WO2008006756A1 - Circuit for controlling light emitting diodes - Google Patents

Circuit for controlling light emitting diodes Download PDF

Info

Publication number
WO2008006756A1
WO2008006756A1 PCT/EP2007/056733 EP2007056733W WO2008006756A1 WO 2008006756 A1 WO2008006756 A1 WO 2008006756A1 EP 2007056733 W EP2007056733 W EP 2007056733W WO 2008006756 A1 WO2008006756 A1 WO 2008006756A1
Authority
WO
WIPO (PCT)
Prior art keywords
current
light emitting
emitting diode
source
transistor
Prior art date
Application number
PCT/EP2007/056733
Other languages
French (fr)
Inventor
Gunther Haas
Heinrich Schemmann
Philippe Le Roy
Original Assignee
Thomson Licensing
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Thomson Licensing filed Critical Thomson Licensing
Publication of WO2008006756A1 publication Critical patent/WO2008006756A1/en

Links

Classifications

    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/22—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources
    • G09G3/30—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels
    • G09G3/32—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED]
    • G09G3/3208—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED]
    • G09G3/3225—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using an active matrix
    • G09G3/3233—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using an active matrix with pixel circuitry controlling the current through the light-emitting element
    • G09G3/3241—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using an active matrix with pixel circuitry controlling the current through the light-emitting element the current through the light-emitting element being set using a data current provided by the data driver, e.g. by using a two-transistor current mirror
    • G09G3/325—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using an active matrix with pixel circuitry controlling the current through the light-emitting element the current through the light-emitting element being set using a data current provided by the data driver, e.g. by using a two-transistor current mirror the data current flowing through the driving transistor during a setting phase, e.g. by using a switch for connecting the driving transistor to the data driver
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/2007—Display of intermediate tones
    • G09G3/2014—Display of intermediate tones by modulation of the duration of a single pulse during which the logic level remains constant
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2300/00—Aspects of the constitution of display devices
    • G09G2300/08—Active matrix structure, i.e. with use of active elements, inclusive of non-linear two terminal elements, in the pixels together with light emitting or modulating elements
    • G09G2300/0809—Several active elements per pixel in active matrix panels
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2300/00—Aspects of the constitution of display devices
    • G09G2300/08—Active matrix structure, i.e. with use of active elements, inclusive of non-linear two terminal elements, in the pixels together with light emitting or modulating elements
    • G09G2300/0809—Several active elements per pixel in active matrix panels
    • G09G2300/0842—Several active elements per pixel in active matrix panels forming a memory circuit, e.g. a dynamic memory with one capacitor
    • G09G2300/0852—Several active elements per pixel in active matrix panels forming a memory circuit, e.g. a dynamic memory with one capacitor being a dynamic memory with more than one capacitor
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2300/00—Aspects of the constitution of display devices
    • G09G2300/08—Active matrix structure, i.e. with use of active elements, inclusive of non-linear two terminal elements, in the pixels together with light emitting or modulating elements
    • G09G2300/0809—Several active elements per pixel in active matrix panels
    • G09G2300/0842—Several active elements per pixel in active matrix panels forming a memory circuit, e.g. a dynamic memory with one capacitor
    • G09G2300/0861—Several active elements per pixel in active matrix panels forming a memory circuit, e.g. a dynamic memory with one capacitor with additional control of the display period without amending the charge stored in a pixel memory, e.g. by means of additional select electrodes
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2310/00—Command of the display device
    • G09G2310/02—Addressing, scanning or driving the display screen or processing steps related thereto
    • G09G2310/0243—Details of the generation of driving signals
    • G09G2310/0259—Details of the generation of driving signals with use of an analog or digital ramp generator in the column driver or in the pixel circuit
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00—Control of display operating conditions
    • G09G2320/02—Improving the quality of display appearance
    • G09G2320/0233—Improving the luminance or brightness uniformity across the screen

