LED BACKLIGHT SYSTEM

• US 20110164069A1
• Filed: 01/06/2010
• Published: 07/07/2011
• Est. Priority Date: 01/06/2010
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Kim, Floriano, Yoshimoto, Mark A., Thompson, Paul M.
• Granted Patent: US 8,907,884 B2
• Time in Patent Office: Days
• US Class Current: 345/690
• CPC Class Codes: G09G 2320/0233 Improving the luminance or brightness uniformity across the screen
• CPC Class Codes: G09G 2320/0247 Flicker reduction other than flicker reduction circuits used for single beam cathode-ray tubes
• CPC Class Codes: G09G 2320/0633 by amplitude modulation of the brightness of the illumination source
• CPC Class Codes: G09G 2320/064 by time modulation of the brightness of the illumination source
• CPC Class Codes: G09G 3/342 using several illumination sources separately controlled corresponding to different display panel areas, e.g. along one dimension such as lines
• CPC Class Codes: H05B 45/38 using boost topology
• CPC Class Codes: Y02B 20/30 Semiconductor lamps, e.g. solid state lamps [SSL] light emitting diodes [LED] or organic LED [OLED]

Abstract

A method and system for modifying the pulse width modulation frequency for controlling the backlit illumination intensity of a liquid crystal display. The modified pulse width modulation frequency may be selected to reduce distortion in the display while allowing for a wide range of dimming settings for the display. A pulse width modulation signal may be also be phase shifted such that a string of light sources may be sequentially activated to generate a effective frequency greater than that of the frequency of the pulse width modulation signal.

Claims

1. An electronic device, comprising:

a display panel comprising a plurality of pixels;

a light source comprising a plurality of light emitting diode (LED) strings adapted to generate light to illuminate the plurality of pixels; and

display control logic adapted to sequentially activate and deactivate each LED string of the plurality of LED strings at a frequency of approximately at least 1 kHz.

2. The electronic device of claim 1, comprising a pulse width modulator configured to generate a pulsed signal and transmit the pulsed signal to the display control logic.

3. The electronic device of claim 2, wherein the display control logic is adapted to receive the pulsed signal and utilize the pulsed signal to sequentially activate and deactivate the plurality of LED strings.

4. The electronic device of claim 2, wherein each LED string of the plurality of LED strings comprises a plurality of LEDs.

5. The electronic device of claim 4, wherein each LED string includes six LEDs coupled in series.

6. The electronic device of claim 1, wherein the display control logic is adapted to sequentially activate and deactivate each LED string at a frequency of at least 8 kHz.

7. An electronic device, comprising:

a pulse width modulator (PWM) adapted to generate an oscillating PWM signal at a frequency of at least approximately 1 kHz and at least at a 10-bit brightness resolution;

a sequencer adapted to receive the oscillating PWM signal and to phase shift the oscillating PWM signal for controlling the activation and deactivation of at least one light emitting diode (LED) string.

8. The electronic device of claim 7, wherein the oscillating PWM signal toggles the activation and deactivation of a plurality of LED strings sequentially.

9. The electronic device of claim 8, comprising a display, wherein the brightness of the display is controlled by adjusting a duty cycle of the oscillating PWM signal.

10. The electronic device of claim 8, wherein the sequencer phase shifts the oscillating PWM cycle to generate an effective frequency of at least approximately 6 kHz.

11. The electronic device of claim 8, wherein the sequencer phase shifts the oscillating PWM cycle to generate an effective frequency of at least approximately 48 kHz.

12. An electronic device, comprising:

a display having a plurality of light emitting diode (LED) strings adapted to generate light to illuminate a plurality of pixels in the display;

a pulse width modulator (PWM) adapted to generate an oscillating PWM signal; and

display control logic adapted to activate and deactivate the plurality of LED strings at approximately at least 1 kHz, wherein the display control logic comprises delay circuitry adapted to phase shift the oscillating PWM signal for activation and deactivation of the light source at an effective frequency of at least approximately 10 kHz.

13. The electronic device of claim 12, wherein each of the LED strings comprises a plurality of LEDs.

14. The electronic device of claim 12, wherein display control logic is adapted to activate and deactivate the plurality of LED strings at approximately at least 8 kHz.

15. The electronic device of claim 12, wherein the PWM modifies a duty cycle of the oscillating PWM signal for control of the brightness of the display.

16. The electronic device of claim 12, wherein the display control logic is configured to lessen a brightness of the display by reducing a level of current through active LED strings if a duty cycle of the PWM signal is to be set below a threshold duty cycle level.

17. The electronic device of claim 12, wherein the display control logic comprises delay circuitry adapted to phase shift the oscillating PWM signal for activation and deactivation of the light source at an effective frequency of at least approximately 48 kHz.

18. A method of providing illumination to a display to reduce visual artifacts, comprising:

generating an oscillating pulse width modulator (PWM) signal in a PWM;

receiving the PWM signal at a sequencer; and

sequentially routing the PWM signal from the sequencer to a plurality of light emitting diode (LED) strings in a display at an effective frequency of at least approximately 6 kHz to reduce optical beating resulting from the frequency of the PWM signal and a refresh frequency of the display.

19. The method of claim 18, comprising adjusting the brightness of the display by adjusting the duty cycle of the PWM signal.

20. The method of claim 19, comprising adjusting the duty cycle of the PWM signal based on user input or based on a determination of the amount of internal power remaining in an internal power source.

21. The method of claim 20, comprising sequentially transmitting the PWM signal to the plurality of LED strings in the display at a frequency of at least approximately 8 kHz.

22. A method for illuminating a display, comprising:

generating light from a light source;

directing the light towards a plurality of pixels in a display; and

toggling the light source on and off at an effective frequency of approximately at least 48 kHz via a phase shifted pulse width modulator (PWM) signal.

23. The method of claim 22, comprising adjusting a duty cycle of the PWM signal in response to user initiated changes of brightness of the display.

24. The method of claim 22, wherein toggling the light source on and off comprises sequentially activating and deactivating at least one light emitting diode (LED) string in the light source.

25. The method of claim 22, wherein toggling the light source on and off comprises sequentially activating and deactivating each of six LED strings in sequence at approximately at least 8 kHz per string.