LED controller

Abstract

Methods and circuits for controlling one or more LEDs are disclosed. In one embodiment, a light emitting diode (LED) driver for driving one or more serially connected LEDs includes a voltage regulator circuit configured to receive a rectified AC voltage, where the voltage regulator circuit includes a depletion device configured to generate an unregulated voltage using the rectified AC voltage, a band gap voltage reference circuit configured to generate one or more reference voltages using the unregulated voltage, and a current setting circuit configured to control the one or more serially connected LEDs using the one or more reference voltages, where the current setting circuit is connected to a circuit ground through a current setting resistor having a fixed resistance value.

Description

FIELD OF THE INVENTION

The present invention relates to the field of electronics. In particular, the present invention relates to methods and circuits for controlling light emitting diodes (LEDs).

BACKGROUND OF THE INVENTION

Conventional LED controllers are typically powered by direct current. Because of this limitation, their applications are limited as they would be battery powered or would require conversion of power produced in other forms to direct current. To work with an alternating current power source, conventional LED controllers would require a power adaptor as a transformer, which increases the cost and limits the usage of LEDs. Therefore, there is a need for an improved LED controller that addresses the limitations of the conventional LED controllers.

LED drivers implemented with integrated circuits would have to deal with inherent variations in manufacturing processes, which can affect the accuracy of the currents used to drive the LEDs. A conventional approach is to use an adjustable current setting resistor external to the LED driver integrated circuit for adjusting the current during the manufacturing of the printed circuit board. There are a few drawbacks with this conventional approach. First, this approach adds an additional component to the printed circuit board, which increases the size and thus the cost of the printed circuit board. Second, this approach requires the adjustable current setting resistor to be fine-tuned during the process of manufacturing of the printed circuit board, which increases the manufacturing cost of such conventional LED drivers. Thus, there is a need for methods and circuits that can address such issues of the conventional LED drivers.

SUMMARY

Methods and circuits for controlling LEDs are disclosed. In one embodiment, a light emitting diode (LED) driver for driving one or more serially connected LEDs includes a voltage regulator circuit configured to receive a rectified AC voltage, where the voltage regulator circuit includes a depletion device configured to generate an unregulated voltage using the rectified AC voltage, a band gap voltage reference circuit configured to generate one or more reference voltages using the unregulated voltage, and a current setting circuit configured to control the one or more serially connected LEDs using the one or more reference voltages, where the current setting circuit is connected to a circuit ground through a current setting resistor having a fixed resistance value.

In another embodiment, method for controlling one or more serially connected LEDs includes receiving a rectified AC voltage by a voltage regulator circuit, where the voltage regulator circuit includes a depletion device configured to generate an unregulated voltage using the rectified AC voltage, generating one or more reference voltages by a band gap voltage reference circuit using the unregulated voltage, connecting a current setting circuit to a circuit ground through a current setting resistor having a fixed resistance value; and controlling the one or more serially connected LEDs by the current setting circuit using the one or more reference voltages.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned features and advantages of the invention, as well as additional features and advantages thereof, will be more clearly understandable after reading detailed descriptions of embodiments of the invention in conjunction with the following drawings.

FIG. 1 illustrates a block diagram of a LED controller according to embodiments of the present invention.

FIG. 2 illustrates an application of the LED controller of FIG. 1 according to an embodiment of the present invention.

FIG. 3 illustrates another application of the LED controller of FIG. 1 according to an embodiment of the present invention.

FIG. 4 illustrates yet another application of the LED controller of FIG. 1 according to an embodiment of the present invention.

FIG. 5 illustrates an exemplary implementation of the disclosed LED controller according to embodiments of the present invention.

FIG. 6 illustrates an exemplary implementation of band gap voltage reference circuit of FIG. 5 according to embodiments of the present invention.

FIG. 7 illustrates an exemplary implementation of current setting circuit of FIG. 5 according to embodiments of the present invention.

FIG. 8 illustrates an exemplary implementation of a multi-stage LED driver according to aspects of the present disclosure.

FIG. 9 illustrates an exemplary implementation of a multi-stage LED driver of FIG. 8 with a TRIAC dimmer circuit according to aspects of the present disclosure.

FIG. 10 illustrates another exemplary implementation of a multi-stage LED driver according to aspects of the present disclosure.

FIG. 11 illustrates an exemplary implementation of a multi-stage LED driver of FIG. 10 with a TRIAC dimmer circuit according to aspects of the present disclosure.

