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Powering OLEDs: the care and feeding of organic displays

Edit: Blaze Display Technology Co.,Ltd      Date: Oct 28, 2015

【R&D Department of Blaze Display】Improving yields and declining manufacturing costs are contributing to a steady ramp in OLED (organic-light-emitting-diode) display usage. In response, several semiconductor manufacturers have begun to offer power-converter ICs for OLED- and LCD-bias supplies that give OEM designers flexibility in how they implement the display power subsystem. Though IC makers have not strictly optimized these power controllers forOLEDs, these devices do help preserve the OLED´s superior energy efficiency and take advantage of the economies of scale that the LCD segment of the display market offers.

Among the first commercial applications for monochrome OLEDs were small, low-resolution, front-panel displays in portable measurement instrumentation and in entertainment devices. Since manufacturing processes have matured, the technology has enjoyed greater commercial success as secondary displays in clam-shell mobile phones.

Color OLEDs first found homes in analog and digital camcorders and in digital cameras as eyepiece viewfinders, in which they fit well with OEMs´trends toward smaller, higher resolution cameras. Since those first color OLEDs, display manufacturers have been improving fabrication processes and display designs to reduce cost, improve performance, and increase robustness.

Unlike LCDs, which behave like voltage-controlled translucent shutters, OLEDs are light emitters and so do not need a backlight. Current OLED displays offer better energy efficiency, image quality, ruggedness, and low-temperature performance than do LCDs. For the time being, they are also more expensive, but continuing yield improvements and market penetration are narrowing the cost difference. Also, as iSuppli Director of Technology and Strategic Research Kimberly Allen points out, "Kodak$$$s original [OLED-technology] patents are beginning to expire." The associated licensing-fee load that OLED vendors have been bearing is likewise expiring. LCD manufacturers have responded, and pricing differences continue to favor their displays, whereas image quality and efficiency go to the OLED displays.

Not all OLED displays are created equal; their power-supply requirements reflect their differences. The two fundamental display structures are passive matrix and active matrix. "The materials are similar...but, because we drive the PMOLED (passive-matrix OLED) with higher current, we have higher voltage drops. So [PMOLEDs require] up to 20V and less than 10V for the AMOLED (active-matrix OLED) because the currents are so much smaller," observes STMicroelectronicsOLED Product Line Manager Joel Roibet. "The currents are in the range of tens or hundreds of microamps per column for the PMOLED display; some tens or even single-digit microamps per column in the AMOLED."

Complicating the power-supply outlook is the fact that "many active-matrix LCD [and] OLED...subsystems in cell phones, digital cameras and other portable devices require positive and negative low-current bias supplies," observes Advanced Analogic Technologies Vice President Jan Nilsson. The AAT3190 from Advanced Analogic Technologies addresses this requirement by generating both positive and negative supplies with one chip using a self-clocking dual-charge-pump architecture that operates at a nominal 1-MHz switching frequency. The charge pump develops adjustable outputs to a maximum of ±25V from a unipolar input supply of 2.7 to 5.5V.

The voltage controller requires no inductors, which often challenge board-height limits in small, portable devices. Instead, each pump drives external diode/capacitor multiplier stages. You can cascade multiplier stages to scale the output voltages. An external resistive divider on each output provides a feedback signal to the converter. The tolerance on the feedback regulation voltage is 50 mV, or about 4% for the positive supply and 100 mV for the negative. The chip´s pump-drive pins can each deliver an absolute maximum 200 mA, and Advanced Analogic Technologies characterizes the part´s efficiency and load regulation to 40 mA. Because the two drive pins pump all of the stages for their given polarity, the load current the multiplier cascade can deliver scales inversely with the number of stages.

The $1.73 (1000) AAT3190 provides soft-start, undervoltage lockout, and a shutdown mode that reduces the converter´s quiescent current from a maximum of 800 µA to no more than 1 µA. Advanced Analogic Technologies offers the AAT3190 in MSOP-8 and TSOP-12 packages.

For unipolar-display applications, Fairchild Semiconductor offers the FAN5331 boost converter. Like the AAT3190, the FAN5331 operates at a high switching rate—in this case, 1.6 MHz, to reduce the size of external reactive components, so, although the boost topology requires an inductor, this converter requires only 10 mH. The small SOT23-5 package and comparatively few external parts help minimize your display´s power supply.

The FAN5331 can deliver a minimum of 35 mA at 15V over its full input range in steady state. With inputs of 3.2V or more, the current capability rises to 50 mA under similar operating conditions. Additionally, the converter´s fractional-ohm output switch can deliver a 1A peak current; a cycle-by-cycle current-limit monitor ensures that the peak output current stays within this limit.

