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IC for Traffic Light Led Driver

Question asked by chatzis.nick on Feb 17, 2015
Latest reply on Feb 17, 2015 by ST Employee Fabrizio
Hi All!!

            We are a company based inThessaloniki Greece, which focuses its activities in the sector of trafficcontrol. Our R&D department is in the design process of a power supplydevice that will be used to drive the LED disks (red, amber and green) in atraffic light device. Having worked in the past with various products of yourcompany and being fully satisfied, we would like you to suggest some of youravailable devices (SMPS, regulators, converters, etc) in order to help uscomplete this project.

            Some of the desirable features ofour device that will help you select the IC you consider suitable for our casewill be described below:


• Input: The operating voltage of the power supply is 230VAC andcomes from an external controller. It can supply our device with three voltagelevels which will determine how the device will work. More specifically, forvoltages <90 VAC the power supply will be in the off-state. For voltages90VAC <Vin <170VAC, output should be in a the low-state (70% of normaloperation). Finally, for voltages 170VAC <Vin <230VAC power supply willoperate normally.

• Output: Each power supply device will drive one of theaforementioned Led discs with constant current. Each disc consists of 10strings connected in parallel and in each string there are 12 Leds connected inseries. The output voltage ranges from 23 to 26 VDC. At this point there arethree main issues:

1. Output current should remain constant for each LED string evenif one or more strings go out of order so that the remaining leds don’toverburden. (Self-Adjusting Current Sharing Mode)

2. If a certain number of leds (e.g. 40%) fail, the power supplyshould sense the change in the output voltage and enter into off-state.

3. Because the operating voltage of led discs differs from colorto color (red, amber, green) the power supply will be able to provide theappropriate current for each disc either automatically (which is the mostdesired) or by using a DIP switch.


            Having studied the respective designguides of your company we have identified some ICs that could meet therequirements of our device, but we also want you to suggest us- based on theabove characteristics – which one of your available solutions you consider themore efficient for our project.


Don’thesitate to ask for further information.

Thankyou in advance!




R&DDep. Simatodotisi