Apparatus and method for ambient light compensation for backlight control in small format displays

• US 7,843,422 B1
• Filed: 11/29/2005
• Issued: 11/30/2010
• Est. Priority Date: 11/29/2005
• Status: Active Grant

• Current Assignee: National Semiconductor Corporation (Texas Instruments, Inc.)
• Original Assignee: National Semiconductor Corporation (Texas Instruments, Inc.)
• Inventors: Ludden, Christopher A., Knausz, Imre
• Primary Examiner(s): Abdulselam; Abbas I
• Application Number:
  US11/289,103
• Time in Patent Office: 1,827 Days
• Field of Search: 345/87, 345/89, 345/102, 345/204, 345/211, 345/212, 345/690, 348/554, 348/729, 349/61
• US Class Current: 345/102
• CPC Class Codes: G09G 2320/0247 Flicker reduction other than flicker reduction circuits used for single beam cathode-ray tubes
• CPC Class Codes: G09G 2320/064 by time modulation of the brightness of the illumination source
• CPC Class Codes: G09G 2360/144 the light being ambient light
• CPC Class Codes: G09G 3/3406 Control of illumination source illumination devices structurally associated with liquid crystal cells G02F1/1336

Abstract

An ambient light compensation circuit for controlling a backlight brightness is provided. The circuit automatically adjusts the LCD backlight brightness based on a reading of the ambient light. The circuit includes a non-linear ADC and a backlight control circuit. The non-linear ADC provides a digital signal from a photodiode signal. The backlight control circuit adjusts the brightness of the backlighting based on the digital signal. The brightness adjustment is performed gradually, e.g., one significant bit of the digital signal per second.

Claims

1. A circuit for back-lighting of a small-format display, comprising:

an analog-to-digital conversion circuit that is operable to receive an analog signal that is based, at least in part, on a sensed illuminance of ambient light; and

to convert the analog signal in a conversion output signal, wherein the conversion output signal is a digital signal having a value, and wherein the analog-to-digital conversion circuit is integrated onto a column driver; and

a backlight control circuit that is operable to receive the digital signal, and to control modulation of brightness of the back-lighting based, in part, on the digital signal, wherein the backlight control circuit is integrated onto the column driver, whereby the control of the modulation of the brightness of the back-lighting is accomplished without involvement of a microprocessor.

2. The circuit of claim 1, wherein the analog-to-digital conversion circuit is operable to perform a non-linear analog conversion such that a value of the digital signal is approximately proportional to a logarithm of a voltage of the analog signal.

3. The circuit of claim 1, wherein the backlight control circuit is operable to provide a backlight control signal such that a parameter of the backlight control signal is modulated based on the conversion output signal, and wherein the brightness of the back-lighting is approximately proportional to the modulated parameter of the backlight control signal.

4. The circuit of claim 3, wherein the parameter of the backlight control signal is at least one of a voltage or a duty cycle of the backlight control signal.

5. The circuit of claim 3, wherein the backlight control circuit is operable to provide the backlight control signal such that, if the digital signal changes, the parameter is gradually adjusted in proportion to the change, but such that the modulated parameter is not adjusted below a minimum bound or above a maximum bound, and such that the modulation is sufficiently gradual that a change in ambient light from 20 lux to 300 lux requires at least three seconds for the gradual modulation of the parameter to be complete.

6. The circuit of claim 1, wherein the column driver includes a gamma reference circuit;

the gamma reference circuit includes a resistor string;

the resistor string includes a first plurality of voltage taps and a second plurality of voltage taps;

the gamma reference circuit is operable to provide a plurality of gamma reference voltages at the first plurality of voltage taps, and to provide a plurality of ambient light sensing reference voltages at the second plurality of voltage taps; and

wherein the analog-to-digital conversion circuit is operable to receive the plurality of ambient light sensing reference voltages, and to perform analog-to-digital conversion employing the plurality of ambient light sensing reference voltages as reference voltages for the analog-to-digital conversion.

7. A circuit for back-lighting of a small-format display, comprising:

a reference voltage circuit that is operable to provide a plurality of reference voltages that span a voltage range non-linearly;

an analog-to-digital converter that is operable receive a photodiode voltage and the plurality of reference voltages, and to provide a digital signal by performing a non-linear analog-to-digital conversion based on the plurality of reference voltages to convert the photodiode voltage into a digital signal such that the digital signal has a value that is approximately proportional to a logarithm of a luminance of ambient light; and

a backlight control circuit that is operable to provide a backlight control signal based, in part, on the digital signal, such that a parameter of the backlight control signal is modulated based, in part, on the value of the digital signal.

