give accurate driving current for
every LED string.

The data sheet say this only for four string for many string to many LM3464 will be used so, the circuit will be more bigger and expensive than simple Series resistance method.
as Red Led work on ~1.9V so 80/1.9= 40 Led per string total 4x40= 160 LEDs
there are few more method which chinese lightning use to have Ac supply in DC with series Resistance and TRIAC PCR 406 To control it