Definitions

  • the present invention relates to a circuit for controlling light emitting diodes, in particular organic light emitting diodes (OLED).
  • the circuit comprises a current source that is adapted to provide an individual predetermined current for each associated light emitting diode.
  • Organic light emitting diodes form part of displays, which are commonly used in flat-panel displays such as video headsets, portable computers, and digital cameras. They have a superior optical performance and lower power consumption in comparison with competing display technologies.
  • OLED technology is based on organic chemical compounds that emit light when an electric current flows through the device.
  • the OLEDS are arranged in an array and constitute the pixels of the micro- displays.
  • Each OLED is a light emitting device, wherefore a light source is not required opposed to conventional liquid crystal displays.
  • a CMOS integrated circuit is coupled to each OLED in the array.
  • the CMOS integrated circuit controls the power to each OLED and controls the light emitted by each pixel at very high speed.
  • a colour display may be provided by using OLEDs emitting white light and appropriate colour filters, or by using OLEDs of respective primary colours.
  • OLED displays use less power and provide brighter and more stylish pictures than liquid crystal displays.
  • the light intensity emitted by each OLED is proportional to the current density flowing through the diode. Light emission begins at diode forward voltages of about 2.5 to 2.8 V and continuously increases with the bias voltage.
  • the currents needed for controlling the brightness of the OLED, in particular in a microdisplay range from hundreds of picoamperes (pA) to tens of nanoamperes (nA), depending on pixel size and required luminance.
  • the luminance of an OLED is proportional to its current density. Therefore, current drivers are preferably used for controlling the brightness of the display.
  • a field effect transistor (FET) may be used as current source in order to control the current density and the respective brightness of the OLED.
  • the source electrode of the field effect transistor constituting the current source is connected to the OLED.
  • the gate of the field effect transistor controls the source-drain-current, which is provided to the diode.
  • the output current characteristic of a field effect transistor is conventionally divided into distinct regions, wherein each region displays a different current vs. voltage characteristic.
  • the first region is called the sub threshold region, because the gate source voltage Vg is below a threshold voltage Vth. If the gate voltage is below the threshold voltage, the transistor is turned off and ideally there is no current from the drain to the source of the transistor.
  • the remaining output current of the FET is in the region of Pico amperes (pA) and nano amperes (nA).
  • the second region is called the saturation region, in which the current vs. voltage characteristic is essentially linear and high output currents are achieved.
  • Traditional FET current sources operating in the saturation region are not compatible with the small currents required for OLED displays. Therefore, the field effect transistor must be operated in the sub threshold region.
  • the drain current in the sub threshold region can described by the following equation:
  • IDS IDO(W/L)e (Vgs - vtO)/( ⁇ kT/q) (1 )
  • VtO is the sub threshold voltage
  • I DO is the drain current for sub threshold voltage normalized to W/L
  • W is the width of the gate
  • L is the length of the gate ⁇ is the sub threshold slope ranging from 1.4 to 1.6
  • the output current of the FET operated as current source depends exponentially on the input gate-source voltage Vg, and the resulting brightness of the OLED is very sensitive to fluctuations in the gate-source voltage of the FET. Furthermore, each OLED used in a display should render the same luminance, when operated with the identical input voltage.
  • the matching of the threshold voltage VtO is a function of the area W * L.
  • the standard deviation of the threshold voltage is given by A V to/(W * L) 1/2 .
  • a V to is approximately equal to
  • the drain current l D s may change by as much as 65%.
  • the current driver comprises a current source FET, which is operated in the sub threshold region.
  • a capacitor is used as a storage device, which holds the pixel data for the OLED.
  • Several transistors are configured as minimum sized switches in order to connect/disconnect the current source FET and storage capacitor to/from the OLED and a programming line.
  • the current source transistor is separated from the OLED during calibration.
  • a fixed voltage offset is applied from the capacitor to the gate of the current source transistor.
  • the offset is equal to ⁇ kT/(q * ln100). Therefore, the drain current is increased by 100 times.
  • the sampled program current is set 100 times greater than the desired pixel current.
  • the respective gate voltage is established and then shifted back by the fixed voltage offset.
  • the calibration procedure does address the problem of differing output currents of a current source FET due to process variations, the use of such current sources still poses daunting problems.
  • the brightness of the OLED and consequently the input current is supposed to be changed according to input control signals. This could be done by increasing or decreasing the voltage provided by the capacitor, which is connected to the gate of the current source FET. Due to the exponential relationship between the gate voltage and the output current, slight changes in the voltage lead to very strong changes in the output current. Capacitor leakage currents can significantly change the drain current of the current source FET. Therefore, it is not desirable to add/subtract charge carriers to the capacitor. Recalibrating the current drive each time a new output current needs to be supplied does not represent a feasible solution, since the access time to the OLED would be considerably reduced.
  • EP 1 455 335 A1 discloses an OLED display device in which a constant current is provided to each OLED.
  • the constant current applied to the OLED for a time period within a frame period, which time period is selected to achieve a perceived average brightness during the frame period in accordance with a video signal for the respective OLED.
  • the circuit for controlling light emitting diodes comprises a current source, which is adapted to provide an individually controllable predetermined current for each associated light emitting diode.
  • the circuit further comprises a modulation means adapted to modulate the predetermined current in such a way that one or plural consecutive current pulses are output by the modulation means to the light emitting diode. In this way, the brightness of the current drive is adjusted without having to recalibrate the current source. The impact of capacitor leakage currents in current source FETs operated in the sub threshold region is reduced.
  • the inventive circuit advantageously allows for adjusting a uniform brightness distribution across the whole display for identical video levels applied to the circuit by correspondingly setting the individual predetermined currents for the current sources.
  • the video signals used for controlling individual light emitting diodes no longer need to be equalised for brightness uniformity prior to applying them to the light emitting diodes. Rather, the video signals may now correspond to the nominal levels as provided by a video source. Control of the brightness may for example be effected using pulse width modulation schemes.
  • the invention also advantageously allows for a balanced driving. Balanced driving may be applied in situations when pulses applied to a pulse width modulator must be very short in order to achieve a desired low brightness level.
  • the constant current supplied to the light emitting diodes could be set to be lower than otherwise required, allowing for a longer pulse to be applied to the pulse width modulator.
  • the current required to achieve a desired brightness level would become too low to be appropriately set in the current source a somewhat higher current could be set and a low perceived brightness could be achieved by accordingly driving the modulator using very short pulses.
  • Known methods of control of brightness are based on the assumption that a predetermined constant output current must be supplied to the organic light emitting diode.
  • the size of the constant output current should relate to the brightness of the display.
  • the same brightness may also be provided by current pulses. If the pulses reoccur fast enough, the human eye integrates the pulses into a perceived constant brightness level and a change in the brightness is imperceptible. Consequently, the organic light emitting diode appears to be just as bright as an OLED driven with a constant current of a corresponding magnitude. Furthermore, the output voltage of the current source may be increased.
  • the perceptible average brightness due to pulse modulated input currents is lower than the brightness provided by driving the OLED with a constant current equal to the peak current of the modulation pulse.
