# Patent

## Title
AC LED lighting systems and control methods efficiently providing operating voltage

## Abstract

A LED lighting system efficiently provides an operating voltage powering integrated circuits. A LED string has LEDs segregated into LED groups connected in series. A LED controller has channel nodes connected to the cathodes of the LED groups respectively, and an output node connected to a capacitor providing the operating voltage. The LED controller drains a channel current from a selected channel node among the channel node. The LED controller regulates the channel current to a channel target value corresponding to the selected channel node, and provides a portion of the channel current as a charging current to power and regulate the operating voltage.

## Background
BACKGROUND
The present disclosure relates generally to Light-Emitting Diode (LED) lighting systems, and more particularly to Alternating Current (AC) driven LED lighting systems and control methods that efficiently provide an operating voltage.
Light-Emitting Diodes or LEDs are increasingly being used for general lighting purposes. In one example, a set of LEDs is powered from an AC power source and the term “AC LED” is sometimes used to refer to such circuit. Concerns for AC LED lighting systems include manufacture cost, power efficiency, power factor, flicker, lifespan, etc.
FIG. 1 demonstrates an AC LED lighting system 100 in the art. The AC LED lighting system 100 employs full-wave rectifier 18 to rectify an AC voltage VAC and provide a DC input voltage VIN at an input power line IN and a ground voltage at a ground line GND, where the ground voltage is deemed to be 0 volt in this system. A string of LEDs are segregated into LED groups 201, 202, 203, and 204, each having one or more LEDs. An integrated circuit 102 performing as a LED controller has pins or channel nodes PIN1, PIN2, PIN3, and PIN4, connected to the cathodes of LED groups 201, 202, 203, and 204 respectively. Inside integrated circuit 102 are path switches SG1, SG2, SG3, and SG4, and a current controller 103 as well. When the input voltage VIN at the input power line IN increases, current controller 103 can adjust the conductivity of path switches SG1, SG2, SG3, and SG4, making more LED groups join to emit light. Operations of integrated circuit 102 have been exemplified in U.S. Pat. No. 7,708,172 and are omitted here for brevity.
There in FIG. 1 includes a low dropout linear regulator (LDO) 112, which drains current from input power line IN to charge capacitor COUT, so operating voltage VCC is built up at a power source line VCC for powering integrated circuit 102 or other integrated circuits, such as microcontroller units. The LDO 112 is power consuming, however. The voltage drop across the LDO 112 could be as high as several hundred volts, so the power consumed by the LDO 112 will become significant.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention can be more fully understood by the subsequent detailed description and examples with references made to the accompanying drawings, wherein:
FIG. 1 demonstrates an AC LED lighting system in the art;
FIG. 2 demonstrates an AC LED lighting system according to embodiments of the invention;
FIG. 3 demonstrates a LED driver in association with the current controller in FIG. 2;
FIG. 4 demonstrates another LED driver in association with the current controller in FIG. 2;
FIG. 5 demonstrates another AC LED lighting system according to embodiments of the invention; and
FIG. 6 demonstrates a LED driver and a LDO, in association with the current controller in FIG. 5.



