Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B45/00—Circuit arrangements for operating light-emitting diodes [LED]
  • H05B45/40—Details of LED load circuits
  • H05B45/44—Details of LED load circuits with an active control inside an LED matrix
  • H05B45/46—Details of LED load circuits with an active control inside an LED matrix having LEDs disposed in parallel lines
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B45/00—Circuit arrangements for operating light-emitting diodes [LED]
  • H05B45/50—Circuit arrangements for operating light-emitting diodes [LED] responsive to malfunctions or undesirable behaviour of LEDs; responsive to LED life; Protective circuits
  • H05B45/56—Circuit arrangements for operating light-emitting diodes [LED] responsive to malfunctions or undesirable behaviour of LEDs; responsive to LED life; Protective circuits involving measures to prevent abnormal temperature of the LEDs
• • G—PHYSICS
  • G05—CONTROLLING; REGULATING
  • G05F—SYSTEMS FOR REGULATING ELECTRIC OR MAGNETIC VARIABLES
  • G05F1/00—Automatic systems in which deviations of an electric quantity from one or more predetermined values are detected at the output of the system and fed back to a device within the system to restore the detected quantity to its predetermined value or values, i.e. retroactive systems
  • G05F1/10—Regulating voltage or current 
  • G05F1/46—Regulating voltage or current  wherein the variable actually regulated by the final control device is DC
  • G05F1/56—Regulating voltage or current  wherein the variable actually regulated by the final control device is DC using semiconductor devices in series with the load as final control devices
  • G05F1/577—Regulating voltage or current  wherein the variable actually regulated by the final control device is DC using semiconductor devices in series with the load as final control devices for plural loads
• • G—PHYSICS
  • G05—CONTROLLING; REGULATING
  • G05F—SYSTEMS FOR REGULATING ELECTRIC OR MAGNETIC VARIABLES
  • G05F3/00—Non-retroactive systems for regulating electric variables by using an uncontrolled element, or an uncontrolled combination of elements, such element or such combination having self-regulating properties
  • G05F3/02—Regulating voltage or current
  • G05F3/08—Regulating voltage or current wherein the variable is DC
  • G05F3/10—Regulating voltage or current wherein the variable is DC using uncontrolled devices with non-linear characteristics
  • G05F3/16—Regulating voltage or current wherein the variable is DC using uncontrolled devices with non-linear characteristics being semiconductor devices
  • G05F3/20—Regulating voltage or current wherein the variable is DC using uncontrolled devices with non-linear characteristics being semiconductor devices using diode- transistor combinations
  • G05F3/26—Current mirrors
  • G05F3/262—Current mirrors using field-effect transistors only
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S362/00—Illumination
  • Y10S362/80—Light emitting diode

