Thermal Limited Backlight Driver

Abstract

A system for powering and controlling an LED backlight, the system comprising: a control circuitry; a plurality of LED strings; a pulse width modulation functionality associated with the control circuitry and arranged to pulse width modulate a current flow through each of the plurality of LED strings; and a plurality of current limiters responsive to the control circuitry, each of the plurality of current limiters being associated with a particular one of the plurality of LED strings and operative to limit current flow of the pulse width modulated current there-through, the control circuitry being operative in the event of a thermal condition of one of the plurality of current limiters to reduce a duty cycle of the pulse width modulation functionality of the current flow through the one of the plurality of current limiters.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings in which like numerals designate corresponding elements or sections throughout.

With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. In the accompanying drawings:

FIG. 1 illustrates a high level block diagram of a backlighting system exhibiting a separate controllable voltage source for each of a plurality of LED strings of a single color according to the principle of the invention;

FIG. 2 illustrates a high level functional block diagram of an LED string controller, a plurality of current limiters, a controllable voltage source, a plurality of LED strings of a single color of the backlighting system of FIG. 1 and a color sensor according to a principle of the invention;

FIG. 3 illustrates a high level flow chart of the operation of the LED string controller of FIGS. 1, 2 to test the LED strings prior to full operation according to a principle of the invention;

FIG. 4 illustrates a high level flow chart of the operation of the LED string controller of FIGS. 1, 2 to control the voltage of the controllable voltage source so as to minimize excess power dissipation while ensuring a balanced current flow through each of the LED strings of the same color, and to further monitor the PWM dynamic range and increase the current flow through the LEDs when the PWM duty cycle has reached a predetermined maximum according to a principle of the invention;

FIG. 5 illustrates a high level flow chart of an initialization operation for the LED string controller of FIGS. 1, 2 and 8 to measure the chrominance impact of a failure of each of the LED strings, calculate the required change in current to compensate for the failure and store the changes according to a principle of the invention;

FIG. 6A illustrates a high level flow chart of the operation of the LED string controller of FIGS. 1, 2 and 8 to periodically check the voltage drop across each of the current limiters and the actual current flow through the LED strings so as to detect one of a short circuited LED and an open circuited LED string, set an error flag in the event that a short circuited LED has been detected, adjust the current of the remaining strings to compensate for the open LED string in accordance with the stored values of FIG. 5 and renter the high level flow chart of FIG. 4 so as to update the control of the controllable voltage source according to a principle of the invention;

FIG. 6B illustrates a high level flow chart of the operation of the LED string controller of FIGS. 1, 2 and 8 to periodically check the voltage drop across each of the current limiters and the actual current flow through the LED strings so as to detect one of a short circuited LED and an open circuited LED string, disable the LED string associated with the detected short circuited LED, adjust the current of the remaining strings to compensate for the open or disabled LED string in accordance with the stored values of FIG. 5 and renter the high level flow chart of FIG. 4 so as to update the control of the controllable voltage source according to a principle of the invention;

FIG. 7 illustrates an arrangement of LED strings in a matrix which allows for improved compensation of a failed LED string by other LED strings according to a principle of the invention;

FIG. 8 illustrates a high level functional block diagram of an LED string controller, a plurality of current limiters, a controllable voltage source, a plurality of white LED strings and a photo-sensor according to a principle of the invention;

FIG. 9 illustrates a high level flow chart of the operation of the LED string controller of FIG. 8 to select a particular LED string, or a function of the LED strings, to feedback for control of the controllable voltage source, and to further monitor the PWM dynamic range and increase the current flow through the LEDs when the PWM duty cycle has reached a predetermined maximum according to a principle of the invention; and

FIG. 10 illustrates a high level flow chart of the operation of the LED string controller of FIG. 2 comprising internal current limiters in accordance with the principle of the current invention to prevent thermal overload resulting from power dissipation of the internal current limiters.

Claims

1. A system for powering and controlling an LED backlight, the system comprising: a control circuitry;a plurality of LED strings;a pulse width modulation functionality associated with said control circuitry and arranged to pulse width modulate a current flow through each of said plurality of LED strings; anda plurality of current limiters responsive to said control circuitry, each of said plurality of current limiters being associated with a particular one of said plurality of LED strings and operative to limit current flow of said pulse width modulated current there-through,said control circuitry being operative in the event of a thermal condition of one of said plurality of current limiters to reduce a duty cycle of said pulse width modulation functionality of said current flow through said one of said plurality of current limiters.

2. A system according to claim 1, wherein said control circuitry is further operative in the event of said thermal condition to reduce the duty cycle of said pulse width modulation functionality of said current flow through said plurality of current limiters.

3. A system according to claim 1, wherein said control circuitry is further operative to increase a current limit value of said one of said plurality of current limiters to thereby at least partially compensate for said reduced duty cycle.

4. A system according to claim 1, further comprising a thermal sensor responsive to at least one of said plurality of current limiters, and wherein said control circuitry is operative responsive to said thermal sensor to detect said thermal condition.