FIG. 12A illustrates another exemplary implementation of a multi-stage LED driver according to aspects of the present disclosure.

FIG. 12B illustrates an exemplary implementation of a bandgap circuit reference voltage circuit according to aspects of the present disclosure.

FIG. 12C illustrates another exemplary implementation of a bandgap circuit reference voltage circuit according to aspects of the present disclosure.

FIG. 13A illustrates an exemplary implementation of a first adjustable load of a bandgap circuit and FIG. 13B illustrates an exemplary implementation of a second adjustable load of the bandgap circuit according to aspects of the present disclosure.

FIG. 14A illustrates an exemplary implementation of fuse units of the first adjustable load of FIG. 13A and FIG. 14B illustrates an exemplary implementation of fuse units of the second adjustable load of FIG. 13B according to aspects of the present disclosure.

FIG. 15A illustrates a method of controlling one or more serially connected LEDs according to aspects of the present disclosure.

FIG. 15B illustrates a method of generating one or more reference voltages according to aspects of the present disclosure.

FIG. 16A illustrates an exemplary implementation of providing a first adjustable load according to aspects of the present disclosure; FIG. 16B illustrates an exemplary implementation of providing a second adjustable load according to aspects of the present disclosure.

Like numbers are used throughout the specification.

DESCRIPTION OF EMBODIMENTS

Methods and circuits are provided for controlling LEDs. The following descriptions are presented to enable any person skilled in the art to make and use the invention. Descriptions of specific embodiments and applications are provided only as examples. Various modifications and combinations of the examples described herein will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other examples and applications without departing from the spirit and scope of the invention. Thus, the present invention is not intended to be limited to the examples described and shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.

Some portions of the detailed description that follows are presented in terms of flowcharts, logic blocks, and other symbolic representations of operations on information that can be performed on a computer system. A procedure, computer-executed step, logic block, process, etc., is here conceived to be a self-consistent sequence of one or more steps or instructions leading to a desired result. The steps are those utilizing physical manipulations of physical quantities. These quantities can take the form of electrical, magnetic, or radio signals capable of being stored, transferred, combined, compared, and otherwise manipulated in a computer system. These signals may be referred to at times as bits, values, elements, symbols, characters, terms, numbers, or the like. Each step may be performed by hardware, software, firmware, or combinations thereof.

FIG. 1 illustrates a block diagram of a LED controller according to embodiments of the present invention. As shown in FIG. 1, the LED controller 100 includes an overshoot voltage protection (OVP) unit 102, a LED current select unit 104, a pulse width modulation dimming unit 106, and a thermal control unit 108. The overshoot voltage protection unit 102 is coupled to a mode pin 103; the LED current select unit 104 is coupled to a current select pin (ISEL) 105; and the pulse width modulation unit 106 is coupled to a dimming control (DIM) pin 107.

The LED controller further includes a voltage regulator (and reference) unit 110, and a current regulator (and LED sequencer) unit 112, a top LED driver unit 114, a center LED driver unit 116, and a bottom LED driver unit 118. The top LED driver unit is coupled to a TOP pin 115; the center LED driver unit 116 is coupled to a center (CNTR) pin 117; and the bottom LED driver unit 118 is coupled to a bottom (BOTM) pin 119. The voltage regulator 110 is coupled to the overshoot voltage protection unit 102, the current regulator unit 112, and the TOP pin 115. The current regulator is coupled to the overshoot voltage protection unit 102, the LED current select unit 104, the pulse width modulation dimming unit 106, the thermal control unit 108, the top LED driver unit 114, the center LED driver unit 116, and the bottom LED driver unit 118. The current regulator 112 is also coupled to an analog ground (AGND) pin 109. The top LED driver unit 114, the center LED driver unit 116, and the bottom LED driver unit 118 are coupled to a digital ground (GND) pin 121.

The LED controller 100 supports both conventional TRIAC dimming, and pulse width modulation dimming. Built-in thermal regulation mechanism may be employed to linearly reduce the LED current when the driver's junction temperature exceeds a preprogrammed temperature, such as 100° C. They may also be configured to shut-down when the junction temperature reaches preprogrammed temperature, for example 150° C., to prevent the system from thermal runaway. The mode pin sets the driver's operating voltage to 110V AC or 220V AC environment. It protects the system from being damaged when power is applied incorrectly. The drivers can withstand up to 400 volts between the TOP and GND pins. It consumes about 150 uA of quiescent current.

