High Gray-Scale Driving Methods and Circuits for Microdisplays

Abstract

마이크로 디스플레이는 광학 시스템을 사용한 영상의 확대가 필요한 매우 작은 디스플레이를 말한다. 대부분의 마이크로 디스플레이는 silicon 칩을 기판으로 사용하며, 대표적인 디스플레이인 thin film transistor liquid crystal display (TFT-LCD) 와 같이 active matrix 형태의 전극을 가진다. 따라서 마이크로 디스플레이는 TFT-LCD에서 요구되는 고화질, 저전력, 저가격을 구현하기 위하여 소면적의 정확한 고계조 구현 회로나 방식을 필요로 한다. 또한 마이크로 디스플레이만이 갖고 있는 demux 방식의 데이터 기입이나 적색 녹색 청색의 색상을 순차적으로 표현하는 field sequential color (FSC) 방식으로 인한 데이터 기입 시간의 감소 문제와 매우 작은 디스플레이 패널을 구동함으로 인한 전압과 전류의 dynamic 범위 감소 문제의 어려움이 있다. 이에, 기존의 TFT-LCD에서 사용되던 방식보다 더욱 정확한 고계조 구동 방식과 회로의 개발이 필요하다. 본 논문은 3가지 종류의 초소형 디스플레이 장치의 고계조 표현을 구현하기 위한 구동회로와 구동방법을 제안한다. 차세대 초소형 디스플레이로 가능성이 있는 spatial optical modulator (SOM) 을 위한 구동 회로와 휴대기기용 프로젝터로 적합한 liquid crystal on silicon (LCoS) 의 고속 구동회로와 디지털 구동방식, 그리고 직시형 초소형 디스플레이로 적합한 organic light emitting diode on silicon (OLEDoS) 의 전류 구동회로, 전류 구동방식과 전류 복사 회로를 제안한다. 첫째로, SOM을 이용한 고해상도 TV에 적용하기 위한 10-bit 구동 드라이버 IC를 제안한다. 구동 드라이버 IC의 면적을 줄이기 위하여, 7-bit 저항열 DAC와 연산증폭기의 선형 보간 특성을 이용하여 3-bit 선형 DAC로 구성한 DAC를 제안하였다. 제안한 10-bit DAC는 기존의 8-bit 저항열 DAC 보다 40% 적은 면적으로 구현되었다. 측정결과 INL 과 DNL 모두 0.13 LSB 이하가 되고, 구동 회로의 출력 편차는 1.3 mV 이내로 얻었다. 두 번째로, 고해상도 LCoS 를 구동하기 위한 구동 IC를 개발하였다. RC-delay로 인한 데이터 기입의 문제를 해결하기 위하여 연산증폭기 회로의 mismatch 를 이용한 선 강조 회로를 제안한다. 제안한 구동회로를 측정한 결과 충, 방전의 최대 에러 전압은 각각 6 mV, 8 mV 를 얻었다. 이는 선 강조 방식을 적용하지 않은 회로에 비하여 출력 전압의 에러를 각각 88% 그리고 94% 적은 수치이다. 또한 디지털 화소를 갖는 LCoS 패널을 위한 디지털 구동 방식을 제안한다. 제안한 디지털 구동 방법은 구동 주파수를 증가하지 않고 계조를 높힐수 있는 방법을 보여준다. 제안한 디지털 방식은 8-bit의 계조를 표현하기 위한 구동 주파수가 75.6 MHz 로 기존의 FRC나 PWM에서 요구되는 주파수의 13.7% 의 주파수로 구현이 가능하게 되었다. 마지막으로 본 논문은 OLEDoS를 위한 전류 구동 방법, 전류 버퍼, 그리고 전류 복사 회로를 제안한다. 제안한 전류 구동 방법은 전류 DAC의 전압 변화를 감지하고, 추가적인 전류원으로 OLEDoS 패널을 충, 방전하는 것이다. 충전과 방전의 결정은 전류 DAC의 전압 변화로 결정된다. 이 전류 DAC 출력은 작은 전압 변화를 갖는데 이를 감지하기 위해서 연산증폭기를 이용한 감지회로 또한 제안하였다. 시뮬레이션 결과 제안한 구동 방식은 20 μs 에 10 nA 크기의 전류를 기입이 가능하다는 것을 확인하였다. 고속으로 전류 프로그램밍을 하기 위하여 연산증폭기와 trans-resistance 연산 증폭기로 구성된 전류 버퍼를 제안한다. 에러를 0.5 LSB로 가정하면, 제안한 전류 버퍼는 기입할 최소 전류가 1 nA 일때 5 μs 내에 전류의 기입이 가능함을 보여준다. 이중 피드백 방식을 이용하며 하나의 입력 트랜지스터를 갖는 전류 복사회로를 제안한다. 시뮬레이션 결과 1 μA 의 전류에서 cascode형 전류 복사회로는 5 GΩ 의 출력저항을 갖는데 반하여 제안한 회로는 250 GΩ 의 출력저항을 가져 더 정확한 전류 복사가 가능하였다.|Microdisplays are display devices that are so small that optical magnification is required. Most microdisplays use a silicon backplane as a substrate. They tend to have the same active matrix addressing found in TFT-LCD, meaning that they require the same small-area and high gray-scale driving circuits and methods that TFT-LCDs require. Microdisplays have unique features that are short driving time and a narrow dynamic range. For these reasons, the development of high gray-scale driving methods and circuits for microdisplay should be seen as highly valuable. This dissertation proposes driving circuits and methods to achieve high gray-scale resolution for three types of microdisplays. A driver IC is proposed for spatial optical modulators (SOM) which may be the next generation in microdisplay technology. A driver IC and a digital driving method are proposed for a liquid crystal on silicon (LCoS) formulation that is best suited to the microdisplay of mobile applications. Driving circuits and methods are proposed for organic light emitting diodes on silicon (OLEDoS), which are suitable for direct view microdisplay. A 10-bit driver IC for full high definition television (HDTV) applications using SOM is developed. To reduce the chip area, a digital-to-analog convertor (DAC) structure, including a 7-bit resistor-string DAC and a unity-gain buffer with its own 3-bit linear DAC, is proposed. The area of the proposed DAC is 40% smaller than that of the typical 8-bit resistor-string DAC. The measured output voltages of integral non-linearity (INL) and differential non-linearity (DNL) are less than 0.13 least significant bit (LSB). The output voltage deviation is 1.3 mV in the proposed driver IC. A 10-bit driver IC for LCoS with full HD resolution is also developed. To reduce charging and discharging errors due to RC-delay, we propose pre-emphasis circuits with an intentional mismatch of input transistors. In the measurement, the charging and discharging maximum error voltages are 6 mV and 8 mV, respectively. The error voltages of the proposed driver IC is 88% and 94% smaller than those of the driver IC without pre-emphasis under charging and discharging conditions, respectively. An LCoS panel with a digital pixel structure is developed using a new digital driving method that increases the gray-scale of the image without increasing the driving frequency. The measured results show that the proposed driving method achieves an 8-bit gray-scale at a driving frequency of 75.6 MHz, which is less than 13.7% of the frequency reported in previously described methods, such as frame rate control and pulse width modulation. Finally, this dissertation proposes a