DETAILED DESCRIPTION
The following embodiments are described in sufficient detail to enable those skilled in the art to make and use the invention. It is to be understood that other embodiments would be evident based on the present disclosure, and that improves or mechanical changes may be made without departing from the scope of the present invention.
In the following description, numerous specific details are given to provide a thorough understanding of the invention. However, it will be apparent that the invention may be practiced without these specific details. In order to avoid obscuring the present invention, some well-known configurations and process steps are not disclosed in detail.
FIG. 2 demonstrates an AC LED lighting system 200 according to embodiments of the invention. The AC LED lighting system 200 has a full-wave rectifier 18 to rectify a sinusoid AC input voltage VAC, and provides a rectified input voltage VIN at the input power line IN and a ground voltage at the ground line GND. The LED groups 201, 202, 203 and 204 together compose a LED string connected in series between the input power line IN and the ground line GND. FIG. 2 exemplifies a LED string with 4 LED groups, but other embodiments might have more or less LED groups to compose a LED string. The LED string in FIG. 2 is deemed to have a most upstream anode connected to the input power line IN and a most downstream cathode connected to channel node PIN4. Each LED group might have only one LED in some embodiments, or consist of several LEDs connected in parallel or in series, depending on its application. The LED group 201 is the most upstream LED group in FIG. 2 as its anode is connected to the rectified input voltage VIN, the highest voltage in the LED string. Analogously, the LED group 204 is the most downstream LED group in FIG. 2. A downstream LED group uses its anode to connect with the cathode of an upstream LED. LED currents ILED1-ILED4 denote the currents passing through LED groups 201-204, respectively.
LDO 201 drains current directly from input power line IN to charge capacitor COUT, so operating voltage VCC is built up at a power source line VCC for powering integrated circuit 202 or other integrated circuits, such as a micro control unit. It will become apparent later that LDO 201 charges capacitor COUT only during startup or when operating voltage VCC is very low. As LDO 201 conducts no current for most of time, it consumes very little or ignorable power.
An integrated circuit 202 performs as a LED controller, and has LED drivers LD1, LD2, LD3 and LD4, and a current controller 203. LED driver LD2 is a relatively upstream LED driver in view of LED driver LD3, and is also a relatively downstream LED driver in view of LED driver LD1, for example. Channel currents IPIN1-IPIN4 denote the currents entering the integrated circuit 202 via channel nodes PIN1-PIN4, respectively. Each of LED drivers LD1, LD2, LD3 and LD4 has an output node OUT commonly short to power source line VCC. As LED drivers LD1, LD2, LD3 and LD4 are similar or the same with each other, one of them might be detailed while the others are comprehensible based on the teaching of the detailed one.
Channel current IPIN1 for instance, flows into LED driver LD1, and splits into charging current IL1 and driving current IC1. Charging current IL1 goes to the output node OUT of LED driver LD1, charging the capacitor COUT, while driving current IC1 follows another path to the ground line GND. In one embodiment, LED drivers LD1 has a LDO using charging current IL1 to power and regulate operating voltage VCC, while the channel current IPIN1 is regulated to be a channel target value IACHL1 represented by target signal ICOM_L1.
FIG. 3 demonstrates a LED driver LDn in association with current controller 203, where n could be 1, 2, 3, or 4, meaning LED driver LDn might embody any one of the LED drivers LD1, LD2, LD3 and LD4 in FIG. 2. The current controller 203 provides target signals ICOM_Ln and ICOM_Cn to LED driver LDn, and receives current sense signals IsEN_Ln and ISEN_cn from LED driver LDn. LED driver LDn drains channel current IPINn and tries to regulate it to be the channel target value IAcHLn represented by target signals ICOM_Ln.
LED driver LDn includes current regulator LGn and LDO LRn, for providing driving current ICn and charging current ILn respectively, each originating from the channel current IPINn.
The LDO LRn has two error amplifiers EALDO and EALMT. Derivable from FIG. 3 in view of FIG. 2, the LDO LRn monitors operating voltage VCC via feedback voltage VFB at a feedback node FB to control LDO switch SWLDO. LDO switch SWLDO and diode DLDO are connected in series between node PINn and the capacitor COUT. If the operating voltage VCC is less than a target voltage VTAR_CC represented by the reference voltage VREF, error amplifiers EALDO turns ON LDO switch SWLDO to conduct charging current ILn as large as possible, where charging current ILE accordingly charges the capacitor COUT and increases the operating voltage VCC. The magnitude of the charging current ILn is limited, though. The error amplifier EALMT senses the charging current ILn via current sense signal ISEN_Ln, and makes the charging current ILn no more than channel target value IAcHLn represented by the target signal ICOM_Ln, because switch SW1 reduces the conductivity of LDO switch SWLDO when sense signal ISEN_Ln exceeds target signal ICOM_Ln.
