Optical and temperature feedbacks to control display brightness

• US 7,391,172 B2
• Filed: 02/26/2007
• Issued: 06/24/2008
• Est. Priority Date: 09/23/2003
• Status: Active Grant

• Current Assignee: Polaris PowerLED Technologies, LLC (Microchip Technology Incorporated)
• Original Assignee: Microsemi Corporation (Microchip Technology Incorporated)
• Inventors: Holliday, Roger, Henry, George C., Ferguson, Bruce R.
• Primary Examiner(s): TRAN, THUY V
• Application Number:
  US11/679,046
• Publication Number:
  US 20070132398A1
• Time in Patent Office: 484 Days
• Field of Search: 315/149, 315/150, 315/156, 315/158, 315/291, 315/307, 315/308, 315/309, 345/102
• US Class Current: 315/308
• CPC Class Codes: G09G 2320/041 Temperature compensation
• CPC Class Codes: G09G 2320/062 Adjustment of illumination source parameters
• CPC Class Codes: G09G 3/3406 Control of illumination source illumination devices structurally associated with liquid crystal cells G02F1/1336
• CPC Class Codes: H05B 41/2856 against internal abnormal circuit conditions
• CPC Class Codes: H05B 41/2858 for protecting the lamp against abnormal operating conditions
• CPC Class Codes: H05B 41/386 for speeding-up the lighting-up
• CPC Class Codes: H05B 41/3922 and measurement of the incident light

Abstract

An illumination control circuit allows a user to set a desired brightness level and maintains the desired brightness level over temperature and life of a light source. The illumination control circuit uses a dual feedback loop with both optical and thermal feedbacks. The optical feedback loop controls power to the light source during normal operations. The thermal feedback loop overrides the optical feedback loop when the temperature of the light source becomes excessive.

Claims

1. An illumination control circuit comprising:

a first optical sensor configured to detect visible light produced by a light source and to generate a first optical sensor output;

an error amplifier configured to generate a control signal based on a comparison of the first optical sensor output to a reference level;

a second optical sensor configured to detect ambient light and to generate a second optical sensor output; and

an inverter controller configured to generate driving signals to control power to the light source, wherein the inverter controller operates in a boost mode to power the light source using a boosted AC current of a substantially constant level when the control signal from the error amplifier indicates that the first optical sensor output is less than the reference level, operates in a normal mode to power the light source using a nominal AC current that has a lower level than the boosted AC current when the control signal indicates that the first optical sensor output is greater than the reference level, and further adjusts power to the light source in response to a change in the second optical sensor output indicating a change in ambient light conditions.

2. The illumination control circuit of claim 1, wherein the light source provides backlight for a liquid crystal display and the second optical sensor is placed in front of the liquid crystal display.

3. The illumination control circuit of claim 1, wherein the error amplifier is an integrating amplifier and the control signal is a substantially DC control voltage that sets the level of a substantially AC current for the light source.

4. The illumination control circuit of claim 1, wherein the reference level corresponds to a desired brightness level of the light source and is variable by a user.

5. The illumination control circuit of claim 1, wherein the level of the boosted AC current is approximately 150% of the level of an initial nominal AC current.

6. The illumination control circuit of claim 1, wherein the first optical sensor comprises a first PIN diode array that outputs a first current source and a first current sink with respective current levels that vary with detected visible light from the light source while the second optical sensor comprises a second PIN diode array that outputs a second current source and a second current sink with respective current levels that vary with sensed ambient light.

7. The illumination control circuit of claim 6, further comprising a low pass filter or a gain amplifier coupled to one of the current sources or one of the current sinks to generate the first and the second optical sensor outputs.

8. The illumination control circuit of claim 1, wherein the light source is a light emitting diode, a cold cathode fluorescent lamp, a hot cathode fluorescent lamp, a Zenon lamp, or a metal halide lamp.

9. The illumination control circuit of claim 1, wherein the first optical sensor output is provided to an inverting input of the error amplifier and the reference level is provided to a non-inverting input of the error amplifier.

10. The illumination control circuit of claim 9, further comprising a low pass filter at an output of the error amplifier.

11. The illumination control circuit of claim 9, further comprising a pull-up resistor coupled between an output of the error amplifier and a pull-up control voltage corresponding to a predetermined maximum AC current for the light source.

12. A method to improve response speed of a light source, the method comprising the steps of:

sensing light produced by the light source with a first visible light sensor;

comparing an output of the first visible light sensor to a predetermined threshold level;

providing a substantially constant boost current to the light source when the output of the first visible light sensor is less than the predetermined threshold level;

providing a preset nominal current to the light source when the output of the first visible light sensor is approximately equal to or greater than the predetermined threshold level, wherein the preset nominal current has a lower average level than the boost current;

sensing ambient light with a second visible light sensor; and

further adiusting power to the light source in response to changes in an output of the second visible light sensor.

13. The method of claim 12, wherein the substantially constant boost current is adjustable to vary the response speed of the light source.

14. The method of claim 12, wherein at least one of the first and the second visible light sensors is substantially immune to infrared light.

15. The method of claim 12, wherein the substantially constant boost current has a level that is at least 1.5 times higher than the level of the preset nominal current.

16. A liquid crystal display monitor comprising:

at least one visible light detector located proximate to one or more backlight lamps to monitor the intensity of the backlight lamps;

an inverter that monitors an output of the visible light detector and provides power to illuminate the backlight lamps, wherein the inverter operates in a boost mode to provide a boosted current to the backlight lamps when the output of the visible light detector is less than a threshold level and operates in a normal mode to provide a nominal current that has a lower level than the boosted current to the backlight lamps when the output of the visible light detector is greater than a threshold level; and

an additional visible light detector located in a corner of the liquid crystal display monitor for monitoring ambient light, wherein said nominal current is adjusted responsive to said additional visible light detector.

17. The liquid crystal display monitor of claim 16, wherein each of the visible light detectors comprises a PIN diode array configured to generate complementary current outputs.

18. The liquid crystal display monitor of claim 16, wherein the inverter decreases brightness of the backlight lamps when an output of the additional visible light detector indicates a relatively dark environment and increases brightness of the backlight lamps when the output of the additional visible light detector indicates a relatively bright environment.

19. The liquid crystal display monitor of claim 16, further comprising embedded stereo speakers and a class-D audio amplifier.

20. The liquid crystal display monitor of claim 16, wherein the backlight lamps comprise a plurality of cold cathode fluorescent lamps.