Method of characterising an LED device

Abstract

A method of characterizing an LED, as well as an integrated circuit using this method, based on a so-called characteristic resistance, in which the LED is operated at a first, relatively low, operating current and then at a second, relatively high, operating current. From the ratio between the difference between the forward voltages at these two operating currents, and the difference between the operating current, the characteristic resistance is determined. The characteristic resistance is measured at two or more moments during the operational lifetime of the device, and a prediction or estimate is made in relation to the total operational lifetime of the devices, from the evolution or change of the characteristic resistance.

The invention claimed is: 
     
       1. A method of characterising an LED device, the method comprising:
 determining a first value and a second value of a characteristic-resistance Ron, by measuring a first voltage and a second voltage across the LED device whilst a respective first current and a second current is passing through the LED device; 
 determining the characteristic-resistance from a ratio of a difference between the first and second voltages, and a difference between the first and the second currents, according to
   Ron=( Vh−Vl )/( ih−il ); and 
 
 predicting an end of an operational lifetime of the device from the first value and the second value of the characteristic-resistance. 
 
     
     
       2. The method of  claim 1 , wherein the first value of the characteristic-resistance is determined at a start of an operational lifetime of the LED device. 
     
     
       3. The method of  claim 1 , wherein the second value of the characteristic-resistance is determined after a part of an operational lifetime of the LED device. 
     
     
       4. The method of  claim 2 , wherein the first value of the characteristic-resistance is determined after a further part of the operational lifetime of the LED device, and further comprising extrapolating to estimate a value of the characteristic at a start of the operational lifetime of the LED device. 
     
     
       5. The method of  claim 1 , further comprising:
 determining at least one further value of the characteristic resistance after respectively at least one further part of the operation life, and wherein predicting an end of an operational lifetime comprising extrapolating an evolution of the characteristic resistance against operational lifetime. 
 
     
     
       6. The method of  claim 1 , further comprising storing at least a value of the characteristic resistance in a memory. 
     
     
       7. The method of  claim 1 , wherein the end of an operation lifetime is predicted to be when the characteristic-resistance Ron differs from its value at the start of the operational lifetime, by a predetermined amount. 
     
     
       8. The method of  claim 1 , wherein the first current is larger than the second current by between 4 and 6 orders of magnitude. 
     
     
       9. The method of  claim 7 , wherein the first current is larger than the second current by 5 orders of magnitude. 
     
     
       10. The method of  claim 1 , further comprising providing a warning signal indicative of the predicted end of an operational lifetime. 
     
     
       11. The method of  claim 1 , wherein the method further comprises selecting one of a plurality of performance bins based on the predicted end of lifetime. 
     
     
       12. An integrated circuit configured to drive an LED device and to operate the method of  claim 1 .