Each switch in this specification could be embodied by a transistor, such as a BJT, a MOS transistor or a JFET.
The current regulator LGn has a channel switch SWCn and an error amplifier EAn. Derivable from FIG. 3, the current regulator LGn is configured for conducting and regulating the driving current ICn to be a supplementary target value IASUPn represented by target signal ICOM_Cn.
Sense signals ISEN_Ln and ISEN_cn, generated by sensing charging current ILn and driving current ICn respectively, are not limited to be generated from the locations specified in FIG. 3. As sense signals ISEN_Ln represents the magnitude of charging current ILn, it could be generated by sensing somewhere in the path connecting the LDO switch SWLDO and diode DLDO, for example.
The current controller 203 controls and provides target signals ICOM_Ln and ICOM_cn. The determination of target signal ICOM_Ln will be detailed later. Target signal ICOM_Cn is determined by the current sense signal ISEN_Ln and the channel target value IACNLn. The supplementary target value IASUPn represented by the target signal ICOM_Cn is equal to the channel target value IACNLn minus the charging current ILn. As the channel current IPINn is the combination of the charging current ILn and driving current ICn, and the driving current ICn is regulated to be the channel target value IACNLn minus the charging current ILn, the channel current IPINn is about regulated to be the channel target value IACNLn, represented by target signal ICOM_Ln.
In other words, the channel current IPINn can be regulated to the channel target value IACNLn, and meanwhile a portion of the channel current IPINn could be directed to be a charging current ILn for charging the capacitor COUT and regulating the operating voltage VCC.
The current controller 203 sends target signal ICOM_Ln to seemingly turn ON or OFF the LED driver LDn. If the channel target value IACNLn represented by target signal ICOM_Ln is 0 mA, the LED driver LDn seems to be turned OFF, because the channel current IPINn is going to be 0 mA. If the channel target value IACNLn is 50 mA, for example, the LED driver LDn seems to be turned ON, trying to regulate the channel current IPINn to be 50 mA. The channel current through a turned-ON LED driver might not be well regulated nevertheless and it depends on whether the voltage at the channel node connected to the turned-ON LED driver is high enough for regulation.
LDO LRN is capable of regulating the operating voltage VCC to the target voltage VTAR_CC only if the LED driver LDn is turned ON by the current controller 203. A turned-OFF LED driver LDn cannot regulate the operating voltage VCC because the charging current ILn will become zero.
The current controller 203 determines the channel target value IACNLn based on the current sense signals of the LED driver LDn and the neighboring, downstream LED driver LDn+1. An initial condition is supposed that the current controller 203 happens to turn ON the LED driver LDn and all the LED drivers relatively downstream to the LED driver LDn, i.e. LDn+1, LDn+2, etc., but turn OFF all the LED drivers relatively upstream to the LED driver LDn, i.e. LDn−1, LDn−2, etc., and the channel target value IACNLn is 50 mA. Meanwhile, the LED driver LDn is the most upstream one among the turned-ON LED drivers, so LED groups 201-20n are driven to illuminate together.
In one case that the channel current IPINn nevertheless is found unable to be regulated, or very below 50 mA, it implies the input voltage VIN is too low for LED driver LDn to generate the channel current IPINn with a magnitude of 50 mA. Based on the finding, the current controller 203 then further turns ON the LED driver LDn−1, which requires a lower input voltage VIN for regulation. Accordingly, LED driver LDn−1 now becomes the most upstream turned-ON LED driver, LED group 20n stops illuminating but LED groups 201-20n−1 continues.
In another case that the channel current IPINn is being well regulated to be the channel target value IACNLn of 50 mA, and the neighboring, downstream channel current IPINn+1 starts increasing from 0 mA, it implies that the input voltage VIN now becomes high enough for the downstream LED driver LDn+1 to regulate the downstream channel current IPINn+1. Accordingly, the current controller 203 then turns OFF the LED driver LDn (by setting the channel target value IACNLn 0 mA) and lets downstream LED driver LDn+1 kept ON. As a result, LED group 20n+1 joins LED groups 201-20n to illuminate.