current driving method, a current buffer, and a current mirror for OLEDoS. In the proposed driving method, the voltage variation of a current DAC is detected and then an additional current source charges or discharges OLEDoS panel. Either charging or discharging operations are selected through the voltage variation of the current DAC. We propose a detection circuit using a unit gain operational amplifier because the voltage variation of the current DAC is small. Simulation results show that the current reaches a value of 10 nA in 20 μs. The proposed current buffer includes an operational amplifier and a trans-resistance amplifier for high-speed current programming. Simulation results show that the settling time of the proposed current buffer is 5 μs when the programmed current is 1 nA and the charging error is 0.5 LSB. The proposed current mirror uses a double loop feedback and has only one input transistor to increase input range. Simulation results show that the output resistances of the cascode current mirror and the proposed current mirror are 5 GΩ and 250 GΩ, respectively.; Microdisplays are display devices that are so small that optical magnification is required. Most microdisplays use a silicon backplane as a substrate. They tend to have the same active matrix addressing found in TFT-LCD, meaning that they require the same small-area and high gray-scale driving circuits and methods that TFT-LCDs require. Microdisplays have unique features that are short driving time and a narrow dynamic range. For these reasons, the development of high gray-scale driving methods and circuits for microdisplay should be seen as highly valuable. This dissertation proposes driving circuits and methods to achieve high gray-scale resolution for three types of microdisplays. A driver IC is proposed for spatial optical modulators (SOM) which may be the next generation in microdisplay technology. A driver IC and a digital driving method are proposed for a liquid crystal on silicon (LCoS) formulation that is best suited to the microdisplay of mobile applications. Driving circuits and methods are proposed for organic light emitting diodes on silicon (OLEDoS), which are suitable for direct view microdisplay. A 10-bit driver IC for full high definition television (HDTV) applications using SOM is developed. To reduce the chip area, a digital-to-analog convertor (DAC) structure, including a 7-bit resistor-string DAC and a unity-gain buffer with its own 3-bit linear DAC, is proposed. The area of the proposed DAC is 40% smaller than that of the typical 8-bit resistor-string DAC. The measured output voltages of integral non-linearity (INL) and differential non-linearity (DNL) are less than 0.13 least significant bit (LSB). The output voltage deviation is 1.3 mV in the proposed driver IC. A 10-bit driver IC for LCoS with full HD resolution is also developed. To reduce charging and discharging errors due to RC-delay, we propose pre-emphasis circuits with an intentional mismatch of input transistors. In the measurement, the charging and discharging maximum error voltages are 6 mV and 8 mV, respectively. The error voltages of the proposed driver IC is 88% and 94% smaller than those of the driver IC without pre-emphasis under charging and discharging conditions, respectively. An LCoS panel with a digital pixel structure is developed using a new digital driving method that increases the gray-scale of the image without increasing the driving frequency. The measured results show that the proposed driving method achieves an 8-bit gray-scale at a driving frequency of 75.6 MHz, which is less than 13.7% of the frequency reported in previously described methods, such as frame rate control and pulse width modulation. Finally, this dissertation proposes a current driving method, a current buffer, and a current mirror for OLEDoS. In the proposed driving method, the voltage variation of a current DAC is detected and then an additional current source charges or discharges OLEDoS panel. Either charging or discharging operations are selected through the voltage variation of the current DAC. We propose a detection circuit using a unit gain operational amplifier because the voltage variation of the current DAC is small. Simulation results show that the current reaches a value of 10 nA in 20 μs. The proposed current buffer includes an operational amplifier and a trans-resistance amplifier for high-speed current programming. Simulation results show that the settling time of the proposed current buffer is 5 μs when the programmed current is 1 nA and the charging error is 0.5 LSB. The proposed current mirror uses a double loop feedback and has only one input transistor to increase input range. Simulation results show that the output resistances of the cascode current mirror and the proposed current mirror are 5 GΩ and 250 GΩ, respectively.