Definitions

• the present invention relates to the field of light emitting diode drive circuits .
• Light emitting diodes are used in many applications to light displays or buttons, such as on cell phones, pagers, computers, etc. In such applications, it is desirable to have a constant illumination when using one or multiple diodes.
• the brightness of a light emitting diode depends upon the current flowing through the diode. Diodes are conventionally biased through a series resistor from a regulated voltage supply. The problem associated with this technique is that the amount of current through the diode depends significantly upon the forward voltage drop of the diode, which varies with size, process, temperature and aging.
• the present invention is directed to methods and apparatus for providing a predetermined current to such diodes in spite of these variations.
• the method comprises providing a predetermined current through a first transistor, and mirroring the current through the first transistor to at least one additional transistor while holding the voltage across the first transistor to a predetermined value, wherein each additional transistor is coupled in series with a light emitting diode.
• Figure 1 is a diagram of an exemplary embodiment of the present invention
• Figure 2 is a diagram showing first exemplary application of the present invention.
• Figure 3 is a diagram showing second exemplary application of the present invention.
• Figure 4 is a diagram showing third exemplary application of the present invention.
• Figure 5 is a diagram showing fourth exemplary application of the present invention.
• the present invention current source methods and apparatus for light emitting diodes is ideally suited for use in battery operated systems, particularly those using white light emitting diodes used for lighting buttons, displays and the like on hand held, laptop and similar devices. Consequently for purposes of illustration and not for purposes of limitation, the exemplary embodiments of the invention are described in a manner consistent with such use, though clearly the invention is not so limited.
• Figure 1 a diagram of an exemplary embodiment of the present invention may be seen. The embodiment shown is intended for fabrication in integrated circuit form for driving up to three light emitting diodes LED1, LED2 and LED3.
• the integrated circuit includes an enable (EN) pin, a SET pin, LED connection pins 0UT1, OUT2 and OUT3 and an integrated circuit ground GND.
• the integrated circuit itself requires very little power, and accordingly, power for the circuit is provided by the enable signal on the enable pin EN.
• the enable signal on the enable pin EN goes high (typically to the system battery voltage or approaching the system battery voltage)
• the reference voltage generator Vref provides a reference voltage to an amplifier Al, in the specific exemplary embodiment shown, a 1.25 volt reference voltage.
• an undervoltage lockout circuit UVLO will detect the undervoltage condition and disable the voltage reference generator Vref.
• the positive input to amplifier Al is taken from the drain of transistor Ql which is connected to an external control voltage Vctrl through a user selectable external resistor Rset .
• Amplifier Al therefore, drives the voltage on the gate of transistor Ql to a level such that the drain voltage of transistor Ql equals the output of the reference voltage generator Vref, in the specific exemplary embodiment being described, 1.25 volts.
• the effect of the amplifier is to hold the voltage across transistor Ql, source to drain, to a predetermined voltage, 1.25 volts in the exemplary embodiment.
• This is to be compared to simply using a current mirror to mirror a current through a resistor coupled to an external regulated voltage to the transistors coupled to the LEDs. With such a current mirror alone, the resistor current and thus the LED current, as well as the LED drive transistor headroom, would be highly dependent on the threshold of the transistors, which has a substantial processing variation.
• Vctrl - Vref drain current of transistor Ql will be equal to .
• control voltage Vctrl may use an existing regulated power supply elsewhere in the system in which the present invention is being used.
• the output of amplifier Al is also used to drive the gates of transistors Q2 , Q3 and Q4.
• These transistors are preferably substantially larger than transistor Ql, transistor Ql only being used to establish the gate voltage for transistors Q2 , Q3 and Q4, while these latter transistors are each sized to conduct the LED current of the LED connected thereto.
• transistors Q2, Q3 and Q4 are approximately 200 times the size of transistor Ql .
• the current through transistor Ql is mirrored to each of transistors Q2, Q3 and Q4 in a ratio of 200:1, at least for those devices to which an LED is connected.
• the circuit of Figure 1 further includes a thermal shutdown circuit of a type well known in the prior art, which will shutdown the circuit to turn off transistors Q2 through Q4 in the event the circuit is subject to an excessive temperature, internally generated or otherwise.
• the integrated circuit When the voltage on the enable pin EN is low, the integrated circuit is disabled and transistors Ql through Q4 are turned off, providing a high impedance on the terminals SET, OUT1, OUT2 and OUT3 , allowing any external circuitry connected thereto to pull the respective pin high without power dissipation.
• the LEDs are supplied and turned on and off by a regulated charge pump, such as the MAX682/3/4, manufactured by Maxim Integrated Products, Inc., the assignee of the present invention.
• a regulated charge pump such as the MAX682/3/4, manufactured by Maxim Integrated Products, Inc., the assignee of the present invention.
• the preferred embodiments of the present invention are used with white LEDs. Due to the typically high forward conduction voltage of white LEDs, the charge pump is used to provide enough voltage headroom such that the LEDs will maintain constant brightness for any battery voltage. When the charge pump is on, the LEDs are on, and when the charge pump is off, the LEDs are off.
• the charge pump's regulated output may also be used as the control voltage Vctrl .
• the use of the MAX682/3/4, which is a 3.3 volt input to a regulated 5 volt output charge pump, is exemplary only, and presented only as an illustration of one application of the present invention. Such an application is shown in Figure 2.
• the MAX682/3/4 would also be supplying other circuits in the system.
• Preferably at least a 4 volt to 5.5 volt regulated supply is used in order to provide enough voltage headroom such that the LEDs will maintain constant brightness for any battery voltage.
• the LEDs may be supplied directly from a battery, such as either a single lithium ion cell or three NiMH (nickel metal halide) cells, as shown in Figure 4. Due to the typically high forward voltage of white LEDs, the LED brightness may slightly dim at the end of battery life. However, the present invention's current regulated ' architecture and low dropout will greatly minimize this effect compared to using a simple ballast resistor for each LED.
• a regulated supply typically already existing in the system in which the present invention is used, such as from an LDO, is used in this application to provide the control voltage Vctrl .
• FIG. 5 illustrates a further alternate application of an exemplary embodiment of the present invention wherein a voltage output digital-to-analog converter (DAC) is used to provide the control voltage Vctrl such that the LED brightness may be factory calibrated, or dynamically adjusted in the field.
• DAC digital-to-analog converter
• a current output DAC may be used in order to eliminate the current setting resistor Rset.
• the LEDs may be supplied directly from the battery (single lithium ion or three NiMH cells) or from a regulated supply. As shown, the LEDs may be turned on and off using the enable pin EN rather than using the DAC in order to minimize supply current and leakage for longer battery life.

Landscapes

• Led Devices (AREA)
• Control Of Electrical Variables (AREA)
• Circuit Arrangement For Electric Light Sources In General (AREA)

Priority Applications (1)

Applications Claiming Priority (2)

Publications (2)

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Family Applications (1)

Country Status (3)

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• 2001
  • 2001-03-30 US US09/822,620 patent/US6538394B2/en not_active Expired - Lifetime
• 2002
  • 2002-03-19 JP JP2002578586A patent/JP2004524704A/ja active Pending
  • 2002-03-19 WO PCT/US2002/008474 patent/WO2002080288A2/en not_active Ceased

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