  • the modulation means has a modulation signal input for receiving a signal, which is to be modulated onto the predetermined current.
  • the modulation means may comprise a first modulation transistor having gate, source and drain electrodes.
  • the gate electrode of the modulation transistor is connected to the modulation signal input.
  • the source electrode is connected to the current source.
  • the drain electrode is connected to the light emitting diode.
  • the first modulation transistor represents a controllable resistor. By controlling the gate-source voltage, the electrical resistance for the current flowing from the current source to the OLED may be adjusted. Therefore, the current may be adjusted.
  • the modulation signal is a voltage signal fed to the gate electrode of the first modulation transistor.
  • the modulation means comprises a second transistor having gate, source and drain electrodes.
  • the source electrode of the second transistor is connected to the current source.
  • the first and second transistors constitute a differential amplifier.
  • a voltage difference between the first transistor gate electrode and the second transistor gate electrode determines the output current of the modulation means applied to the light emitting diode.
  • the current from the current source is directed through the first or second transistor depending on the gate voltages applied to the respective gate electrodes. If the source-drain channel of the first transistor is closed due to the gate voltage, then the output current of the current source is directed through the second transistor.
  • the brightness of an organic light emitting diode may be controlled using the current source driver.
  • the first step is to calibrate the current source used for providing a current to the light emitting diode to a predetermined calibration current.
  • the output current from the current source may be set to a predetermined level. Variations in the electrical properties of the current source may be compensated using the calibration step.
  • the output current from the current source is modulated in order to provide one or plural consecutive current pulses. These current pulses are output by the modulation means to the light emitting diode. Consequently, the radiation intensity of the OLED depends on the alternating current made up of a plurality of current pulses. If the frequency of the current pulses is sufficiently high variations in the radiation intensity are imperceptible to the human eye. The brightness appears to be constant. This is in particular the case, if the pulse frequency is well above 60 Hz.
  • the brightness of the organic light emitting diode is preferably adjusted by adjusting width and/or magnitude of the consecutive current pulses.
  • the waveform may also take into account nonlinear characteristics of the light emitting diode. If the brightness of the OLED does not depend in a linear fashion on the diode current, then an accordingly selected waveform of the current pulses may compensate for the non-linear dependence.
  • the consecutive current pulses preferably constitute a ramp or saw tooth waveform having a predetermined inclination.
  • the brightness of the organic light emitting diode is adjusted by adjusting the inclination of the ramp or saw tooth waveform.
  • the modulation means is adapted to at least partly divert the current away from the light emitting diode in response to the modulation signal. This advantageously allows for essentially maintaining a constant current from the current source.
  • Fig. 1 shows a schematic diagram of a circuit for controlling light emitting diodes according to the preferred embodiment of the present invention, which is connected to an organic light emitting diode.
  • Fig. 2 shows a diagram of a ramp wave used in the circuit of Fig. 1
  • the circuit shown in Fig. 1 comprises two main functional components, namely a current source 10 and a current modulator 20.
  • the current source 10 outputs a predetermined constant current to the modulator 10 via line 50.
  • a modulated current is input to an OLED 30 via line 60.
  • the waveform of the output current running through line 60 is determined by the signal input to the current modulator 20 via line "Ramp".
  • Current source 10 comprises a current source transistor P1 , which is operated in the sub threshold region.
  • the drain current of the current source transistor P1 is the output current of the current source 10.
  • the magnitude of the current source output current is determined by the voltage applied to the source, drain and gate electrodes of current source transistor P1.
  • the source voltage is defined by supply voltage Vdd in Fig. 1.
  • Capacitor CO is connected to the gate electrode of the current source transistor P1.
  • the charge stored in capacitor CO defines the gate-source voltage of the current source transistor.
  • Two further transistors, P2 and P3, are provided in the current source 10 of Fig. 1.
  • Transistors P2 and P3 are switches, which either connect or disconnect the drain electrode of the current source transistor P1 to line "Data".
  • the output current on line 50 from the current source is set to a predetermined level. This is achieved by finding the appropriate gate voltage for providing the desired output current.
  • the gate voltage is adjusted for a high drain current and then shifted to the corresponding calibration voltage for the desired low drain current. This is accomplished by applying a shifting signal to the capacitor CO via line Vcap. Thereafter the switch transistors P3 and P2 are closed. Consequently, the gate of current source transistor P1 is connected to the line designated "Data" in Fig. 1. The gate voltage is adjusted until the desired high current is flowing through the current source transistor. Then, switches P3 and P2 are opened for storing the charge on capacitor CO.
  • the gate voltage of the current source transistor P1 is shifted again by accordingly applying the shifting signal to line Vcap.
  • the resulting drain current is determined by the exponential function given in equation 1.
  • Modulation means 20 in Fig. 1 includes transistors P4 and P5. Both transistors in Fig. 1 are preferably matched or identical, i.e. they provide essentially the same drain current, if the same gate, source and drain voltages are applied to them.
  • the source electrodes of both transistors are connected to line 50 carrying the output current from current source 10.
  • the constant output current from the current source 10 is equal to the sum of the currents flowing through the source of transistors P4 and P5 of modulation means 20. Therefore, the amount of current depends essentially on the electrical resistance of transistors P4 and P5. The resistance may be controlled by the gate-source voltages applied to transistors P4 and P5.
  • Capacitor C1 provides a constant voltage corresponding to the desired brightness or video signal level to the gate electrode of transistor P5 during operation. It may be connected and disconnected using switch transistor P6 from line data in order to store charge carriers in accordance with the desired video signal.
  • the drain electrode of transistor P5 is connected to the opposite side of capacitor C1 , which is connected to line GND, which is connected to ground voltage.
  • Line “Ramp” is connected to the gate electrode of transistor P4. A voltage signal is applied in operation to line Ramp. The voltage difference between line “Ramp” and capacitor C1 determines the current flowing through OLED 30. The peak current on line 60 is equal to the current on line 50 supplied by current source 10 and otherwise determined by the ratio of the gate voltages applied to the gate electrodes of transistors P4 and P5. Therefore, the modulator circuit 20 effectively scales down the current from current source 10.
  • Fig. 2 schematically shows the ramp signal, which may be applied to the line "Ramp" in for controlling the current flowing to the OLED 30.
  • the abscissa designates the time in seconds and the axis of ordinate represents the Ramp voltage.
  • the signal is essentially periodic having a period T.
  • the waveform during a single period resembles a ramp.
  • the slope of the ramp is given by angle ⁇ and the peak voltage is represented by V p .
  • Different modifications of the signal may be used for changing the brightness of the OLED.
  • the peak voltage V p may be increased.
  • the brightness tends to increase.
  • the period T may be changed.
  • the slope of the ramp signal may be adjusted by increasing or decreasing the angle ⁇ .
  • the waveform used as a modulation signal may also be a double slope ramp or any arbitrary signal.
  • the resulting brightness largely depends on the average voltage that is applied as signal.
  • the average voltage is represented by the area under the signal pulse divided by the period T.
  • the drive circuit and method described in the foregoing is particularly suitable for an OLED microdisplay, or for an OLED on IC device.
  • the drive circuitry is produced on a silicon substrate like known from common ICs, and the active OLED layer is applied thereon.