8. The circuit of claim 7, wherein the backlight control circuit is operable to provide the backlight control signal such that, if the digital signal changes, the parameter is gradually adjusted in proportion to the change, but such that the modulated parameter is not adjusted below a minimum bound or above a maximum bound.

9. The circuit of claim 7,

wherein the reference voltage circuit includes a resistor with a plurality of voltage taps spaced in a non-linear manner to provide the plurality of references voltages.

10. The circuit of claim 7, further comprising a white LED driver and a column driver, wherein the analog-to-digital converter, the backlight controller circuit, the LED driver, and the column driver are all included on a single integrated circuit;

and wherein a resistor string used in a gamma reference circuit of the column driver is also employed as the reference voltage circuit.

11. The circuit of claim 7, wherein the backlight control circuit is arranged to store the value of the digital signal at a plurality of times, and to modulate the parameter of the backlight control signal based on a median value of the digital signal over a pre-determined period of time.

12. The circuit of claim 7, wherein the backlight control circuit is arranged to store the value of the digital signal at a plurality of times, and to modulate the parameter of the backlight control signal based on an average value of the digital signal over a pre-determined period of time.

13. The circuit of claim 12, wherein the backlight control circuit is operable to provide the backlight control signal such that, if the average value changes, the parameter is gradually adjusted in proportion to the change, but such that the modulated parameter is not adjusted below a minimum bound or above a maximum bound, and such that the modulation is sufficiently gradual that a change in ambient light from 20 lux to 300 lux requires at least three seconds for the gradual modulation of the parameter to be complete.

14. The circuit of claim 12, wherein

the modulated parameter of the backlight control signal is the duty cycle of the backlight control signal;

and

wherein the backlight control circuit includes;

a backlight controller that is operable to receive the digital signal and to provide a digital PWM input signal based, in part, on the average value of the digital signal; and

a PWM circuit that is operable to provide the backlight control signal such that the duty cycle of the backlight control signal is pulse-width modulated based on the digital PWM input signal.

15. The circuit of claim 14,

wherein the backlight controller is operable to receive a digital minimum bound signal indicating the minimum bound, to receive a digital maximum bound signal indicating the maximum bound, and to provide the digital PWM input signal value such that:

if the average value of the digital signal increases, and the digital PWM input signal has not reached the maximum bound, the value of the digital PWM input signal increases by an amount corresponding to one least significant bit of the digital signal;

if the average value of the digital signal decreases, and the digital PWM input signal has not reached the minimum bound, the value of the digital PWM input signal decreases by an amount corresponding to one least significant bit of the digital signal, such that the digital PWM input signal docs not change by more than one least significant bit of the digital signal for every pre-determined time interval.

16. The circuit of claim 15, further comprising a histogram block, wherein the histogram block is operable to determine whether a displayed image is a high-contrast image, and to provide the digital maximum bound signal such that the maximum bound is a first value if it is determined that the displayed image is a high-contrast image, and such that the maximum bound is a second value if it is determined that the displayed image is not a high-contrast image, wherein the second value is greater than the first value.

17. The circuit of claim 15, wherein the pre-determined time interval is at least 0.1 seconds.

18. A method for backlight modulation, comprising:

back-lighting a display such that a brightness of the display is based, in part, on a backlight control signal;

sensing an illuminance of ambient light;

providing an analog signal based on the illuminance of ambient light;

performing an analog-to-digital conversion on the analog signal to provide a digital signal having a value; and

providing a backlight control signal based, in part, on the digital value such that a parameter of the backlight control signal is gradually modulated based, in part, on the digital value; and

such that the modulation is sufficiently gradual that a change in ambient light from 20 lux to 300 lux requires at least three seconds for the gradual modulation of the parameter to be complete.

19. The method of claim 18, wherein providing the backlight control signal includes:

providing an average digital value based on a plurality of readings of the digital signal over time;

during each pre-determined interval;

if the average digital value is greater than a value corresponding to the current brightness and less than a maximum bound, incrementing a PWM input signal; and

if the average digital value is less than a value corresponding to the current brightness and greater that a minimum bound, decrementing the PWM input signal; and

providing the backlight control signal such that a duty cycle of the backlight control signal is approximately proportional to a value of the PWM input signal.

20. The method of claim 19, wherein the pre-determined time interval is at least 0.2 seconds.