Brief Introduction of ICs Required for AMOLED Display Power Supply

OLED screens are classified into two types: passive matrix (PMOLED) and active matrix (AMOLED). PMOLED displays are cheaper and easier to manufacture. However, the resolution and screen refresh rate are limited. Most OLED displays today use passive matrix, but active matrix displays are growing rapidly. High color saturation, thin size, high contrast, fast response time and low power consumption are the factors that contribute to this trend.

　　PMOLED only needs positive voltage to supply the power required by the row and column drivers VOH_r and VOH_c. Many boost converters can supply the boost required for PMOLED displays, but AMOLED displays have a fast switching TFT matrix to control the OLED. Therefore, to drive the OLED of an AMOLED display, positive and negative voltages are required.

　　AMOLED requires less current and lower voltage. In fact, for a PMOLED display of the same size, only an order of magnitude less power is needed (think of the difference between microwatts and milliwatts). In the past, designers would search IC manufacturers' websites to try to find devices that could power OLED displays, but only a few devices were suitable. Those devices were specifically marked as "suitable for OLED powering purposes." To power an AMOLED, the right IC is needed to get the power levels right and minimize the external circuitry to provide the negative and positive voltages.

　　A new generation of power ICs is needed

　　The continued development of AMOLED displays requires a new generation of power ICs. Some ICs control two independent power loops using two inductors to meet the requirements for positive and negative voltages.

　　Using a single independent inductor for multiple output converters can achieve a smaller solution size, fewer components, and lower design cost. With relatively close matching of the load current between the positive and negative voltages, both ends of the inductor can be used to provide two regulated electrodes. Taking the converter used in Texas Instruments TPS65136 as an example, a single inductor multiple output converter helps AMOLED displays achieve high-quality performance. With only a 2.2μH inductor, it can operate in a buck-boost topology to generate positive and negative output voltages. This technology can also achieve excellent line and load regulation, thereby avoiding display interference during signal transmission in mobile phones.

　　To maintain high efficiency over the entire load current range, the converter reduces the switching frequency as the load current decreases. When the frequency is too low, the converter can generate audible noise in the form of vibrations in the ceramic output capacitors. The TPS65136 uses a voltage controlled oscillator (VCO) to keep the switching frequency above the range where audible noise is generated, thus avoiding this condition. This is particularly important when using devices with small AMOLED displays, such as mobile phones and portable media players.

　　Today, AMOLED panels and system integrators are turning to the development of second and third generation products. Mobile phones and portable media players will promote mass production, achieve economies of scale, and benefit other devices. These industrial developments highlight the importance of power IC evolution and obsolescence for display operation and innovation.