Redundant operation of a backlight unit of a display device under a shorted LED condition

• US 9,076,357 B2
• Filed: 11/16/2012
• Issued: 07/07/2015
• Est. Priority Date: 11/16/2012
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Hussain, Asif, Pandya, Manisha P.
• Primary Examiner(s): Amadiz, Rodney
• Application Number:
  US13/679,745
• Publication Number:
  US 20140139499A1
• Time in Patent Office: 963 Days
• Field of Search: 345/102, 345/204, 315121-123, 315291-311
• US Class Current: 1/1
• CPC Class Codes: G09G 2300/0426 Layout of electrodes and connections
• CPC Class Codes: G09G 2310/0264 Details of driving circuits
• CPC Class Codes: G09G 2330/04 Display protection
• CPC Class Codes: G09G 2330/08 Fault-tolerant or redundant circuits, or circuits in which repair of defects is prepared
• CPC Class Codes: G09G 3/3406 Control of illumination source illumination devices structurally associated with liquid crystal cells G02F1/1336
• CPC Class Codes: G09G 5/00 Control arrangements or circuits for visual indicators common to cathode-ray tube indicators and other visual indicators image data processing or generation, in general G06T
• CPC Class Codes: H05B 45/38 using boost topology
• CPC Class Codes: H05B 45/46 having LEDs disposed in parallel lines
• CPC Class Codes: H05B 45/58 involving end of life detection of LEDs

Abstract

Disclosed embodiments relate to techniques for operating a backlight unit of a display device in a redundant mode and a non-redundant mode in the event of a shorted LED failure condition. For instance, in a redundant mode, multiple LED strings arranged in an end-to-end configuration are each driven to provide a first quantity of light, such that the combined output from all LED strings is capable of providing a total light output corresponding to a maximum brightness setting for the display device. In the case that an LED on one of the strings fails due to a shorted LED failure condition, the remaining functional LEDs of the affected string may be driven to provide a second quantity of light, such that the combined output from the affected strings and the non-affected strings may still provide the same total light output for achieving the maximum brightness setting. The second quantity of light is greater than the first quantity.

Claims

1. A method comprising:

controlling at least one light-emitting diode (LED) string of a backlight unit in a first mode, wherein the LED string comprises a plurality of LEDs electrically connected in series, such that all of the LEDs of the LED string are controlled to each provide a first light output, wherein the combined first light output from all of the LEDs provides a target luminance output for the LED string;

determining whether a shorted LED failure condition occurs in any of the LEDs in the LED string; and

in response to detecting a shorted LED failure condition across at least one of the LEDs in the LED string, controlling at least one other LED string in a second mode to compensate for the shorted LED failure condition such that the remaining operational LEDs of the LED string are controlled to each provide the first light output, and LEDs of the at least one other LED string are each controlled to provide a second light output greater than the first light output, wherein the second light output of the LEDs of the at least one other LED string compensates for a difference between the target luminance output for the LED string and the combined first light output from the remaining operational LEDs of the LED string.

2. The method of claim 1, wherein backlight unit is configured to provide 1D scanning, and wherein the LED string and the other LED string are the only LED strings of the backlight unit.

3. The method of claim 1, wherein the backlight unit comprises a plurality of identically configured LED strings configured in an end-to-end series arrangement.

4. The method of claim 1, wherein, when controlling the LED strings in the first mode, the remaining operational LEDs on the LED string on which a shorted LED failure condition occurs are controlled to continue to provide the first light output, while the LEDs on all of the other LED strings on which a short circuit condition does not occur are controlled to provide the second light output.

5. The method of claim 1, wherein controlling the LED string in the first mode such that each of the LEDs provide the first light output comprises driving the LED string using first boost voltage generated by a first pulse-width modulation (PWM) signal having a first duty cycle, and wherein controlling the at least one other LED string in the second mode such that each of the LEDs of the at least one other LED string provide the second light output comprises driving the at least one other LED string using a second boost voltage generated by a second PWM signal having a second duty cycle, wherein the second duty cycle is greater than the first duty cycle.

6. The method of claim 1, wherein controlling the LED string in the first mode such that each of the LEDs provide the first light output comprises driving the LED string using a first current, and wherein controlling the at least one other LED string in the second mode such that each of the LEDs of the at least one other LED string provide the second light output comprises driving the at least one other LED string using a second current, wherein the second current is greater than the first current.

7. The method of claim 1, wherein determining whether a short circuit condition occurs in the LED string comprises monitoring at least one of a voltage feedback signal, a current sink signal, and a temperature reading provided by the LED string.