Landscapes

  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Computer Hardware Design (AREA)
  • General Physics & Mathematics (AREA)
  • Theoretical Computer Science (AREA)
  • Electroluminescent Light Sources (AREA)
  • Control Of Indicators Other Than Cathode Ray Tubes (AREA)

Abstract

The present invention relates to a circuit for controlling light emitting diodes, in particular organic light emitting diodes. The circuit according to the present invention comprises a current source. The current source is adapted to provide a predetermined current that can be individually set for a respective associated light emitting diode. The current drive further comprises modulation means adapted to modulate the predetermined current in such a way that one or plural consecutive current pulses are output by the modulation means to the light emitting diode.

Description

Circuit for controlling light emitting diodes
The present invention relates to a circuit for controlling light emitting diodes, in particular organic light emitting diodes (OLED). The circuit comprises a current source that is adapted to provide an individual predetermined current for each associated light emitting diode.
Organic light emitting diodes form part of displays, which are commonly used in flat-panel displays such as video headsets, portable computers, and digital cameras. They have a superior optical performance and lower power consumption in comparison with competing display technologies.
OLED technology is based on organic chemical compounds that emit light when an electric current flows through the device. Using OLED-on-Silicon technology, the OLEDS are arranged in an array and constitute the pixels of the micro- displays. Each OLED is a light emitting device, wherefore a light source is not required opposed to conventional liquid crystal displays. A CMOS integrated circuit is coupled to each OLED in the array. The CMOS integrated circuit controls the power to each OLED and controls the light emitted by each pixel at very high speed. A colour display may be provided by using OLEDs emitting white light and appropriate colour filters, or by using OLEDs of respective primary colours. OLED displays use less power and provide brighter and more colourful pictures than liquid crystal displays.
The light intensity emitted by each OLED is proportional to the current density flowing through the diode. Light emission begins at diode forward voltages of about 2.5 to 2.8 V and continuously increases with the bias voltage. The currents needed for controlling the brightness of the OLED, in particular in a microdisplay, range from hundreds of picoamperes (pA) to tens of nanoamperes (nA), depending on pixel size and required luminance. The luminance of an OLED is proportional to its current density. Therefore, current drivers are preferably used for controlling the brightness of the display. A field effect transistor (FET) may be used as current source in order to control the current density and the respective brightness of the OLED. In one of multiple alternative embodiments the source electrode of the field effect transistor constituting the current source is connected to the OLED. The gate of the field effect transistor controls the source-drain-current, which is provided to the diode. However, the required currents are extremely small, in particular for microdisplays. The output current characteristic of a field effect transistor is conventionally divided into distinct regions, wherein each region displays a different current vs. voltage characteristic. The first region is called the sub threshold region, because the gate source voltage Vg is below a threshold voltage Vth. If the gate voltage is below the threshold voltage, the transistor is turned off and ideally there is no current from the drain to the source of the transistor. However, in reality the remaining output current of the FET is in the region of Pico amperes (pA) and nano amperes (nA). The second region is called the saturation region, in which the current vs. voltage characteristic is essentially linear and high output currents are achieved. Traditional FET current sources operating in the saturation region are not compatible with the small currents required for OLED displays. Therefore, the field effect transistor must be operated in the sub threshold region.
The drain current in the sub threshold region can described by the following equation:
IDS=IDO(W/L)e(Vgs-vtO)/(ηkT/q) (1 )
VtO is the sub threshold voltage
I DO is the drain current for sub threshold voltage normalized to W/L
W is the width of the gate
L is the length of the gate η is the sub threshold slope ranging from 1.4 to 1.6
Hence, the output current of the FET operated as current source depends exponentially on the input gate-source voltage Vg, and the resulting brightness of the OLED is very sensitive to fluctuations in the gate-source voltage of the FET. Furthermore, each OLED used in a display should render the same luminance, when operated with the identical input voltage. However, the matching of the threshold voltage VtO is a function of the area W*L. The standard deviation of the threshold voltage is given by AVto/(W*L)1/2. AVto is approximately equal to
10 mV*μm. If the transistor is sized with a width W equal to 6 μm and length L equal to 400 nm, then the resulting voltage offset of 6.5 mV. In return, the drain current lDs may change by as much as 65%.
In order to address the problem of variations for drive current FETs, it has been proposed to calibrate the FETs in a display device. Thereby, differences in the output drain current IDS may be compensated.
The publication "An 852*600 Pixel OLED-on-Silicon Colour Micro display using CMOS Sub threshold-Voltage-Scaling Current Drivers", Gary B. Levy et. Al, IEEE Journal of Solid-State Circuits, Vol. 37, No. 12, December 2002 discloses a current driver, which is calibrated. The current driver comprises a current source FET, which is operated in the sub threshold region. A capacitor is used as a storage device, which holds the pixel data for the OLED. Several transistors are configured as minimum sized switches in order to connect/disconnect the current source FET and storage capacitor to/from the OLED and a programming line. The current source transistor is separated from the OLED during calibration. A fixed voltage offset is applied from the capacitor to the gate of the current source transistor. The offset is equal to ηkT/(q*ln100). Therefore, the drain current is increased by 100 times. The sampled program current is set 100 times greater than the desired pixel current. The respective gate voltage is established and then shifted back by the fixed voltage offset.
Although the calibration procedure does address the problem of differing output currents of a current source FET due to process variations, the use of such current sources still poses formidable problems. In particular, the brightness of the OLED and consequently the input current is supposed to be changed according to input control signals. This could be done by increasing or decreasing the voltage provided by the capacitor, which is connected to the gate of the current source FET. Due to the exponential relationship between the gate voltage and the output current, slight changes in the voltage lead to very strong changes in the output current. Capacitor leakage currents can significantly change the drain current of the current source FET. Therefore, it is not desirable to add/subtract charge carriers to the capacitor. Recalibrating the current drive each time a new output current needs to be supplied does not represent a feasible solution, since the access time to the OLED would be considerably reduced.
