The operation of OLEDs with electronic drivers is similar to anorganic LEDs for the most part. In a majority of applications, standard LED drivers may be used also for OLEDs. Nevertheless, there are some important basic rules and OLED specific characteristics, that have to be considered.
For correct operation of more than one OLED at one single constant current (CC) driver, the OLEDs have to be connected in series connection. Alternatively, multi channel drivers with more than one individually current controlled output channel may be used.
Due to degradation processes, the luminous output of the OLED decreases with increasing operation time. Additionally, the static resistance of the OLED increases.
In constant voltage operation, the rise of the static resistance leads to a decreasing operation current with increasing operation time. As this happens additionally to the normal luminous degradation, the L70 point of luminous output is reached earlier (figure 1). Hence, constant voltage mode results in reduced lifetime and is not recommended.
Knowing this, constant current operation is suggested as an optimal solution: The OLED current will be kept constant over the full operation time, while the forward voltage of the OLED increases due to the rise of the static resistance (so-called voltage ageing). The L70 point of luminous output will be reached by far later, the operation time of the OLED is maximized (figure 2).
The luminous flux of the OLED is proportional to the operation current, but not proportional to the operating voltage. The Voltage-Luminance characteristic is very steep at the nominal operating point. This means, that very small changes of the operation voltage or small changes of the characteristic (by temperature, production tolerances) will result in significantly high changes in the luminous output. The Current-Luminance characteristic is much more flat at the nominal operating point.
This means that small changes in operation current or small changes of the characteristic (by temperature, production tolerances) will only lead to small changes of the luminous output (figure 3). Comparing both characteristics leads to the result, that current controlled mode is the best solution for driving OLEDs.
Production tolerances result in much more luminance deviations in constant voltage mode than in constant current operation. Hence, constant voltage drivers would need to be adjusted extremely exact to the correct OLED voltage, which is not possible with most of the available devices.
Also constant voltage operation combined with a series resistor allows only very imprecise adjustment of the operation point and leads – especially due to the voltage ageing – to an early decrease of the luminance.
Constant current operation ensures maximum OLED lifetime and keeps the luminance stable over a wider variation of environmental parameters like: