METHOD AND APPARATUS TO CONTROL DISPLAY BRIGHTNESS WITH AMBIENT LIGHT CORRECTION

• US 20090091560A1
• Filed: 12/17/2008
• Published: 04/09/2009
• Est. Priority Date: 02/09/2004
• Status: Active Grant

• Current Assignee: Polaris PowerLED Technologies, LLC (Microchip Technology Incorporated)
• Original Assignee: Microsemi Corporation (Microchip Technology Incorporated)
• Inventors: FERGUSON, BRUCE R.
• Granted Patent: US 8,223,117 B2
• Time in Patent Office: Days
• US Class Current: 345/207
• CPC Class Codes: G09G 2300/0456 with a reflective area and a transmissive area combined in one pixel, such as in transflectance pixels
• CPC Class Codes: G09G 2320/0606 Manual adjustment
• CPC Class Codes: G09G 2320/0626 for control of overall brightness
• CPC Class Codes: G09G 2360/144 the light being ambient light
• CPC Class Codes: G09G 3/22 using controlled light sources
• CPC Class Codes: G09G 3/3406 Control of illumination source illumination devices structurally associated with liquid crystal cells G02F1/1336
• CPC Class Codes: G09G 3/36 using liquid crystals

Abstract

An ambient light sensor produces a current signal that varies linearly with the level of ambient light. The current signal is multiplied by a user dimming preference to generate a brightness control signal that automatically compensates for ambient light variations in visual information display systems. The multiplying function provides noticeable user dimming control at relatively high ambient light levels.

Claims

1. A visual information display system with ambient light correction comprising:

a visible light sensor configured to output a sensor current signal in proportion to the level of ambient light;

a dimming control input determined by a user;

a multiplier circuit configured to generate a brightness control signal based on a product of the sensor current signal and the dimming control input; and

a display driver configured to adjust brightness levels of one or more light sources in response to the brightness control signal.

2. The visual information display system of claim 1, further comprising a dark level bias circuit configured to maintain the brightness control signal above a predetermined level when the ambient light level decreases to approximately zero.

3. The visual information display system of claim 1, further comprising an overdrive clamp circuit configured to limit the brightness control signal to be less than a predetermined level.

4. The visual information display system of claim 1, further comprising an automatic shutdown circuit configured to turn off auxiliary light sources in a transflective display system when the ambient light is greater than a predefined level.

5. The visual information display system of claim 1, wherein the visible light sensor comprises an array of PIN diodes on a single substrate that produces a current which is amplified to be the sensor current signal.

6. The visual information display system of claim 1, wherein the visible light sensor has an adjustable response time using a capacitor.

7. The visual information display system of claim 1, wherein the multiplier circuit further comprises:

a pair of current steering diodes configured to multiple the sensor current signal by a pulse-width-modulation signal representative of the dimming control input;

a network of resistors configured to scale the brightness control signal; and

at least one capacitor coupled to the network of resistors and configured as a low pass filter for the brightness control signal.

8. The visual information display system of claim 7, wherein the dimming control input is a DC voltage and the pulse-width-modulation signal is generated using a comparator circuit and a saw-tooth ramp signal.

9. The visual information display system of claim 1, wherein the dimming control input corresponds to a setting of a potentiometer and the multiplier circuit further comprises:

an isolation diode coupled between the visible light sensor output and the potentiometer, wherein the potentiometer conducts a portion of the sensor current signal to generate the brightness control signal;

a network of resistors configured to scale the brightness control signal; and

an optional output capacitor configured as a low pass filter for the brightness control signal.

10. The visual information display system of claim 1, wherein the dimming control input is provided as a digital word and the multiplier circuit further comprises:

a digital-to-analog converter configured to receive the digital word and to output the brightness control signal;

an isolation diode coupled between the visible light sensor and a network of resistors, wherein the network of resistors conducts the sensor current signal to generate a reference voltage for the digital-to-analog converter; and

an optional output capacitor configured as a low pass filter for the reference voltage.

11. The visual information display system of claim 1, wherein the display driver is an inverter and the light sources are fluorescent lamps for backlighting a liquid crystal display.

12. The visual information display system of claim 1, wherein the light sources are light emitting diodes for backlighting a liquid crystal display.

13. A method to adjust display brightness over ambient light variations, the method comprising the steps of:

sensing ambient light with a visible light detector, wherein the visible light detector outputs a current signal that varies linearly with the ambient light level;

multiplying the current signal with a user-adjustable dimming control input to generate a brightness control signal; and

providing the brightness control signal to a display driver.

14. The method of claim 13, wherein the visible light detector has an adjustable response time to allow the current signal to remain substantially unchanged during transient variations of less than a predefined duration in the ambient light.

15. The method of claim 13, wherein software algorithm is used to multiply the current signal with the user-adjustable dimming control input.

16. The method of claim 13, wherein the user-adjustable dimming control input is a pulse-width-modulation logic signal and the multiplying step further comprises the steps of:

steering the current signal to a network of resistors to generate the brightness control signal when the pulse-width-modulation logic signal is a first logic level;

steering the current signal away from the network of resistors when the pulse-width-modulation logic signal is a second logic level; and

using at least one capacitor as a smoothing filter for the brightness control signal.

17. The method of claim 13, wherein the user-adjustable dimming control input adjusts a potentiometer and the multiplying step is accomplished by driving the potentiometer and a resistor network with the output of the visible light detector.

18. The method of claim 13, wherein the user-adjustable dimming control input is a digital word and the multiplying step further comprises the steps of:

providing the digital word to a digital-to-analog converter for conversion to an analog voltage; and

generating a reference voltage for the digital-to-analog converter by driving a resistor network with the output of the visible light detector.

19. The method of claim 13, further comprising the step of shutting off the display driver when the ambient light level is above a predetermined threshold.

20. A visual information display system with ambient light correction comprising:

means for monitoring ambient light and generating a current signal with an amplitude proportional to the ambient light level;

means for multiplying the current signal by a dimming control input to generate a brightness control signal; and

means for adjusting display brightness with the brightness control signal.