OLED screens are divided into passive matrix (PMOLED) and active matrix (AMOLED). PMOLED displays are less expensive and easier to manufacture. However, resolution and screen update frequency are limited. Most OLED displays today use passive matrices, but active matrix displays are accelerating. High color saturation, thin volume, high contrast, fast response time and low power consumption are the factors that contribute to this trend.

PMOLED requires only the 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 that can control OLEDs. Therefore, to drive an 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, it is only necessary to supply a lower level of power (think of the difference between the microwatts and the milliwatts in each column). Previously, designers searched the IC manufacturer's website to try to find a device that could power an OLED display, but only a few devices were suitable. These devices are specifically marked as "suitable for OLED power supply applications." In order to supply power to the AMOLED, a suitable IC is required to make the power level correct, and to minimize the external circuit that supplies the negative and positive voltages.

Need a new generation of power ICs

The ever-evolving AMOLED display requires a new generation of power ICs. Some ICs control two independent power loops that use two inductors to meet the need for positive and negative voltages.

Smaller output converters with a single, independent inductor allow for smaller solution sizes, fewer component counts, and lower design costs. Both ends of the inductor can be used to provide two regulated electrodes through a relatively tightly matched load current between the positive and negative voltages. Taking the converter used by Texas Instruments TPS65136 as an example, a single inductor multiple output converter can help AMOLED displays achieve high image quality. It can operate in a buck boost topology with only a 2.2μH inductor, producing positive and negative output voltages. This technology also achieves excellent line-to-line and load regulation, thus avoiding display interference from mobile phones during signal transmission.

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 emits audible noise with the vibration of the ceramic output capacitor. The TPS65136 uses a voltage controlled oscillator (VCO) to keep the switching frequency above the range where audible noise is emitted, thus avoiding this type of situation. This is especially important when using devices with small AMOLED displays, such as mobile phones and portable media players.

Today, AMOLED panel and system integrators are turning to the development of second and third generation products. Mobile phones and portable media players will boost volume production, achieve economies of scale and benefit other devices. The development of these industries is highlighting the importance of the evolution and elimination of power ICs for display operations and innovation.

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