Adaptive switch mode LED driver

• US 8,334,662 B2
• Filed: 09/11/2009
• Issued: 12/18/2012
• Est. Priority Date: 09/11/2009
• Status: Active Grant

• Current Assignee: Dialog Semiconductor Incorporated (Renesas Electronics Corporation)
• Original Assignee: iWatt, Inc. (Renesas Electronics Corporation)
• Inventors: Jin, Xuecheng, Chang, Yu Cheng, Li, Yang, Lan, Maofeng, Kesterson, John W., Wang, Xiaoyan, Pan, Chenghung
• Primary Examiner(s): Tran, Anh
• Application Number:
  US12/558,275
• Publication Number:
  US 20110062872A1
• Time in Patent Office: 1,194 Days
• Field of Search: None
• US Class Current: 315/299
• CPC Class Codes: H01L 22/14 for electrical parameters, e.g. resistance, deep-levels, CV, diffusions by electrical means
• CPC Class Codes: H01L 2924/00 Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
• CPC Class Codes: H01L 2924/0002 Not covered by any one of groups H01L24/00, H01L24/00 and H01L2224/00
• CPC Class Codes: H05B 45/325 Pulse-width modulation [PWM]
• CPC Class Codes: H05B 45/38 using boost topology
• CPC Class Codes: H05B 45/395 Linear regulators
• CPC Class Codes: H05B 45/46 having LEDs disposed in parallel lines
• CPC Class Codes: Y02B 20/30 Semiconductor lamps, e.g. solid state lamps [SSL] light emitting diodes [LED] or organic LED [OLED]

Abstract

An adaptive switch mode LED driver provides an intelligent approach to driving multiple strings of LEDs. The LED driver determines an optimal current level for each LED channel from a limited set of allowed currents. The LDO driver then determines a PWM duty cycle for driving the LEDs in each LED channel to provide precise brightness control over the LED channels. Beneficially, the LED driver minimizes the power dissipation in the LDO circuits driving each LED string, while also ensuring that the currents in each LED string are maintained within a limited range. A sample and hold LDO allows PWM control over extreme duty cycles with very fast dynamic response. Furthermore, fault protection circuitry ensures fault-free startup and operation of the LED driver.

Claims

1. A light-emitting diode (LED) driver for

driving one or more LED strings, the LED driver comprising:

a power converter configured to receive an input voltage and generate an output voltage applied to a first LED string;

a first channel switch configured to switch the first LED string on or off according to a first duty cycle signal applied to the first channel switch;

a first channel regulator configured to receive a first programmed current level signal and regulate current through the first LED string according to the first programmed current level signal; and

a luminance controller configured to generate the first programmed current level signal for the first LED string to correspond to one of a limited set of programmable current levels, and generating the first duty cycle signal for driving the first channel switch as a function of the first programmed current level signal,

wherein the first channel switch is coupled in between and in series with the first channel regulator and the first LED string.

3. The LED driver of claim 1, wherein the first channel switch is coupled in series with the first LED string.

4. The LED driver of claim 1, wherein the luminance controller generates the first programmed current level to be a highest one of the limited set of programmable current levels at which the current through the first LED string can be regulated by the first channel regulator under the output voltage applied to the first LED string.

5. The LED driver of claim 1, wherein generating the first duty cycle signal comprises:

determining a ratio of the first programmed current level signal to a baseline current level; and

multiplying the ratio by a baseline duty cycle.

6. The LED driver of claim 1, further comprising:

a second channel switch configured to switch a second LED string on or off according to a second duty cycle signal applied to the second channel switch, the second LED string having different current-voltage characteristics than the first LED string;

a second channel regulator configured to receive a second programmed current level signal from the luminance controller and regulate current through the second LED string according to the second programmed current level signal, wherein the first and second programmed current level signals correspond to different programmed current levels; and

wherein the luminance controller is further configured to generate the second programmed current level signal for the second LED string to correspond to another one of the limited set of programmable current levels, and to determine the second duty cycle signal for driving the second channel switch as a function of the second programmed current level signal.

7. The LED driver of claim 6, wherein the first and second LED strings are switched on or off according to different programmed current levels and different duty cycles but have substantially same average current levels.

13. The LED driver of claim 1, further comprising:

a feedback path transmitting a feedback signal from the first channel regulator representative of a sensed current through the first channel regulator to the luminance controller,

wherein the luminance controller generates the first programmed current level signal based in part on the feedback signal.

