LED CURRENT CONTROL

• US 20140159598A1
• Filed: 11/06/2013
• Published: 06/12/2014
• Est. Priority Date: 12/07/2012
• Status: Active Grant

• Current Assignee: NXP B.V. (NXP Semiconductors NV)
• Original Assignee: NXP B.V. (NXP Semiconductors NV)
• Inventors: BOEZEN, Hendrik
• Granted Patent: US 9,204,512 B2
• Time in Patent Office: Days
• US Class Current: 315/193
• CPC Class Codes: H05B 45/10 Controlling the intensity of the light
• CPC Class Codes: H05B 45/14 using electrical feedback from LEDs or from LED modules
• CPC Class Codes: H05B 45/375 using buck topology
• CPC Class Codes: H05B 45/48 having LEDs organised in strings and incorporating parallel shunting devices
• CPC Class Codes: Y02B 20/30 Semiconductor lamps, e.g. solid state lamps [SSL] light emitting diodes [LED] or organic LED [OLED]

Abstract

An LED control circuit for controlling a current supplied to an LED unit is disclosed. A sense resistor is included for sensing an output current provided from an output node. A comparator circuit is provided for detecting upper and lower thresholds in the output current based on the voltages at ends of the sense resistor, and for controlling a current supply to maintain the current between the thresholds. The comparator circuit comprises a reference current source and reference resistors, for generating upper and lower threshold references. The comparator circuit further comprises a comparator for detecting a mid-range voltage dependent on the output current and deriving a control signal which indicates if the detected voltage is above or below the mid-range voltage. The control signal is used to adjust the reference current source, thereby to vary the upper and lower threshold references to maintain a constant median or average output current.

Claims

1. An LED control circuit for controlling a current supplied to an LED unit, the control circuit comprising:

a sense resistor for sensing an output current provided from an output node; and

a comparator circuit for detecting upper and lower thresholds in the output current based on the voltages at each end of the sense resistor, and for controlling a current supply to maintain the current between the thresholds, wherein the comparator circuit comprises a reference current source and reference resistors, for generating upper and lower threshold references,

wherein the comparator circuit further comprises a comparator for detecting a mid-range voltage dependent on the output current and deriving a control signal which indicates if the detected voltage is above or below the mid-range voltage,

and wherein the control signal is used to adjust the reference current source, thereby to vary the upper and lower threshold references to maintain a constant median or average output current.

2. A circuit as claimed in claim 1, wherein the comparator is for detecting a mid-range voltage across the sense resistor.

3. A circuit as claimed in claim 2, wherein the mid-range voltage comprises the voltage across the sense resistor corresponding to a desired average or median output current.

4. A circuit as claimed in claim 1, comprising an inductor in series with the sense resistor to the output node.

5. A circuit as claimed in claim 1, comprising a duty cycle circuit for determining the duty cycle of the control signal.

6. A circuit as claimed in claim 5, wherein the comparator circuit comprises means for adjusting the reference current to maintain the duty cycle at 50%.

7. A circuit as claimed in claim 5, wherein the duty cycle circuit comprises:

a reference clock;

a first counter (counter1) for counting the number of clock cycles during a predetermined number of periods of the control signal;

a second counter (counter2) for counting the number of clock cycles during the high phases with the predetermined number of periods of the control signal; and

a comparator for comparing the counter outputs to derive duty cycle information.

8. A circuit as claimed in claim 7, wherein the duty cycle circuit comprises a divider for dividing the control signal to derive a count window corresponding in length to a multiple of period of the control signal.

9. An LED circuit comprising:

an LED control circuit as claimed in claim 1; and

a string of series-connected LEDs provided at the control circuit output.

10. A circuit as claimed in claim 9, wherein the string of LEDs comprises at least one bypass switch in parallel with a sub-set of one more LEDs of the strings, thereby to provide controllable dimming of the sub-set of LEDs.

11. A method of controlling the current supplied to an LED unit, the method comprising:

sensing an output current provided to the LED unit;

detecting upper and lower thresholds in the output current by comparing the voltages at each end of a sense resistor with upper and lower threshold references which are generated using a reference current;

controlling a current supply to maintain the current between the thresholds,

wherein the method further comprises;

detecting a mid-range voltage dependent on the output current and deriving a control signal which indicates if the detected voltage is above or below the mid-range voltage; and

adjusting the reference current source to vary the upper and lower threshold references to maintain a constant median or average output current.

12. A method as claimed in claim 11, wherein the mid-range voltage is detected across the sense resistor and comprises the voltage across the sense resistor corresponding to a desired average or median output current.

13. A method as claimed in claim 11, comprising determining the duty cycle of the control signal.

14. A method as claimed in claim 13, comprising adjusting the reference current to maintain the duty cycle at 50%.

15. A method as claimed in claim 11, comprising deriving a calibration function from the reference current source adjustments, such that the calibration function determines a required relationship between the control signal and the required reference current source setting.