FIG. 4 demonstrates another LED driver LDx in association with current controller 203, where x could be 1, 2 3, or 4, meaning LED driver LDx could embody any one of the LED drivers LD1, LD2, LD3 and LD4 in FIG. 2. Different from FIG. 3, where the channel node PINn is a common node connecting LDO switch SWLDO and channel switch SWcn, FIG. 4 has LDO switch SWLDO and channel switch SWCx connected in series between channel node PINx and the ground line GND. The operation of LED driver LDx in FIG. 4 is comprehensible based on the teaching of LED driver LDn in FIG. 3, and is omitted herein for brevity.
Please refer to FIG. 2 in view of FIG. 3 or 4. In one embodiment, LDO 201 is configured for regulating the operating voltage VCC to a target voltage, which for example is 4.5V, and the LDOs in LED drivers LD1-LD4 are all configured for regulating the operating voltage VCC to another target voltage, which is 5V for example. During a startup procedure when the operating voltage VCC is below 4.5V, LDOs 201 and LR1-LR4 all work together to pull up operating voltage VCC. When the operating voltage Vcc exceeds 4.5V, LDO 201 stops charging the capacitor COUT but at least one of LDOs LR1-LR4 continues regulating the operating voltage VCC to 5V. During a normal operation, the operating voltage VCC remains 5V and is powered by the LDO of the most upstream one among turned-ON LED drivers. For example, if LED drivers LD3 and LD4 are ON and LED drivers LD1 and LD2 OFF, then the LDO LR3 in LED driver LD3 is substantially in charge of regulating the operating voltage VCC to 5V while LDOs LR1 and LR2 are OFF and LDO LR4 hardly provides any charging current because of the too-low voltage at the channel node PIN4. As one LDO in LED drivers LD1-LD4, if turned ON, could power the operating voltage VCC by providing a charging current from a corresponding channel node, whose voltage is at least several forward voltages lower than input voltage VIN, the LDO in one LED driver works much more efficient than LDO 201 whose efficiency suffers due to the high voltage difference between input voltage VIN and operating voltage VCC.
Each of LED drivers LDn and LDx in FIGS. 3 and 4 includes a LDO, but this invention is not limited to however. In some embodiments of the invention, a LED driver might have no LDO. FIG. 5 demonstrates another AC LED lighting system 300 according to embodiments of the invention, where an integrated circuit 302 as a LED controller has LED drivers LDX1, LDX2, LDX3 and LDX4, a LDO LR0, and a current controller 303. Please note that each of LED drivers LDX1, LDX2, LDX3 and LDX4 in FIG. 5 has no LDO.
FIG. 6 demonstrates a LED driver LDXn and the LDO LR0, in association with current controller 303, where LED driver LDXn could embody any one of the LED drivers LDX1, LDX2, LDX3 and LDX4 in FIG. 5. Current controller 303 sends target signal ISEN_L0 to LDO LR0 and receives current sense signal ISEN_L0 from it. Current controller 303 further sends control signal Vin and target signal ICOM_L0 to LED driver LDXn and receives current sense signal ISEN_Cn from it.
LED driver LDXn is turned OFF by turning OFF selection switch SWLn and setting target signal ICOM_Cn to represent 0 mA. LED driver LDXn is turned ON by turning ON the selection switch SWLn via control signal VLn. As for the target signal ICOM_Cn, if LED driver LDXn is the most upstream one among the turned-ON LED drivers, then target signal ICOM_L0 is set to represent channel target value IACNLn, which is 50 mA for example, and the target signal ICOM_Cn is set to represent the channel target value IACNLn minus the current sense signal ISEN_L0, so as to regulate the channel current IPINn to the channel target value IACNLn. If LED driver LDXn is a turned-ON one but not the most upstream turned-ON one, then the target signal ICOM_Cn is set to represent 10 mA, for example, so current controller 303 could receive current sense signal ISEN_Cn to determine whether input voltage is high enough for further driving another LED group.
Please refer to FIG. 5 in view of FIG. 6. Analogous to the current controller 203 in FIG. 2, current controller 303 can further turn ON a neighboring, upstream LED driver if input voltage VIN falls and the channel current of the most upstream turned-ON LED driver almost diminishes. Similarly, current controller 303 can turn OFF the most upstream turned-ON LED driver and let a neighboring downstream LED driver take over if the channel current of the neighboring downstream LED driver increases to a certain level.
FIG. 5, similar with FIG. 2, is beneficial in efficiency of generating operating voltage VCC. Operating voltage VCC is regulated to 5 volts by LDO LR0 inside the integrated circuit 302, and LDO 201 normally does not power operating voltage VCC, voiding high power consumption. The charging current IL0 that LDO LR0 provides for regulation of operating voltage VCC is from the most upstream LED driver among the turned-ON ones, and has been utilized efficiently for driving at least one LED group.
While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.