14. The LED driver of claim 1, wherein the luminance controller generates the first duty cycle signal as a function of the first programmed current level signal such that average current is substantially matched between each of the one or more LED strings configured for a same relative brightness with respect to a maximum brightness of each of the one or more LED strings.

15. The LED driver of claim 1, wherein the luminance controller determines the first duty cycle signal as a function of the first programmed current level signal based in part on a luminance transfer function such that luminous flux is substantially matched between at least some of the LED strings configured for a same relative brightness with respect to a maximum brightness of each of the one or more LED strings.

16. The LED driver of claim 15, wherein the luminance controller further comprises a temperature input receiving a temperature measurement, and wherein the luminance transfer function includes a temperature compensation function for compensating for temperature variations between the one or more LED strings.

2. A light-emitting diode (LED) driver for driving one or more LED strings, the LED driver comprising:

a power converter configured to receive an input voltage and generate an output voltage applied to a first LED string;

a first channel switch configured to switch the first LED string on or off according to a first duty cycle signal applied to the first channel switch;

a first channel regulator configured to receive a first programmed current level signal and regulate current through the first LED string according to the first programmed current level signal, wherein the first channel regulator comprises;

a pass transistor coupled in series with the first LED string and the first channel switch; and

a feedback loop configured to sense the current through the first LED string, compare the sensed current to an analog current reference, and control the pass transistor to reduce the sensed current responsive to the sensed current exceeding the analog current reference; and

a luminance controller configured to generate the first programmed current level signal for the first LED string to correspond to one of a limited set of programmable current levels, and generating the first duty cycle signal for driving the first channel switch as a function of the first programmed current level signal.

8. The LED driver of claim 2, wherein the feedback loop comprises:

a sense resistor coupled in series to a source of the pass transistor; and

an operational amplifier having a negative input terminal coupled to the source of the pass transistor, a positive input terminal coupled to receive the analog current reference, and an output terminal coupled to a gate of the pass transistor.

9. The LED driver of claim 8, further comprising:

a comparator configured to compare an output of the operational amplifier to a reference voltage for detecting when the sensed current exceeds the programmed current level signal and output a comparison signal to the luminance controller.

10. The LED driver of claim 9, wherein the luminance controller determines that an LED in the first LED string is shorted to ground responsive to the comparison signal indicating that the sensed current exceeds the programmed current level signal during normal operation of the LED driver following calibration.

11. The LED driver of claim 8, wherein the first channel regulator comprises a sample and hold regulator including:

a first sample and hold component for sampling an output voltage of the operational amplifier prior to the first channel switch turning off, and holding the output voltage of the operational amplifier until the first channel switch turns back on; and

a second sample and hold component for sampling a negative input terminal voltage of the operational amplifier prior to the first channel switch turning off, and holding the sampled negative input terminal voltage until the first channel switch turns back on.

12. The LED driver of claim 2, further comprising:

a digital-to-analog converter (DAC) configured to receive the programmed current level signal in digital form from the luminance controller and output the analog current reference to the first channel regulator.

17. A light-emitting diode (LED) driver for driving one or more LED strings, the LED driver comprising:

a power converter configured to receive an input voltage and generate an output voltage applied to a first LED string;

a first channel switch configured to switch the first LED string on or off according to a first duty cycle signal applied to the first channel switch;

a first channel regulator configured to receive a first programmed current level signal and regulate current through the first LED string according to the first programmed current level signal; and

a luminance controller configured to generate the first programmed current level signal for the first LED string to correspond to one of a limited set of programmable current levels, and generating the first duty cycle signal for driving the first channel switch as a function of the first programmed current level signal;

a fault detection circuit for detecting at least one of an open circuit and a short circuit in the one or more LED strings, and turning off the first channel switch to disable the first LED string responsive to detecting the open circuit or the short circuit, wherein the fault detection circuit comprises;

a first current source supplying a first test current to the first channel regulator during a short circuit detection test;

a first comparator comparing a test point voltage of the first channel regulator to a short threshold voltage, wherein the first comparator indicates the short circuit responsive to the test point voltage exceeding the short threshold voltage;

a second current source drawing a second test current from the first channel regulator during an open circuit detection test; and

a second comparator comparing the test point voltage to an open circuit threshold voltage, wherein the second comparator indicates the open circuit responsive to the test point voltage falling below the open circuit threshold voltage.

