Adaptive switch mode LED driver

Patent metadata

Field	Value
Publication number	US-9451664-B2
Application number	US-201213675515-A
Country	US
Kind code	B2
Filing date	Nov 13, 2012
Priority date	Sep 11, 2009
Publication date	Sep 20, 2016
Grant date	Sep 20, 2016

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.

First claim

What is claimed is:

1. A light-emitting diode (LED) driver for driving one or more LED strings, the LED driver comprising:
a first channel switch in series with a first LED string of the one or more LED strings, the 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 coupled in series with the first LED string and the first channel switch, the first channel regulator configured to receive a first signal at a first input terminal of the first channel regulator and to regulate current through the first LED string, the first channel switch, and the first channel regulator according to the first signal, the current flowing from a second input terminal of the first channel regulator to an output terminal of the first channel regulator; and
a luminance controller configured to select a current level from a limited set of programmable current levels, to generate the first signal based on the selected current level from the limited set of programmable current levels to control the first channel regulator to regulate the current through the first LED string, the first channel switch and the first channel regulator, and to generate the first duty cycle signal for driving the first channel switch as a function of the selected current level.

2. The LED driver of claim 1 , 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 luminance controller generates the first signal based on the selected 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 an output voltage applied to the first LED string.

4. The LED driver of claim 1 , wherein generating the first duty cycle signal comprises:
determining a ratio of the selected current level to a baseline current level; and
multiplying the ratio by a baseline duty cycle.

5. The LED driver of claim 1 , further comprising:
a second channel switch configured to switch a second LED string of the one or more LED strings 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; and
a second channel regulator configured to receive a second signal from the luminance controller and regulate current through the second LED string according to the second signal, wherein the first and second signals correspond to different current levels,
wherein the luminance controller is further configured to select another current level from the limited set of programmable current levels, to generate the second signal based on the selected another current level from the limited set of programmable current levels to control the second channel regulator to regulate the current through the second LED string, the second channel switch and the second channel regulator, and to generate the second duty cycle signal for driving the second channel switch as a function of the selected another current level.

6. The LED driver of claim 5 , wherein the first and second LED strings are switched on or off according to different current levels and different duty cycles but have substantially same average current levels.

7. The LED driver of claim 1 , 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.

8. The LED driver of claim 7 , 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 selected current level 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 selected current level 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 7 , further comprising:
a digital-to-analog converter (DAC) configured to receive the first signal in digital form from the luminance controller and output the analog current reference to the first channel regulator.

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 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 selected current level 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 selected current level based in part on a luminance transfer function such that luminous flux is substantially matched between at least some 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.

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.

17. A light-emitting diode (LED) driver for driving one or more LED strings, the LED driver comprising:
a first channel switch configured to switch a first LED string on or off according to a first duty cycle signal applied to the first channel switch;
a first channel regulator coupled in series with the first LED string and the first channel switch, the first channel regulator configured to receive a first programmed current l

Assignees

• Dialog Semiconductor Inc

Inventors

• Jin Xuecheng
• Chang Yu Cheng
• Li Yang
• Lan Maofeng
• Kesterson John W
• Wang Xiaoyan
• Pan Chenghung

Classifications

• H10P74/207 Primary

  Electrical properties, e.g. testing or measuring of resistance, deep levels or capacitance-voltage characteristics · CPC title

• H01L2924/0002

  Electricity · mapped topic

• H05B33/0827

  Electricity · mapped topic

• H01L2924/00

  Electricity · mapped topic

• H05B33/0842 Primary

  Electricity · mapped topic