EP 1 455 335 A1 discloses an OLED display device in which a constant current is provided to each OLED. The constant current applied to the OLED for a time period within a frame period, which time period is selected to achieve a perceived average brightness during the frame period in accordance with a video signal for the respective OLED.
It is an object of the present invention to provide an improved circuit for controlling light emitting diodes in a display, which circuit is suited for providing ultra small currents in the region of picoamperes (pA) and nanoamperes (nA) in order to adjust the brightness of the light emitting diodes.
This object is achieved by the circuit for controlling light emitting diodes, in particular organic light emitting diodes, according to claim 1. The circuit comprises a current source, which is adapted to provide an individually controllable predetermined current for each associated light emitting diode. The circuit further comprises a modulation means adapted to modulate the predetermined current in such a way that one or plural consecutive current pulses are output by the modulation means to the light emitting diode. In this way, the brightness of the current drive is adjusted without having to recalibrate the current source. The impact of capacitor leakage currents in current source FETs operated in the sub threshold region is reduced. The inventive circuit advantageously allows for adjusting a uniform brightness distribution across the whole display for identical video levels applied to the circuit by correspondingly setting the individual predetermined currents for the current sources. In other words, the video signals used for controlling individual light emitting diodes no longer need to be equalised for brightness uniformity prior to applying them to the light emitting diodes. Rather, the video signals may now correspond to the nominal levels as provided by a video source. Control of the brightness may for example be effected using pulse width modulation schemes. The invention also advantageously allows for a balanced driving. Balanced driving may be applied in situations when pulses applied to a pulse width modulator must be very short in order to achieve a desired low brightness level. In this case the constant current supplied to the light emitting diodes could be set to be lower than otherwise required, allowing for a longer pulse to be applied to the pulse width modulator. On the other hand, if the current required to achieve a desired brightness level would become too low to be appropriately set in the current source a somewhat higher current could be set and a low perceived brightness could be achieved by accordingly driving the modulator using very short pulses.
Known methods of control of brightness are based on the assumption that a predetermined constant output current must be supplied to the organic light emitting diode. The size of the constant output current should relate to the brightness of the display. However, the same brightness may also be provided by current pulses. If the pulses reoccur fast enough, the human eye integrates the pulses into a perceived constant brightness level and a change in the brightness is imperceptible. Consequently, the organic light emitting diode appears to be just as bright as an OLED driven with a constant current of a corresponding magnitude. Furthermore, the output voltage of the current source may be increased. The perceptible average brightness due to pulse modulated input currents is lower than the brightness provided by driving the OLED with a constant current equal to the peak current of the modulation pulse.
Preferably, the modulation means has a modulation signal input for receiving a signal, which is to be modulated onto the predetermined current. Thereby, any convenient waveform may be chosen for modulating the current input to the OLED. The modulation means may comprise a first modulation transistor having gate, source and drain electrodes. The gate electrode of the modulation transistor is connected to the modulation signal input. The source electrode is connected to the current source. The drain electrode is connected to the light emitting diode. The first modulation transistor represents a controllable resistor. By controlling the gate-source voltage, the electrical resistance for the current flowing from the current source to the OLED may be adjusted. Therefore, the current may be adjusted. The modulation signal is a voltage signal fed to the gate electrode of the first modulation transistor.
Preferably, the modulation means comprises a second transistor having gate, source and drain electrodes. The source electrode of the second transistor is connected to the current source. The first and second transistors constitute a differential amplifier. A voltage difference between the first transistor gate electrode and the second transistor gate electrode determines the output current of the modulation means applied to the light emitting diode. The current from the current source is directed through the first or second transistor depending on the gate voltages applied to the respective gate electrodes. If the source-drain channel of the first transistor is closed due to the gate voltage, then the output current of the current source is directed through the second transistor.
The brightness of an organic light emitting diode may be controlled using the current source driver. The first step is to calibrate the current source used for providing a current to the light emitting diode to a predetermined calibration current. Thereby, the output current from the current source may be set to a predetermined level. Variations in the electrical properties of the current source may be compensated using the calibration step. The output current from the current source is modulated in order to provide one or plural consecutive current pulses. These current pulses are output by the modulation means to the light emitting diode. Consequently, the radiation intensity of the OLED depends on the alternating current made up of a plurality of current pulses. If the frequency of the current pulses is sufficiently high variations in the radiation intensity are imperceptible to the human eye. The brightness appears to be constant. This is in particular the case, if the pulse frequency is well above 60 Hz.
The brightness of the organic light emitting diode is preferably adjusted by adjusting width and/or magnitude of the consecutive current pulses. The waveform may also take into account nonlinear characteristics of the light emitting diode. If the brightness of the OLED does not depend in a linear fashion on the diode current, then an accordingly selected waveform of the current pulses may compensate for the non-linear dependence.
The consecutive current pulses preferably constitute a ramp or saw tooth waveform having a predetermined inclination. The brightness of the organic light emitting diode is adjusted by adjusting the inclination of the ramp or saw tooth waveform.