18. A method for driving a first LED string from a set of one or more LED strings in a power converter configured to receive an input voltage and generate an output voltage applied to the first LED string, the method comprising:

selecting, by a luminance controller, a first programmed current level for the first LED string from a limited set of programmable current levels;

regulating, by a first channel regulator, current through the first LED string according to the first programmed current level;

determining, by the luminance controller, a first duty cycle for switching the first LED string as a function of the first programmed current level; and

switching the first LED string on or off according to the first duty cycle by controlling switching of a first channel switch coupled in between and in series with the first channel regulator and the first LED string.

19. The method of claim 18, wherein selecting the first programmed current level for the first LED string comprises:

adjusting a supply voltage to the one or more LED strings such that a weakest string of the one or more LED strings operate at a baseline current; and

selecting the first programmed current level for the first LED string as corresponding to a highest programmable current level at which the first LED string maintains regulation of the programmed current level.

20. The method of claim 18, wherein selecting the first programmed current level for the first LED string comprises:

configuring each of the one or more LED strings to operate at a maximum current from the set of programmable current levels;

adjusting a supply voltage to the one or more LED strings such that each of the one or more LED strings operate at or above a baseline current;

sampling a sensed current through the first LED string;

rounding the sensed current down to a nearest one of the set of programmable current levels; and

storing the nearest one of the set of programmable current levels.

21. The method of claim 18, wherein selecting the programmed current level comprises:

configuring each of the one or more LED strings to operate at a minimum current from the set of programmable current levels;

adjusting a supply voltage to the one or more LED strings such that each of the one or more LED strings operate at or above a baseline current;

monitoring a feedback signal indicating whether or not a regulator regulating the first LED string is operating in saturation;

responsive to the feedback signal indicating that the regulator is operating in saturation, increasing the programmed current level for the first LED string; and

storing a highest programmed current level under which the regulator operates in saturation.

22. The method of claim 18, wherein determining the first duty cycle for switching the first LED string comprises:

determining the first duty cycle such that average currents substantially match through each of the one or more LED strings configured for a same relative brightness with respect to a maximum brightness of each of the one or more LED strings.

23. The method of claim 18, wherein determining the first duty cycle for switching the first LED string comprises:

determining the first duty cycle based in part on a luminance transfer function such that luminous flux outputted substantially matches from each of the one or more LED strings configured for a same relative brightness with respect to a maximum brightness.

24. The method of claim 23, wherein the luminance transfer function includes a temperature compensation function for compensating for temperature variations between each of the one or more LED strings.

25. A method for driving a first LED string from a set of one or more LED strings in a power converter configured to receive an input voltage and generate an output voltage applied to the first LED string, the method comprising:

selecting, by a luminance controller, a first programmed current level for the first LED string from a limited set of programmable current levels;

regulating, by a first channel regulator, current through the first LED string according to the first programmed current level, where regulating the current comprises;

sensing via a feedback loop, the current through the first LED string;

comparing the sensed current to an analog current reference; and

controlling a pass transistor to reduce the sensed current responsive to the sensed current exceeding the analog current reference, the pass transistor coupled in series with the first LED string and a first channel switch;

determining, by the luminance controller, a first duty cycle for switching the first LED string as a function of the first programmed current level; and

switching the LED string on or off according to the first duty cycle by controlling switching of the first channel switch.

26. A method for driving a first LED string from a set of one or more LED strings in a power converter configured to receive an input voltage and generate an output voltage applied to the first LED string, the method comprising:

selecting, by a luminance controller, a first programmed current level for the first LED string from a limited set of programmable current levels;

regulating, by a first channel regulator, current through the first LED string according to the first programmed current level;

determining, by the luminance controller, a first duty cycle for switching the first LED string as a function of the first programmed current level; and

switching the LED string on or off according to the first duty cycle by controlling switching of a first channel switch;

detecting, by a fault detection circuit, at least one of an open circuit and a short circuit in the one or more LED strings, and turning off the first channel switch to disable the first LED string responsive to detecting the open circuit or the short circuit, wherein detecting comprises;

supplying a first test current to the first channel regulator during a short circuit detection test;

comparing a test point voltage of the first channel regulator to a short threshold voltage,

indicating that a short circuit is detected responsive to the test point voltage exceeding the short threshold voltage;

drawing a second test current from the first channel regulator during an open circuit detection test;

comparing the test point voltage to an open circuit threshold voltage; and

indicating that the open circuit is detected responsive to the test point voltage falling below the open circuit threshold voltage.