how to extent output led string voltage range with constant current current

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Sir,

    we are into led driver development using LinkswitchPH devices. I have a question. Say in RDR195 output voltage is 28v. I understand the driver will work satisfactorily with 500ma for a led string voltage range of 25.2 to 29.26v. My requirement is that the same driver should work for a range of 18v to 28v by keeping the current constant(500ma). Please advice. 

Thanks

Stalin.

If your application is non-dimming, you'll get better current regulation for wider output voltage range. Please verify if the output current variations for the said voltage range is within your requirements. If you need tighter regulation, one way to do it is to implement an additional output current feedback circuit and feed this information to FB pin in order to tightly regulate the output current.