In one embodiment of the inventive circuit and driving method the modulation means is adapted to at least partly divert the current away from the light emitting diode in response to the modulation signal. This advantageously allows for essentially maintaining a constant current from the current source.
A preferred embodiment of the present invention is described with reference to the accompanied drawings. The preferred embodiment merely exemplifies the invention. Plural possible modifications are apparent to the skilled person. The gist and scope of the present invention is defined in the appended claims of the present application.
Fig. 1 shows a schematic diagram of a circuit for controlling light emitting diodes according to the preferred embodiment of the present invention, which is connected to an organic light emitting diode.
Fig. 2 shows a diagram of a ramp wave used in the circuit of Fig. 1 The circuit shown in Fig. 1 comprises two main functional components, namely a current source 10 and a current modulator 20. The current source 10 outputs a predetermined constant current to the modulator 10 via line 50. A modulated current is input to an OLED 30 via line 60. The waveform of the output current running through line 60 is determined by the signal input to the current modulator 20 via line "Ramp".
Current source 10 comprises a current source transistor P1 , which is operated in the sub threshold region. The drain current of the current source transistor P1 is the output current of the current source 10. The magnitude of the current source output current is determined by the voltage applied to the source, drain and gate electrodes of current source transistor P1. The source voltage is defined by supply voltage Vdd in Fig. 1. Capacitor CO is connected to the gate electrode of the current source transistor P1. The charge stored in capacitor CO defines the gate-source voltage of the current source transistor. Two further transistors, P2 and P3, are provided in the current source 10 of Fig. 1. Transistors P2 and P3 are switches, which either connect or disconnect the drain electrode of the current source transistor P1 to line "Data".
Prior to the use of the current source 10, the output current on line 50 from the current source is set to a predetermined level. This is achieved by finding the appropriate gate voltage for providing the desired output current. In order to define the correct sub threshold voltage, the gate voltage is adjusted for a high drain current and then shifted to the corresponding calibration voltage for the desired low drain current. This is accomplished by applying a shifting signal to the capacitor CO via line Vcap. Thereafter the switch transistors P3 and P2 are closed. Consequently, the gate of current source transistor P1 is connected to the line designated "Data" in Fig. 1. The gate voltage is adjusted until the desired high current is flowing through the current source transistor. Then, switches P3 and P2 are opened for storing the charge on capacitor CO. Finally, the gate voltage of the current source transistor P1 is shifted again by accordingly applying the shifting signal to line Vcap. The resulting drain current is determined by the exponential function given in equation 1. Once the output voltage of the current source has been established, the current source is left unchanged during the operation of the OLED 30. Switch transistors P2 and P3 make sure, that the output current is not altered due to leakage currents from capacitor CO. The actual brightness of the OLED is controlled by modulation circuit 20. If the OLED 30 forms part of an array of OLEDs, then the brightness of the OLED corresponds to the gray or colour value of the corresponding picture element.
Modulation means 20 in Fig. 1 includes transistors P4 and P5. Both transistors in Fig. 1 are preferably matched or identical, i.e. they provide essentially the same drain current, if the same gate, source and drain voltages are applied to them. The source electrodes of both transistors are connected to line 50 carrying the output current from current source 10. The constant output current from the current source 10 is equal to the sum of the currents flowing through the source of transistors P4 and P5 of modulation means 20. Therefore, the amount of current depends essentially on the electrical resistance of transistors P4 and P5. The resistance may be controlled by the gate-source voltages applied to transistors P4 and P5. If the gate-source voltage of transistor P4 is smaller than the gate-source voltage of transistor P5, then the majority of the current from the current source 10 will flow through transistor P5 to ground. Capacitor C1 provides a constant voltage corresponding to the desired brightness or video signal level to the gate electrode of transistor P5 during operation. It may be connected and disconnected using switch transistor P6 from line data in order to store charge carriers in accordance with the desired video signal. The drain electrode of transistor P5 is connected to the opposite side of capacitor C1 , which is connected to line GND, which is connected to ground voltage.
Line "Ramp" is connected to the gate electrode of transistor P4. A voltage signal is applied in operation to line Ramp. The voltage difference between line "Ramp" and capacitor C1 determines the current flowing through OLED 30. The peak current on line 60 is equal to the current on line 50 supplied by current source 10 and otherwise determined by the ratio of the gate voltages applied to the gate electrodes of transistors P4 and P5. Therefore, the modulator circuit 20 effectively scales down the current from current source 10.
Fig. 2 schematically shows the ramp signal, which may be applied to the line "Ramp" in for controlling the current flowing to the OLED 30. In Fig. 2 the abscissa designates the time in seconds and the axis of ordinate represents the Ramp voltage. The signal is essentially periodic having a period T.
The waveform during a single period resembles a ramp. The slope of the ramp is given by angle α and the peak voltage is represented by Vp. Different modifications of the signal may be used for changing the brightness of the OLED.
In particular the peak voltage Vp may be increased. Thereby, the brightness tends to increase. The period T may be changed. Preferably, the resulting Signal frequency f=1/T is chosen in such a way that the resulting brightness flicker is imperceptible. This is the case, if the frequency is well above 60 Hz. Finally, the slope of the ramp signal may be adjusted by increasing or decreasing the angle α.
It is apparent to the skilled person that other waveforms than in Fig. 2 may be chosen. In particular, the waveform used as a modulation signal may also be a double slope ramp or any arbitrary signal. The resulting brightness largely depends on the average voltage that is applied as signal. The average voltage is represented by the area under the signal pulse divided by the period T.
The drive circuit and method described in the foregoing is particularly suitable for an OLED microdisplay, or for an OLED on IC device. In an OLED on IC device, the drive circuitry is produced on a silicon substrate like known from common ICs, and the active OLED layer is applied thereon.

