OLED Power Supply Design

The biggest feature of organic light-emitting diode (OLED) is that it is self-luminous, does not require backlight and color filter, and is thinner than LCD; in addition, it has a wider viewing angle range, faster response speed, lower driving voltage, and higher color and contrast than LCD. In theory, it can achieve lower power consumption and simple design. It is a widely recognized display star after LCD. Although Oled has so many advantages, its lifespan is shorter than LCD because OLED is a current-driven self-luminous body, and its material and component lifespan is relatively shortened.

OLED power supply specifications

Generally, small-sized OLEDs require a set of positive voltage (Vdd) and a set of negative voltage (Vss) for power supply. The power supply architecture can be divided into two types: digital camera and mobile phone architecture. The power supply specification of digital cameras is: Vdd voltage range is 3V to 6V, Vss voltage range is -7V to -10V; the power supply specification of mobile phones is: Vdd voltage is about 2.5V, Vss voltage range is -7V to -10V.

The input power source is usually a lithium battery with a voltage range of about 3V to 4.2V.

Digital Camera Vdd Solution

Since the Vdd voltage range is 3V to 6V, the Vdd power supply architecture should be Buck/Boost or Boost. If we cannot find a Buck/Boost architecture power supply for the time being, we can also use the very common Buck architecture and design it into a Buck/Boost structure. Using a set of ordinary step-down power supply control ICs, as long as a MOSFET and an output diode are added, it can be designed into a Buck/Boost output, as shown in Figure 1. The working principle of this regulator is that when Lx is a high voltage, the inductor current increases according to the slope of Vin/L; when Lx is a low voltage, the inductor current decreases according to the slope of (Vout+VD)/L. The input and output currents are discontinuous, which allows the output voltage to be higher or lower than the input voltage.

Figure 1. Designing a step-up/step-down power supply using a step-down power supply IC

Designers can also directly use a set of Buck/Boost power supply ICs to generate the required voltage output. Figure 2 is a set of direct step-up/step-down conversion ICs. It combines a set of boost converters and linear regulators to provide a conversion solution that can both step up and step down. This converter can provide a stable output when the input voltage is higher or lower than the output voltage. The input range is allowed to be: 1.8V to 11V, which can be preset to 3.3V or 5V output. Two voltage divider resistors can also be used to obtain an adjustable output voltage: 1.25V to 5.5V, with an efficiency of up to 85%. If an output between 3.5V and 4V is required, it can be achieved by combining a boost converter with a linear regulator. For example: a combination of the MAX1606 boost converter and the MAX8512 linear regulator.

Figure 2: Step-up/step-down power supply

From the perspective of reducing costs, you can choose a charge pump without an inductor and output diode. For example, the MAX1759 charge pump uses a step-up/step-down structure to produce a stable output. Although its operating frequency is higher than 1.5MHz, it can still maintain a quiescent current as low as 50uA.

In order to pursue higher conversion efficiency, some designers choose to boost the voltage to produce a stable output higher than the input, as shown in the boost architecture in Figure 3. Since the power switch MOSFET is external, it can provide a larger output power. If the output power permits, you can also choose a boost converter with a built-in MOSFET power switch, such as the MAX1722, which can effectively save space and reduce costs.

Figure 3: Boost power converter

Mobile Phone Vdd Solution

For the mobile phone Vdd, a Buck circuit can be selected to provide the required voltage. Figure 4 is a synchronous buck structure with a built-in MOSFET power switch, which can provide an output current of 400mA. The operating frequency is as high as 1.2MHz, allowing designers to choose small-sized inductors and output capacitors, and the efficiency is as high as over 90%.

Negative voltage Vss solution

As mentioned above, if the designer cannot find a suitable negative voltage output power IC temporarily, a Buck power IC can also be used. As shown in Figure 5, a negative voltage Vss is generated by a floating ground structure. The principle is: connect the normal output to the ground of the input power supply, and get a stable negative voltage output from the ground end of this converter. If a different output voltage is required, two resistors can be connected between the output and FB to set the output voltage.

Figure 5. MAX1836/MAX1837 inverting configuration

You can also choose a negative voltage output converter. Figure 6 is a commonly used PWM negative voltage converter. It has few external components, simple design, and a fixed-frequency PWM structure with less interference.

Figure 6. Inverting converter

in conclusion

Although OLED is an emerging technology industry and its specifications are not yet very clear, the power supply architecture remains basically unchanged. iSuppli research shows that driven by the demand for display conversion from monochrome to color, the OLED display market is expected to reach a scale of US$1 billion in 2006.

OLED has the advantages of self-luminescence and ultra-thinness, making it an ideal choice for products such as DSC, handheld game consoles or mobile phones. These products have great development prospects. In short, the technology and market potential

Strength is a key factor in the growth of an industry. In terms of technology, the efforts of domestic OLED manufacturers have been highly recognized. If they can further enhance their brand effect and product positioning, they will surely establish a prominent position in the gradually taking off OLED industry.