The 50-cent (1000) IC draws 2 µA in shutdown mode. A resistive divider allows you to set the output voltage to between the input voltage and the converter´s 20V maximum. The nominal 1.23V feedback voltage has a tolerance of 25 mV.

Fast clock rates and correspondingly small inductors indicate a trend in boost converters for thin portable devices, whereas traditionally lower clock speeds have called for larger magnetics, which posed layout- and mechanical-design challenges. Despite these and other challenges, small-supply designers have long appreciated the boost converter´s performance advantages. As Linear Technology´s Senior Design Engineer Eddy Wells points out, "A traditional boost converter offers more efficient operation and a greater range of step-up ratios, but boost-[converter supplies] typically take more space, and systems issues, such as inrush current and short-circuit protection, are often solved with added external circuitry."

Linear Technology´s LTC3459 exemplifies a group of boost converters that address these application concerns. The 3459 provides a burst mode that maintains the converter´s efficiency with light load currents and features inrush current limiting, short-circuit protection, and load isolation during shutdown. Despite the feature list, a typical application circuit requires only three capacitors, two resistors, and a boost inductor in the switching circuit.Wells observes that, because "the converter operates with a low peak current of approximately 75 mA, an 0805 miniature inductor...[facilitates] a footprint similar to an [integrated] charge pump."

The SOT23-6 IC can operate on a 1.5 to 5.5V input and provide a 2.5 to 10V output—appropriate for active-matrix OLED displays. Unlike many other low-power converters that operate with a fixed clock frequency, the 3459 uses a variable switching rate that self-adjusts depending on the input-output differential from about 0.6 to more than 2.6 MHz. The feedback reference is 1.22V with a tolerance of 30 mV. The $1.95 (1000) converter requires a maximum 20-µA quiescent current. When the device is in shutdown mode, the residual operating current reduces to less than 1 µA.

Maxim´s MAX8570 is one of a quintet of boost converters for low-current applications, including OLED displays and LCDs. The 8570 features an adjustable output voltage, and Maxim characterizes the converter at load currents as large as 5 mA. They characterize the other members of the family, which include the 8571 and 8573 to 15 mA and the 8574 and 8575 to 25 mA. They offer models with either adjustable or fixed 15V outputs at the two larger currents. The boost controllers operate on input supplies of 2.7 to 5.5V; the adjustable models offer an output range of 3 to 28V and have a reference tolerance of 19 mV. The 857x family of converters provide soft start, undervoltage lockout, and current limiting. An active-low shutdown pin reduces the device$$$s quiescent current from 50 µA to 1 µA. Maxim offers the $1.25 (1000) 857x family in SOT23-6 packages.

Comparing the three boost converters from Maxim, Linear Technology, and Fairchild reveals subtle benefits to both greater and lesser levels of integration, depending on your application and again shows that, even in products as conceptually simple as a boost converter, there´s no such thing as one best approach to the topology. The LTC3459 isolates its output from the boost inductor with an on-chip PMOS device that has a typical channel resistance on the order of 4Ω. As a result, the application circuit does without the Schottky diode common to many boost-converter circuits, and correspondingly reduces layout area and the bill-of-materials and assembly costs, if only incrementally.

The Fairchild FAN5331 and the Maxim MAX8570 and its kin use an external Schottky, which imposes a junction-voltage overhead on the order of 400 mV as well as an incremental forward resistance of several ohms. The advantage of the external Schottky diode, however, is that it brings the switching waveform off chip where you can use it, for example, to charge-pump a coarse negative supply as Maxim points out in one of its application circuits.

The TPS65130 boost controller/inverter from Texas Instruments can develop ±15V output rails from a 2.7 to 5.5V input. The control topology uses a fixed-frequency, 1.25-MHz PWM switching signal. A low-power mode uses pulse skipping to supply light load currents. The TPS65130 can deliver load currents as large as 200 mA, and the converter´s 500-µA quiescent current falls to 1.5 µA in shutdown mode.

The $2.95 (1000) IC is a more "pinny" device than most others, residing as it does in a QFN-24, but the extra connections also provide extra features, such as independent positive- and negative-supply enable inputs that allow you to control supply sequencing. An output to control an external PMOS device can isolate the battery from the boost circuit. In addition to the external PMOS device, however, the application circuit includes two inductors, two Schottky diodes, five resistors, and eight capacitors. These external parts may seem like a lot, particularly in comparison with the lower current single-output boost converters, but the parts count is on the same order as that for the dual charge pump.

The variety of power converters for OLED applications is bound to continue expanding, particularly in current capability and features as OLEDs establish a stronger market position in the overall display-technology mix. "The market for OLED displays is expected to top $1 billion by 2006, with rapid growth driven by a shift from monochrome to color," predicts iSuppli´s Allen. In addition to the shift toward color displays, the clear trend is toward active-matrix OLEDs that can support much larger screens than can passive-matrix displays.

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