Claims

Claims
1. A circuit for controlling individual light emitting diodes (30) arranged in a matrix and forming a display comprising: - a current source (10) associated with each individual light emitting diode
(30), and
- modulation means (20) associated with each individual light emitting diode (30) and arranged in series connection with the current source (10), said modulation means being adapted to receive a predetermined current from the current source (10) and modulate the predetermined current in such a way that one or plural consecutive current pulses are output by the modulation means (20) to the light emitting diode (30), characterised in that
- the current source (10) associated with each individual light emitting diode (30) is a controllable current source adapted to provide an individually programmable current.
2. The circuit according to claim 1 , wherein said modulation means (20) has a modulation signal input (Ramp) for receiving a signal by means of which the predetermined current is modulated.
3. The circuit according to claim 1 or 2, wherein the modulation means (20) is adapted to at least partly divert the current away from the light emitting diode in response to the modulation signal (Ramp).
4. The circuit according to any one of claims 1 to 3, wherein the modulation means (20) comprises a first modulation transistor (P4) having gate, source and drain electrodes, wherein the gate electrode of the modulation transistor (P4) is connected to the modulation signal input (Ramp), the source electrode is connected to the current source (10) and the drain electrode is connected to the light emitting diode (30).
5. The circuit according to claim 4, wherein the modulation means (20) comprises a second transistor (P4) having gate, source and drain electrodes, wherein the source electrode of the second transistor is connected to the current source (10), wherein the first and second transistors (P4, P5) constitute a differential amplifier, wherein a voltage difference between the first transistor gate and the second transistor gate controls the output current of the modulation means.
6. Light emitting display comprising a plurality of light emitting diodes arranged in a matrix and circuits for controlling individual light emitting diodes (30) according to one of the claims 1 to 5.
7. Method for controlling the brightness of light emitting diodes (30) arranged in a matrix and forming a display comprising the steps of - individually programming current sources associated with respective light emitting diodes and providing a current to the respective associated light emitting diode, and
- modulating the predetermined current in such a way that one or plural consecutive current pulses are output by the modulation means to the light emitting diode.
8. The method of claim 7, wherein the brightness of the light emitting diode is controlled by controlling width and/or magnitude of the consecutive current pulses.
9. The method of claim 7 or 8, wherein the one or plural consecutive current pulses constitute a ramp or saw tooth waveform having an inclination, and wherein the brightness of the light emitting diode is adjusted by adjusting the inclination of the ramp or saw tooth waveform.
10. The method of any one of claims 7 to 9, wherein the modulating step includes at least partly diverting the predetermined current away from the light emitting diode.
PCT/EP2007/056733 2006-07-10 2007-07-04 Circuit for controlling light emitting diodes WO2008006756A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
EP06300785A EP1879170A1 (en) 2006-07-10 2006-07-10 Current drive for light emitting diodes
EP06300785.0 2006-07-10

Publications (1)

