OLED power supply more energy efficient design solution

It is the so-called organic light-emitting diode. Its biggest feature is that it is a self-luminous body, so it does not need a backlight (Backlight) and a color filter (Color Filter) and other structures, so it can be thinner than LCD. In addition, the advantages of wider viewing angle, fast response speed, low driving voltage, higher color and contrast than LCD, theoretically lower power consumption and simpler process make OLED the most promising display technology star after LCD. However, OLED also has the disadvantage of shorter lifespan than LCD. This is because OLED is a current-driven self-luminous body, so the lifespan of its materials and components is relatively shortened.

OLED power supply requirements

Generally, the power supply of a small-sized OLED requires a positive voltage (Vdd) output and a negative voltage (Vss) input. The power supply architecture can be divided into two types: digital camera and mobile phone architecture. The power supply specification of a digital camera has a Vdd voltage range of 3V to 6V, and a Vss voltage range of -7V to -10V. The power supply specification of a mobile phone has a Vdd voltage range of approximately 2.5V, and a Vss voltage range of -7V to -10V. The input power supply of these two products is usually a lithium battery, so the voltage range is approximately 3V to 4.2V.

Digital Camera Vdd Solution

Since the Vdd voltage range of digital cameras is 3V to 6V, the Vdd power supply architecture should be a Buck/Boost or Boost architecture. If you cannot find a Buck/Boost architecture power output, you can also use the very common Buck architecture to design a Buck/Boost architecture. Just use a set of ordinary step-down power supply control ICs, plus a MOSFET and an output diode to design 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 with the slope of Vin/L. When Lx is a low voltage, the inductor current decreases with the slope of (Vout+VD)/L. The input and output currents are intermittent, which allows the output voltage to be larger or smaller than the input voltage. Its output voltage is a function of the input voltage and cycle power:

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And the cycle power formula is:

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Figure 1: Buck-boost design using a step-down power supply IC

From the above formula, we can know the relationship between the output voltage, input voltage and cycle. To get a higher or lower output voltage, we only need to control the ratio of 1/1-D. Designers can also directly use a set of Buck/Boost power ICs to generate the required voltage output. Figure 2 shows a set of direct step-up and step-down ICs. It combines a set of boost converters and linear regulators to provide a voltage converter that can step up or step down. This converter provides a stable output voltage for inputs below and above the output voltage. It can have an input range of 1.8V to 11V and a preset output of 3.3V or 5V. It is also possible to use two resistors to divide this output voltage from 1.25V to 5.5V, and its efficiency can be as high as 85%. If the required output voltage is between 3.5V and 4V, a set of step-up and step-down outputs can be generated in a combined manner. Designers only need a set of boost converters and a set of linear regulators, such as the combination of the MAX1606 boost converter and the MAX8512 linear regulator.

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Figure 2 Buck-boost power supply IC

If cost is a consideration, then the Charge-Pump architecture is suitable for low-cost solutions. Its architecture can save an inductor and an output diode. For example, the MAX1759 uses the Charge-Pump method to generate a set of buck-boost output voltages. Maxim's unique Change-Pump architecture allows the input voltage to be higher or lower than the output voltage. Although its operating frequency is higher than 1.5MHz, it still maintains a low quiescent supply current of 50uA.

Some designers choose to use a boost method to generate a set of input voltage higher than the output voltage to improve efficiency because of high efficiency considerations. For example, the boost architecture in Figure 3 requires an external MOSFET as a switch, so it can provide a larger output power. If it is due to space limitations, adding an external MOSFET switch and output diode will become a burden for the designer. At this time, a boost DC-DC converter with a built-in MOSFET switch and output diode, such as the MAX1722, is suitable for this application. It not only saves space and has good performance, but also saves costs.

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Figure 3 Boost power converter

Solution for mobile phone Vdd

Therefore, we choose to provide the required voltage of Vdd in Buck mode. As shown in Figure 4, it is a DC converter with a synchronous buck structure and a built-in MOSFET switch, which can provide an output current of 400mA. Moreover, the operating frequency is as high as 1.2MHz, and the designer can choose a small-sized inductor and output capacitor, and the efficiency is also as high as 90% or more.

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Figure 4 Step-down power converter

Negative voltage Vss solution

After introducing the positive voltage Vdd output of OLED, the negative voltage Vss output of OLED is introduced next. As described in the previous article, if the designer cannot find a suitable negative voltage output power IC temporarily, a Buck architecture power IC can also be used. As shown in Figure 5, the floating ground wire architecture is used to generate negative voltage Vss. The principle is: through the normal output, connected to the supply voltage ground wire, the converter ground wire is forced to stabilize and generate a set of negative voltage outputs. If a different output voltage is required, just connect two resistors across the output capacitor.