Light-emitting diode backlighting systems

• US 8,143,792 B2
• Filed: 08/19/2009
• Issued: 03/27/2012
• Est. Priority Date: 08/19/2009
• Status: Active Grant

• Current Assignee: Analog Devices, Inc.
• Original Assignee: Analog Devices, Inc.
• Inventors: Joo, Chulmin, Shin, Hyunick
• Primary Examiner(s): Le, Don
• Application Number:
  US12/543,722
• Publication Number:
  US 20110043118A1
• Time in Patent Office: 951 Days
• Field of Search: 315/185.R, 315/291, 315/294, 315/297, 315/307, 315/308, 345/102, 345/84, 345/204
• US Class Current: 315/185.R
• CPC Class Codes: H05B 45/46 having LEDs disposed in parallel lines

Abstract

Lighting system embodiments are provided to energize and calibrate strings of light-emitting diodes. These embodiments are particularly useful for calibration of strings of light-emitting diodes that are arranged to provide backlighting of liquid crystal displays. The systems are structured around the use of a single comparator that is multiplexed to facilitate calibration of a plurality of current sources. The systems can be adapted for use in displays in which different techniques (e.g., “analog dimming” and “pulse-width modulation”) are used to vary the brightness of the display. The systems remove the need for special structures (e.g., fuse arrays, special test equipment, and interfaces).

Claims

1. A lighting system to energize strings of light-emitting diodes, comprising:

current sources each configured to provide to a respective one of said strings a current having an amplitude responsive to a control voltage and to provide a monitor voltage representative of said amplitude;

control voltage sources each configured to provide the control voltage of a respective one of said current sources;

a comparator to compare a voltage at an input port of said comparator to a reference voltage; and

a multiplexer arranged to couple the monitor voltage of any selected one of said current sources to said input port;

an output signal of said comparator thereby indicating a calibration state of each of said current sources in response to its respective control voltage.

2. The system of claim 1, wherein said control voltage sources are each configured to vary its control voltage in response to a control signal and further including calibration logic configured to provide said control signal in response to said output signal.

3. The system of claim 1, wherein each of said current sources includes:

a transistor having a control terminal and first and second current terminals responsive to said control terminal wherein said second current terminal is available to provide said current;

a resistor coupled to said first current terminal to provide said monitor voltage; and

a differential amplifier arranged to drive said control terminal and having a positive input terminal to receive said control voltage and having a negative input terminals coupled to said resistor.

4. The system of claim 3, wherein said control terminal is a gate and said first and second current terminals are respectively a source and a drain.

5. The system of claim 1, wherein each of said control voltage sources comprises:

a string of resistors to receive a resistor current; and

a control multiplexer configured to provide the control voltage adjacent any one of said resistors in response to said control signal.

6. The system of claim 5, further including a current mirror to provide said resistor current.

7. The system of claim 5, wherein said string of resistors is arranged to provide voltages above and below said reference voltage.

8. A lighting system to energize strings of light-emitting diodes, comprising:

current sources each configured to include;

a transistor having a control terminal and first and second current terminals responsive to said control terminal wherein said second current terminal is available for coupling to a respective one of said strings;

a resistor coupled to said first current terminal; and

a differential amplifier arranged to drive said control terminal and having positive and negative input terminals with said negative input terminal coupled to said first current terminal;

control voltage sources each configured to provide a selectable control voltage to the positive terminal of the differential amplifier of a respective one of said current sources;

a comparator having a first input terminal and having a second input terminal for reception of a reference voltage; and

a multiplexer arranged to couple the first current terminal of any selected one of said current sources to said first input terminal;

an output signal of said comparator thereby indicating a calibration state of each of said current sources in response to its respective trim voltages.

9. The system of claim 8, wherein said control terminal is a gate and said first and second current terminals are respectively a source and a drain.

10. The system of claim 8, further including calibration logic configured to command said selectable control voltage in response to said output signal.

11. The system of claim 8, wherein each of said voltage sources comprises:

a string of resistors to receive a current; and

a trim multiplexer configured to provide the control voltage between a selected pair of said resistors.

12. The system of claim 11, further including a current mirror to provide said current.

13. The system of claim 11, wherein said string of resistors is arranged to provide voltages above and below said reference voltage.

14. A method to energize strings of light-emitting diodes, comprising the steps of:

providing to each of said strings a current having an amplitude responsive to a control voltage and further providing a monitor voltage representative of said amplitude;

varying the control voltage corresponding to a respective one of said strings;

comparing a voltage at a comparison port to a reference voltage; and

multiplexing the monitor voltage corresponding to any selected one of said currents to said comparison port;

the comparing step thereby indicating a calibration state of the current of each strings in response to its respective control voltage.

15. The method of claim 14, wherein said control voltage is responsive to a control signal and further including the step of varying said control signal in response to said comparing step.