Temperature compensated parallel LED drive circuit

• US 6,717,559 B2
• Filed: 08/23/2001
• Issued: 04/06/2004
• Est. Priority Date: 01/16/2001
• Status: Active Grant

• Current Assignee: Visteon Global Technologies Incorporated (Visteon Corporation)
• Original Assignee: Visteon Global Technologies Incorporated (Visteon Corporation)
• Inventors: Weindorf, Paul F. L.
• Primary Examiner(s): Shankar, Vijay
• Assistant Examiner(s): Patel, Nitin
• Application Number:
  US09/939,493
• Publication Number:
  US 20020135572A1
• Time in Patent Office: 957 Days
• Field of Search: 345/102, 345/82, 340/716, 359/487, 315/291
• US Class Current: 345/82
• CPC Class Codes: F21Y 2115/10 Light-emitting diodes [LED]
• CPC Class Codes: G02B 6/0031 Reflecting element, sheet or layer
• CPC Class Codes: G02B 6/0068 Arrangements of plural sources, e.g. multi-colour light sources
• CPC Class Codes: G02B 6/0083 Details of electrical connections of light sources to drivers, circuit boards, or the like
• CPC Class Codes: G02B 6/0085 Means for removing heat created by the light source from the package heat extraction or cooling elements for semiconductor light sources in general H01L33/64
• CPC Class Codes: G02F 1/133615 Edge-illuminating devices, i.e. illuminating from the side
• CPC Class Codes: G09G 2320/041 Temperature compensation
• CPC Class Codes: G09G 2330/06 Handling electromagnetic interferences [EMI], covering emitted as well as received electromagnetic radiation
• CPC Class Codes: G09G 3/3406 Control of illumination source illumination devices structurally associated with liquid crystal cells G02F1/1336
• CPC Class Codes: H05B 45/18 using temperature feedback
• CPC Class Codes: H05B 45/46 having LEDs disposed in parallel lines

Abstract

A light emitting diode drive circuit provides each of the plurality of parallel light emitting diodes substantially uniform current, causing the light emitting diodes to each have substantially the same brightness. An optional control circuit controls the current flow as a function of an external control signal. Optional temperature derating circuits adjust the intensity of the light emitting diodes as a function of a measured temperature. The measured temperature may be provided externally via a temperature output signal. The light emitting diode drive circuit may be specially adapted for automotive applications.

Claims

1. A light emitting diode device, comprising:

a plurality of light emitting diodes each having anode and cathode terminals; and

a current source circuit comprising a plurality of current source transistors;

wherein the light emitting diodes are connected in parallel such that the anode terminal of each light emitting diode is electrically connected with a different current source transistor and the cathode terminal of each light emitting diode is electrically coupled with a ground node; and

wherein a base terminal of each of the current source transistors is connected with a current control node.

2. The device of claim 1, further comprising a control circuit that maintains a voltage at the current control node substantially at a desired voltage level.

3. The device of claim 2, wherein the control circuit is responsive to an input intensity level signal received at an input node, wherein the desired voltage level at the current control node is a function of the input intensity level signal.

4. The device of claim 3, wherein the control circuit comprises a sample current source circuit that provides a reference current that is substantially similar to the current at the anode node of each of the LEDs.

5. The device of claim 4, wherein the sample current source circuit comprises a current source transistor with a base terminal connected with the current control node.

6. The device of claim 5, wherein the control circuit comprises a feedback control circuit connected with the sample current source circuit that maintains the voltage at the current control node at the desired voltage level after a failure of one or more of the plurality of light emitting diodes.

7. The device of claim 6, wherein the intensity level signal comprises a pulse width modulated signal.

8. The device of claim 6, wherein the intensity level signal comprises an analog DC voltage signal.

9. The device of claim 6, further comprising a temperature derating circuit electrically connected with the control circuit, wherein the temperature derating circuit adjusts the input intensity level signal such that the desired voltage level at the current control node is a function of the input intensity level signal and a temperature derating level.

10. The device of claim 9, wherein the temperature derating circuit decreases a brightness of the light emitting diodes when the temperature derating circuit detects a temperature increase above a temperature threshold.

11. The device of claim 9, wherein the temperature derating circuit comprises a substantially linear temperature sensing device connected with the ground node;

wherein the temperature sensing device measures the temperature of solder connecting the cathode terminal of the light emitting diodes to the ground node.

12. The device of claim 11, wherein the linear temperature sensing device comprises a temperature dependant resistor.

13. The device of claim 12, further comprising a temperature monitoring circuit that provides a temperature output signal indicating the temperature of the solder connecting the cathode terminal of the light emitting diodes to the ground node.

14. The device of claim 13, wherein the light emitting diode control circuit is a band limited low electromagnetic interference circuit.

15. The device of claim 14 wherein the light emitting diode device operates with a supply voltage of less than ten volts.

16. The device of claim 14, wherein the light emitting diode device operates with a supply voltage of substantially five volts.

17. The device of claim 14, wherein the control circuit is tolerant of malfunctioning light emitting diodes.

18. The device of claim 17, wherein malfunction comprises a short in a plurality of the light emitting diodes.

19. The device of claim 17, wherein malfunction comprises an open in a plurality of the light emitting diodes.

20. The device of claim 1, wherein the plurality of light emitting diodes are adapted to provide back lighting for an active matrix liquid crystal display.