Publication Number Publication Date
WO2008006756A1 true WO2008006756A1 (en) 2008-01-17

Family

ID=37057335

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/EP2007/056733 WO2008006756A1 (en) 2006-07-10 2007-07-04 Circuit for controlling light emitting diodes

Country Status (2)

Country Link
EP (1) EP1879170A1 (en)
WO (1) WO2008006756A1 (en)

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN101593488B (en) * 2008-05-30 2012-07-18 索尼株式会社 Electronic circuit and panel having the same
WO2019130138A1 (en) * 2017-12-25 2019-07-04 株式会社半導体エネルギー研究所 Display and electronic apparatus comprising display
US11450709B2 (en) 2019-11-12 2022-09-20 Semiconductor Energy Laboratory Co., Ltd. Functional panel, display device, input/output device, and data processing device
US11610877B2 (en) 2019-11-21 2023-03-21 Semiconductor Energy Laboratory Co., Ltd. Functional panel, display device, input/output device, and data processing device
US11908850B2 (en) 2018-09-05 2024-02-20 Semiconductor Energy Laboratory Co., Ltd. Display device, display module, electronic device, and method for manufacturing display device

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN110033731B (en) * 2018-04-18 2020-09-25 友达光电股份有限公司 Composite driving display panel

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20030142048A1 (en) * 2002-01-31 2003-07-31 Shigeyuki Nishitani Display device employing current-driven type light-emitting elements and method of driving same
EP1455335A1 (en) * 2001-12-14 2004-09-08 Sanyo Electric Co., Ltd. Digitally driven type display device

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1455335A1 (en) * 2001-12-14 2004-09-08 Sanyo Electric Co., Ltd. Digitally driven type display device
US20030142048A1 (en) * 2002-01-31 2003-07-31 Shigeyuki Nishitani Display device employing current-driven type light-emitting elements and method of driving same

Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN101593488B (en) * 2008-05-30 2012-07-18 索尼株式会社 Electronic circuit and panel having the same
WO2019130138A1 (en) * 2017-12-25 2019-07-04 株式会社半導体エネルギー研究所 Display and electronic apparatus comprising display
JPWO2019130138A1 (en) * 2017-12-25 2021-01-14 株式会社半導体エネルギー研究所 A display and an electronic device equipped with the display
US11222583B2 (en) 2017-12-25 2022-01-11 Semiconductor Energy Laboratory Co., Ltd. Display and electronic device including the display
US11783757B2 (en) 2017-12-25 2023-10-10 Semiconductor Energy Laboratory Co., Ltd. Display and electronic device including the display
JP7394627B2 (en) 2017-12-25 2023-12-08 株式会社半導体エネルギー研究所 Displays and electronic devices equipped with such displays
US11908850B2 (en) 2018-09-05 2024-02-20 Semiconductor Energy Laboratory Co., Ltd. Display device, display module, electronic device, and method for manufacturing display device
US11450709B2 (en) 2019-11-12 2022-09-20 Semiconductor Energy Laboratory Co., Ltd. Functional panel, display device, input/output device, and data processing device
US11742379B2 (en) 2019-11-12 2023-08-29 Semiconductor Energy Laboratory Co., Ltd. Functional panel, display device, input/output device, and data processing device
US11610877B2 (en) 2019-11-21 2023-03-21 Semiconductor Energy Laboratory Co., Ltd. Functional panel, display device, input/output device, and data processing device

Also Published As

Publication number Publication date
EP1879170A1 (en) 2008-01-16

Similar Documents

Publication Publication Date Title
US11257426B2 (en) Electronic devices with low refresh rate display pixels
US10089927B2 (en) Active matrix organic light emitting diode display
JP5218879B2 (en) Electroluminescence display device and driving method thereof
EP1132882B1 (en) Active driving circuit for display panel
US6335713B1 (en) Drive apparatus which detects spatial charge voltage on charge storage light-emitting device and controls voltage and current based on the detection while drive current is blocked
US7477248B2 (en) Display device, electric device comprising such a display device and method for driving a display device
US20050190177A1 (en) Current drive circuit and display device using same, pixel circuit, and drive method
US20070057874A1 (en) Display device and control circuit for a light modulator
EP2239723B1 (en) Pixel circuit and display device
US7285797B2 (en) Image display apparatus without occurence of nonuniform display
US7812349B2 (en) Display apparatus
WO2008006756A1 (en) Circuit for controlling light emitting diodes
US20040246202A1 (en) Display element and display device
GB2364593A (en) Pixel driver for an organic electroluminescent device
JP5153331B2 (en) Active matrix image display device and control method thereof
US20080117196A1 (en) Display device and driving method thereof
KR101072757B1 (en) Driving Circuit of Passive Matrix Organic Electroluminescent Display Device
KR100692862B1 (en) Electro-Luminescence Display Device And Driving Method Thereof
KR100675318B1 (en) Driving Circuit For Electro Luminescence Panel
KR100606411B1 (en) Electro-Luminescence Display Device And Driving Method Thereof

Legal Events

Date Code Title Description
121 Ep: the epo has been informed by wipo that ep was designated in this application

Ref document number: 07787047

Country of ref document: EP

Kind code of ref document: A1

NENP Non-entry into the national phase

Ref country code: DE

NENP Non-entry into the national phase

Ref country code: RU

122 Ep: pct application non-entry in european phase

Ref document number: 07787047

Country of ref document: EP

Kind code of ref document: A1