Design Articles

The White Light-Emitting Diode (WLED) is becoming more and more popular for LCD back lighting in visual display electronics. Whether for lighting up LCDs in PDAs, cell phones or remote controls, the luminescence of the WLED makes the LCD displays bright and pleasant to the human eye. In order to make the WLEDs emit light, a forward voltage must be applied across the anode and the cathode of the WLEDs. The MIC2297 is an excellent WLED driver that can drive up to 10 WLEDs in series. In this applications note, how the MIC2297 is used to drive up to 60 WLEDs using a very simple current mirror technique is demonstrated.

What is a White Light-Emitting Diode (WLED)?

The WLED is a type of semiconductor diode that emits light when a forward bias voltage is applied to the p-n junction that forms the diode. The positive (p+) of the LED is called the anode and the negative (n-) portion of the LED is called the cathode. Once a forward bias voltage is applied, the LED will emit an electroluminescent light with a specific wavelength, depending upon what material was used to create it. The wavelength is what determines the color emitted by the diode. The WLED is a popular choice in electronic backlighting because it generates a colorless brightness that does not alter the original intended color from the LCD.

What is a WLED Driver?

It only takes around 3 volts to forward bias a WLED and to turn it on. To drive a series of 10 WLED, it would take 30V across all of them. A WLED driver, for example Micrel’s MIC2297, is a boost regulator that can drive WLEDs in series. The boost regulator is a DC-to-DC converter that takes a low input voltage (2.5V to 10V) and converts it to a high output voltage (up to 40V). This high output voltage is then applied across a series of WLEDs. The MIC2297 can easily boost the output voltage to an appropriate level to forward bias all the WLEDs.