21. The device of claim 1, wherein the plurality of light emitting diodes comprise a plurality of colored light emitting diodes.

22. The device of claim 1, wherein the plurality of light emitting diodes comprise a plurality of white light emitting diodes.

23. The device of claim 22, wherein the plurality of light emitting diodes comprises at least five white light emitting diodes.

24. A display unit adapted for an automotive application, comprising:

a liquid crystal display;

a backlighting light emitting diode array comprising a plurality of light emitting diodes in a parallel configuration, where a cathode terminal of each light emitting diode is connected with a common ground node, where the backlighting light emitting diode array backlights the liquid crystal display;

a current source circuit that maintains substantially identical amounts of electrical current supplied to each light emitting diode;

a temperature derating circuit electrically connected with the current source circuit;

wherein the temperature derating circuit reduces the light emitting diode array'"'"'s intensity as a function of a temperature measured at the common ground node after a threshold is reached.

25. The display unit of claim 24 further comprising:

a light pipe in proximity to the liquid crystal display and the light emitting diode array and a housing encasing at least a portion of the liquid crystal display the light pipe and the light emitting diode array.

26. The display unit of claim 25, further comprising:

a light emitting diode controller that provides a pulse width modulated signal that controls the intensity of the light emitting diode array.

27. The display unit of claim 25, further comprising:

a light emitting diode controller that provides an analog direct current voltage signal that controls the intensity of the light emitting diode array.

28. The display unit of claim 25, wherein the light emitting diode controller comprises a microprocessor.

29. A method of controlling a parallel light emitting diode array, comprising:

monitoring a temperature of a light emitting diode array at a node connected with a light emitting diode;

providing a temperature adjusted signal to the light emitting diode;

monitoring a sample current source; and

adjusting the temperature adjusted signal as a function of a current from the sample current source.

30. A display unit, comprising:

a liquid crystal display;

a light pipe located behind the liquid crystal display;

a light emitting diode array;

an specular reflector that reflects light from the light emitting diode into the light pipe;

a circuit board having first and second sides, where the light emitting diode array connected with the first side and a light emitting diode array controller circuit connected to the other side, wherein the circuit board is within a light cavity of the display unit;

a frame housing the liquid crystal display, light pipe, light emitting diode array, and the circuit board, wherein the circuit board is thermally connected with the frame.

31. The display unit of claim 30 wherein the specular reflector is angled at substantially 45 degrees to reflect light from the light emitting diode array into the light pipe.

32. The display unit of claim 31, wherein the circuit board conducts heat from the light emitting diode array and the light emitting diode array controller circuit toward the frame.

33. The display unit of claim 32, wherein the circuit board is thermally connected with the frame in a plurality of locations.

34. The display unit of claim 33, further comprising a diffuser between the light pipe and the liquid crystal display.

35. The display unit of claim 34, further comprising a reflective polarizer between the diffuser and the liquid crystal display.

36. The display unit of claim 35, wherein the light emitting diode array comprises a plurality of side LEDs.

37. A light emitting diode device, comprising:

at least five white light emitting diodes each having anode and cathode terminals; and

a current source circuit comprising a plurality of current source transistors;

wherein the at least five white light emitting diodes are connected in parallel such that the anode terminal of each light emitting diode is electrically connected with a different current source transistor and the cathode terminal of each light emitting diode is electrically coupled with a ground node.

38. A display unit adapted for an automotive application, comprising:

a liquid crystal display;

a backlighting light emitting diode array comprising a plurality of light emitting diodes in a parallel configuration, where a cathode terminal of each light emitting diode is connected with a common ground node, where the backlighting light emitting diode array backlights the liquid crystal display;

a current source circuit that maintains substantially identical amounts of electrical current supplied to each light emitting diode;

a temperature derating circuit electrically connected with the current source circuit, wherein the temperature derating circuit reduces the light emitting diode array'"'"'s intensity as a function of a temperature measured at the common ground node after a threshold is reached;

a light pipe in proximity to the liquid crystal display and the light emitting diode array;

a housing encasing at least a portion of the liquid crystal display, the light pipe, and the light emitting diode array; and

a light emitting diode controller that provides a pulse width modulated signal that controls the intensity of the light emitting diode array.

39. A display unit adapted for an automotive application, comprising:

a liquid crystal display;

a backlighting light emitting diode array comprising a plurality of light emitting diodes in a parallel configuration, where a cathode terminal of each light emitting diode is connected with a common ground node, where the backlighting light emitting diode array backlights the liquid crystal display;

a current source circuit that maintains substantially identical amounts of electrical current supplied to each light emitting diode;

a temperature derating circuit electrically connected with the current source circuit, wherein the temperature derating circuit reduces the light emitting diode array'"'"'s intensity as a function of a temperature measured at the common ground node after a threshold is reached;

a light pipe in proximity to the liquid crystal display and the light emitting diode array;

a housing encasing at least a portion of the liquid crystal display, the light pipe, and the light emitting diode array; and

a light emitting diode controller that provides an analog direct current voltage signal that controls the intensity of the light emitting diode array.

40. The display unit of claim 39, wherein the light emitting diode controller comprises a microprocessor.