8. A method comprising:

controlling a plurality of light-emitting diode (LED) strings of an edge-lit backlight unit in a redundant mode, wherein the plurality of LED strings are configured in an interleaved arrangement along an edge of the backlight unit, by driving each of the LED strings at a first driving strength such that each LED within each LED string outputs a first quantity of light in the redundant mode and such that the combined light output from each LED string corresponds to a target luminance output of the backlight unit;

detecting the occurrence of a short circuit condition in one of the LED strings;

controlling the plurality of LED strings in a non-redundant mode in response to detecting the short circuit condition by identifying a first LED string containing an LED that is directly adjacent to an LED of the LED string with the short circuit condition, identifying a second LED string containing an LED that is directly adjacent to an LED of the LED string with the short circuit condition, driving the first and second LED strings at a second driving strength, and driving the remaining LED strings at the first driving strength, such that each LED of the first and second LED strings outputs a second quantity of light and each LED of each remaining LED string outputs the first quantity of light, wherein the combined light output from each LED string corresponds to the target luminance output of the backlight unit.

9. The method of claim 8, wherein the second quantity of light is greater than the first quantity of light.

10. The method of claim 8, wherein driving the LED strings at the first driving strength comprises using a first pulse-width modulation (PWM) signal having a first duty cycle and wherein driving the first and second LED strings at the second driving strength comprises using a second PWM signal having a second duty cycle that is greater than the first duty cycle.

11. The method of claim 8, wherein detecting the occurrence of the short circuit condition comprises receiving a feedback signal from each of the plurality of LED strings and comparing the respective feedback signals from the LED strings against each other.

12. A method comprising:

providing a plurality of light-emitting diodes (LEDs) arranged in independently controllable LED strings;

configuring the LED strings in an end-to-end arrangement along an edge of a backlight unit, wherein each LED string comprises a total number of LEDs including a first set of LEDs corresponding to a minimum number of LEDs for achieving a target light output from the LED string and a second set of at least one redundant LED;

determining a first duty cycle for a pulse-width modulation (PWM) signal used to cause each of the LED strings to provide the target light output when controlled in a redundant mode, wherein all of the LEDs of each LED string are functional in the redundant mode, and wherein the first duty cycle is determined as a ratio of the number of LEDs in the first set to the total number of LEDs;

determining a second duty cycle for a pulse-width modulation signal used to cause an LED string with a short circuit condition to provide the same target light output when controlled in a non-redundant mode in which at least one LED of the LED string is non-functional due to the short circuit condition, and wherein the second duty cycle is determined as a ratio of the number of LEDs in the first set to a number of remaining functional LEDs in the LED string.

13. The method of claim 12, comprising:

providing a backlight driver circuit and configuring the backlight driver circuit to control the LED strings using the first duty cycle when no short circuit conditions are present and to control the LED string with the short circuit condition using the second duty cycle when when a short circuit condition is present in one of the LED strings.

14. A display device comprising:

a liquid crystal display (LCD) panel;

a backlight configured to provide light to the LCD panel, wherein the backlight comprises a plurality of light-emitting diodes (LEDs) arranged in independently controllable groups, wherein each of the independently controllable groups comprises a plurality of LEDs; and

a display controller comprising;

display driving circuitry configured to provide image signals and scanning signals to the LCD panel; and

a backlight driver configured to control each independently controllable group of LEDs in a first manner to provide a target luminous flux output from the backlight when no short circuit conditions are present in any of the independently controllable groups of LED, and if a short circuit condition is detected in one of the independently controllable groups of LEDs, driving the independently controllable group of LEDs having the short circuit condition in a first manner while driving the remaining independently controllable groups of LEDs without a short circuit condition in the second manner to provide the same target luminous flux output from the backlight.

15. The method of claim 14, comprising configuring the backlight driver circuit such that the LED strings without a short circuit condition continue to be controlled using the first duty cycle in the non-redundant mode.

16. The display device of claim 14, wherein the backlight comprises an edge-lit backlight, and wherein the independently controllable groups of LEDs are configured in an end-to-end series arrangement along an edge of the backlight.

17. The display device of claim 14, wherein the backlight driver comprises a pulse-width modulation (PWM) signal generator, and wherein driving an independently controllable group of LEDs in the first manner comprises using a PWM signal having a first duty cycle, and wherein driving an independently controllable group of LEDs in the second manner comprises using a PWM signal having a second duty cycle that is greater than the first duty cycle.

18. The display device of claim 14, wherein the backlight driver comprises boost converter circuitry, wherein the boost converter circuitry is configured to receive a voltage feedback signal and a peak current feedback signal, and wherein the backlight driver is configured to detect a short circuit condition by monitoring the states of the voltage feedback signal and the peak current feedback signal.

19. The display device of claim 14, wherein the backlight driver comprises current sink circuitry configured to receive a respect current sink signal from each of the independently controllable groups of LEDs, and wherein the backlight driver is configured to detect a short circuit condition by monitoring the states of the current sink signals.

20. The display device of claim 14, wherein the backlight driver comprises a temperature-sensing device configured to provide a temperature value associated with each of the independently controllable groups of LEDs, and wherein the backlight driver is configured to detect a short circuit condition by monitoring the temperature values.

21. The display device of claim 20, wherein the temperature-sensing device comprises a thermocouple or an external temperature sensor, or some combination thereof.