5. A system according to claim 1, further comprising a voltage sensor arranged to output an indication of the voltage drop across each of said plurality of current limiters, said voltage sensor being in communication with said control circuitry, and wherein said control circuitry is operative responsive to said voltage sensor to detect said thermal condition.

6. A system according to claim 1, further comprising a voltage sensor arranged to output an indication of the voltage drop across each of said current limiters and a current sensor arranged to output an indication of the current flow through each of said current limiters, said voltage sensor an d said current sensor being in communication with said control circuitry, and wherein said control circuitry is operative responsive to said voltage sensor and said current sensor, to detect said thermal condition.

7. A system according to claim 1, wherein said control circuitry is further operative to: monitor said pulse width modulation functionality, andin the event the duty cycle of said pulse width modulation functionality exceeds a maximum, to adjust the current limit of at least one of said current limiters and reduce the duty cycle of said pulse width modulation functionality thereby maintaining a predetermined luminance.

8. A system according to claim 7, wherein the adjustment of the current limit of said at least one of said current limiters is by a predetermined amount.

9. A system according to claim 7, wherein said current is adjusted and said pulse width modulation duty cycle is reduced so as to maintain said predetermined luminance while reducing the maximum duty cycle to a predetermined amount.

10. A system according to claim 7, wherein said current is adjusted and said pulse width modulation duty cycle is reduced so as to maintain said predetermined luminance while reducing the maximum duty cycle by a predetermined amount.

11. A system according to claim 1, wherein said control circuitry is further operative to monitor an electrical characteristic of each of said plurality of LED strings and determine, responsive to said monitored electrical characteristic, if any of said plurality of LED strings exhibits and open circuit condition.

12. A system according to claim 11, wherein responsive to said determined open circuit condition, said control circuitry is further operative to adjust the current of at least one of the remaining LED strings by a predetermined amount to at least partially compensate for said determined open circuit condition.

13. A system according to claim 12, wherein said plurality of LED strings are arranged in a matrix such that said at least partial compensation maintains a substantial uniform color.

14. A method for powering and controlling an LED backlight comprising: providing a plurality of LED strings;providing a plurality of current limiters, each of said provided plurality of current limiters limiting a current flow through a particular one of said provided plurality of LED strings to a settable value;pulse width modulating said current flow through each of said provided plurality of LED strings;monitoring a thermal condition associated with at least one of said provided plurality of current limiters; andin the event of a predetermined thermal condition of one of said provided plurality of current limiters, reducing a duty cycle of said pulse width modulating of said current flow through said one of said plurality of current limiters.

15. A method according to claim 14, further comprising in the event of said predetermined thermal condition, reducing a duty cycle of said pulse width modulating of said current flow through said provided plurality of current limiters.

16. A method according to claim 14, further comprising: increasing said settable value of said one of said plurality of current limiters to thereby at least partially compensate for said reduced duty cycle.

17. A method according to claim 14, further comprising: providing a thermal sensor responsive to at least one of said provided plurality of current limiters,wherein said predetermined thermal condition is determined responsive to said provided thermal sensor.

18. A method according to claim 14, further comprising: providing a voltage sensor arranged to output an indication of the voltage drop across each of said provided plurality of current limiters,wherein said predetermined thermal condition is determined responsive to said output of said provided voltage sensor.

19. A method according to claim 14, further comprising: providing a voltage sensor arranged to output an indication of the voltage drop across each of said provided plurality of current limiters;providing a current sensor arranged to output an indication of the current flow through each of said provided plurality of current limiters,wherein said predetermined thermal condition is determined responsive to said output of said provided voltage sensor and said output of said provided current sensor.

20. A method according to claim 14, further comprising: monitoring said pulse width modulating, andin the event the duty cycle of said pulse width modulation functionality exceeds a predetermined maximum, adjusting said settable value of least one of said provided current limiters and reducing the duty cycle of said pulse width modulating thereby maintaining a predetermined luminance.

21. A method according to claim 20, wherein said adjustment of said settable value of said at least one of said provided current limiters is by a predetermined amount.

22. A method according to claim 20, wherein said adjustment of said settable value of said at least one of said provided current limiters and said reducing the duty cycle of said pulse width modulating maintains said predetermined luminance while reducing the maximum duty cycle to a predetermined amount.

23. A method according to claim 20, wherein said adjustment of said settable value of said at least one of said provided current limiters and said reducing the duty cycle of said pulse width modulating maintains said predetermined luminance while reducing the maximum duty cycle by a predetermined amount.

24. A method according to claim 14, further comprising: monitoring an electrical characteristic of each of said provided plurality of LED strings; anddetermining, responsive to said monitoring, if any of said provided plurality of LED strings exhibits and open circuit condition.

25. A method according to claim 24, further comprising responsive to said determined open circuit condition: adjusting one of said settable value and the duty cycle of said pulse width modulating of at least one of the remaining provided LED strings by a predetermined amount to at least partially compensate for said determined open circuit condition.

26. A method according to claim 24, further comprising: arranging said provided plurality of LED strings in a matrix such that said at least partial compensation maintains a substantial uniform color.