Optical and temperature feedbacks to control display brightness

• US 20050088102A1
• Filed: 09/09/2004
• Published: 04/28/2005
• 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: Henry, George C., Ferguson, Bruce R., Holliday, Roger
• Granted Patent: US 7,183,727 B2
• Time in Patent Office: Days
• US Class Current: 315/149
• 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:

an optical sensor configured to detect visible light produced by a light source;

a thermal sensor configured to indicate temperature of the light source;

a first feedback loop configured to generate a brightness control signal based on comparing an output of the optical sensor to a predefined brightness setting; and

a second feedback loop configured to override the first feedback loop when the thermal sensor indicates that the temperature of the light source is above a predefined temperature limit.

2. The illumination control circuit of claim 1, further comprising a control circuit configured to receive the brightness control signal and to adjust current conducted by the light source.

3. The illumination control circuit of claim 1, wherein the optical sensor is mounted in the back of a liquid crystal display monitor to detect visible light produced by a cold cathode fluorescent lamp.

4. The illumination control circuit of claim 1, wherein the first feedback loop adjusts current provided to the light source to substantially equalize the output of the optical sensor and the predefined brightness setting, and the second feedback loop reduces the current to the light source when the temperature of the light source is above the predefined temperature limit.

5. The illumination control circuit of claim 1, wherein the predefined brightness setting is variable by a user.

6. The illumination control circuit of claim 1, wherein the first feedback loop comprises:

a gain amplifier coupled to the output of the optical sensor; and

a first error amplifier with an inverting input coupled to an output of the gain amplifier and a non-inverting input configured to receive the predefined brightness setting.

7. The illumination control circuit of claim 6, wherein the second feedback loop comprises:

a second error amplifier with an inverting input coupled to an output of the thermal sensor and a non-inverting input configured to receive the predefined temperature limit; and

a pair of OR-ing diodes coupled between the respective error amplifiers and a common node for the brightness control signal.

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 2, wherein the control circuit operates the light source at a boosted current level during warm-up.

10. A backlight system comprising:

a high voltage circuit configured to generate a substantially AC voltage signal to produce a substantially AC lamp current through a fluorescent lamp for lighting a display panel;

a controller configured to drive the high voltage circuit in response to a brightness control input; and

a dual feedback loop configured to generate the brightness control input based on lamp brightness and lamp temperature.

11. The backlight system of claim 10, wherein the controller drives the high voltage circuit to produce a relatively high predetermined lamp current when the lamp temperature is relatively low.

12. The backlight system of claim 10, wherein the lamp illuminates a television display, a handheld device, a computer notebook screen, a computer monitor, or an automotive display.

13. A method to control brightness of a lamp and prolong lamp life, the method comprising the acts of:

detecting visible light produced by the lamp;

comparing the detected visible light level to a desired brightness level;

generating a first control signal to adjust a lamp current based on the comparison of the detected visible light level to the desired brightness level;

detecting operating temperature of the lamp;

comparing the detected operating temperature to a selected limit; and

overriding the first control signal to reduce the lamp current when the detected operating temperature is above the selected limit.

14. 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 visible light sensor;

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

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

15. The method of claim 14, wherein the substantially constant boost current is adjustable to vary the response speed of the light source.

16. The method of claim 14, further comprising the step of providing a preset nominal current to the light source when the output of the light sensor is approximately equal to the predetermined level.

17. A method to operate a lamp in boost mode, the method comprising the steps:

sensing temperature of the lamp;

comparing the sensed temperature to a predetermined threshold; and

providing a substantially constant boost current to the lamp when the sensed temperature is less than the predetermined threshold.

18. The method of claim 17, wherein the lamp is driven by an inverter and the temperature of the lamp is sensed indirectly by monitoring the temperature of a component in the inverter.

19. The method of claim 18, wherein the component is a switching transistor or a transformer.

20. An illumination control circuit comprising:

means for monitoring brightness of a light source;

means for monitoring temperature of the light source;

means for adjusting power to the light source to achieve a predefined brightness level using an optical feedback loop; and

means for transferring control to a thermal feedback loop to reduce power to the light source if the temperature of the light source is greater than a predefined threshold.