## Claims

1. An LED controller for driving a LED string with LEDs connected in series to have anodes and cathodes, wherein a most upstream anode among the anodes is coupled to an input power line, comprising:
a first LED driver, coupled to a first cathode among the cathodes, for draining a first channel current from the first cathode, and providing a first charging current from the first channel current to charge a capacitor for powering an operating voltage power line with an operating voltage; and
a current controller for controlling the first LED driver;
wherein the current controller controls the first LED driver to regulate the first channel current to a first channel target value; and
wherein the first LED driver comprises a primary low dropout linear regulator (LDO) monitoring the operating voltage to control the first charging current and making the first charging current not more than the first channel target value.

2. The LED controller of claim 1, wherein the current controller provides a first target signal to the primary LDO, and the first target signal corresponds to the first channel target value.

3. The LED controller of claim 2, wherein the primary LDO comprises a LDO switch and a diode connected in series between the first cathode and the capacitor, and the first LED driver further comprises a first channel switch for conducting and regulating a first driving current from the first channel current.

4. The LED controller of claim 3, wherein the LDO switch is for regulating the first charging current, and the first cathode is a common node connecting the LDO switch and the first channel switch.

5. The LED controller of claim 3, wherein the LDO switch and the first channel switch are connected in series between the first cathode and a ground line, and the LDO switch is for regulating the first channel current.

6. The LED controller of claim 3, wherein the first LED driver comprises a first current regulator with the first channel switch, and the current controller provides to the first current regulator a second target signal corresponding to the first channel target value minus the first charging current.

7. The LED controller of claim 1, comprising:
a plurality of LED drivers, each coupled to a corresponding cathode, for draining a channel current from the corresponding cathode, and providing a charging current to charge the capacitor for powering the operating voltage power line with the operating voltage;
wherein the current controller controls the LED drivers;
wherein the charging current is sensed to control a driving current from the channel current, so as to make the channel current about a corresponding channel target value.

8. The LED controller of claim 7, wherein each LED driver comprises the primary LDO monitoring the operating voltage to control the charging current and making the charge current not more than the corresponding channel target value.

9. A LED lighting system, comprising:
the LED string and the LED controller of claim 1, wherein the primary LDO regulates the operating voltage to a first target voltage; and
a secondary LDO connected to the most upstream anode for charging the capacitor and regulating the operating voltage to a second target voltage less than the first target voltage.

10. A LED lighting system, comprising:
a LED string with LEDs segregated into LED groups connected in series, each LED group having a cathode and an anode;
a LED controller comprising:
a first LED driver, coupled to a first cathode among the cathodes, for draining a first channel current from the first cathode, and providing a first charging current from the first channel current to charge a capacitor for powering an operating voltage power line with an operating voltage; and
a current controller for controlling the first LED driver;
wherein the current controller controls the first LED driver to regulate the first channel current to a first channel target value; and
wherein the first LED driver comprises a primary low dropout linear regulator (LDO) monitoring the operating voltage to control the first charging current and making the first charging current not more than the first channel target value.

11. The LED lighting system of claim 10, wherein the current controller provides a first target signal to the primary LDO, and the first target signal corresponds to the first channel target value.

12. The LED lighting system of claim 11, wherein the primary LDO comprises a LDO switch and a diode connected in series between the first cathode and the capacitor, and the first LED driver further comprises a first channel switch for conducting and regulating a first driving current from the first channel current.

13. The LED lighting system of claim 12, wherein the LDO switch is for regulating the first charging current, and the first cathode is a common node connecting the LDO switch and the first channel switch.

14. The LED lighting system of claim 12, wherein the LDO switch and the first channel switch are connected in series between the first cathode and a ground line, and the LDO switch is for regulating the first channel current.

15. The LED lighting system of claim 12, wherein the first LED driver comprises a first current regulator with the first channel switch, and the current controller provides to the first current regulator a second target signal corresponding to the first channel target value minus the first charging current.

16. The LED lighting system of claim 10, comprising:
a plurality of LED drivers, each coupled to a corresponding cathode, for draining a channel current from the corresponding cathode, and providing a charging current to charge the capacitor for powering the operating voltage power line with the operating voltage;
wherein the current controller controls the LED drivers;
wherein the charging current is sensed to control a driving current from the channel current, so as to make the channel current about a corresponding channel target value.

17. The LED lighting system of claim 16, wherein each LED driver comprises the primary LDO monitoring the operating voltage to control the charging current and making the charge current not more than the corresponding channel target value.

18. The LED lighting system of claim 10, wherein the primary LDO regulates the operating voltage to a first target voltage, and the LED lighting system further comprises:
a secondary LDO connected to a most upstream anode for charging the capacitor and regulating the operating voltage to a second target voltage less than the first target voltage.