FIG. 2 illustrates an application of the LED controller of FIG. 1 according to an embodiment of the present invention. In this exemplary implementation, the LED controller 100 is used to control a series of LEDs. In particular, the top pin 115, the center pin 117, and the bottom pin 119 of the LED controller 100 are coupled to output of LEDs 202, 204, and 206 respectively, in the series of LEDs. The digital ground pin 121 of the LED controller 100 is coupled to a rectifier 208 configured with four diodes. The rectifier may be configured to receive power from either 110V AC or 220V AC. The rectifier is also coupled to the top of the series of LEDs being controlled.

The LED controller 100 supports a series (also referred to as a string) of LEDs operating at a current of 30 mA. It may sink 30 mA of constant current and sequentially turns on/off the LED string sequentially according to pre-determined input voltages. The string of LEDs light up in the order of top, center, then bottom, and shut off in the reverse order when the LED string is powered directly from a full-wave rectifier off an AC line. The programmable LED current provides user the flexibility to adjust the LED current within a +/−10% range.

FIG. 3 illustrates another application of the LED controller of FIG. 1 according to an embodiment of the present invention. In this exemplary implementation, the LED controller 100 is used to control three parallel strings of LEDs. In particular, the top pin 115 is coupled to output of LEDs 302a, 302b, and 302c from each of the three strings of LEDs. The center pin 117 is coupled to output of LEDs 304a, 304b, and 304c from each of the three strings of LEDs. The bottom pin 119 is coupled to output of LEDs 306a, 306b, and 306c from each of the three strings of LEDs as shown in FIG. 3. The digital ground pin 121 of the LED controller 100 is coupled to a rectifier 308 configured with four diodes. The rectifier 308 may be configured to receive power from either 110V AC or 220V AC. The rectifier 308 is also coupled to the top LED of each of the string of LEDs being controlled. This application supports three strings of LEDs with each string of LEDs operating at a current of about 30 mA, thus providing a combined output current of about 90 mA. Note that the LED controller 100 may deliver up to about 100 mA of LED current, and may be used for high power applications.

FIG. 4 illustrates yet another application of the LED controller of FIG. 1 according to an embodiment of the present invention. The setup of this example is similar to that of the FIG. 2, except that a pulse width modulation signal is coupled to the DIM pin 107 of the LED controller 100. By using the pulse width modulation signal applied, the LED controller 100 is configured to control the dimming (or brightness) of the series of LEDs. In particular, the top pin 115 is coupled to output of LED 402. The center pin 117 is coupled to output of LED 404. The bottom pin 119 is coupled to output of LED 406. The digital ground pin 121 of the LED controller 100 is coupled to a rectifier 408 configured with four diodes as shown in FIG. 4. The rectifier 408 may be configured to receive power from either 120V AC or 240V AC. The rectifier 408 is also coupled to the top LED 402.

The following table lists pin definition of the LED controller according to embodiments of the present invention.

Pin No.	Pin Name	Pin Descriptions
1	ISEL	Select LED current, HIGH: +10%, LOW: −10%
2	CNTR	Cathode of the center LED
3	BOTM	Cathode of the bottom LED
4	DIM	PWM imming control input
5	MODE	Low: For 110 V, Open: For 220 V
6	GND	Power Ground
7	TOP	Cathode of the top LED
8	AGND	Analog Ground

The LED controller 100 may be implemented in a package of a SOP-8 exposed pad. Specifically, the current select (ISEL) pin 105 is assigned to pin 1; the center pin 117 is assigned to pin 2; the bottom pin 119 is assigned to pin 3, the dim pin 107 is assigned to pin 4, the mode pin 103 is assigned to pin 5; the digital ground (GND) pin 121 is assigned to pin 6; the top pin 115 is assigned to pin 7; and the analog ground (AGND) pin 109 is assigned to pin 8. A digital ground is applied to the center of the package, as shown with the dotted rectangle.

The following table lists exemplary electrical specifications of the LED controller.

TJ = 25° C., unless otherwise specified
	Test
Parameter	Conditons	Symbol	Min	Typ	Max	Unit
Operating Voltage		VTOP	5		400	V
Quietsent Current		IQ		150		uA
LED Current		ILEDx		30		mA
(MIKxxxx)
LED Current		ILEDy		100		mA
(MIKyyyy)
LED Current			−10		10	%
Accuracy
LED Current		IADJ	−10		10	%
Adjustment Range
Thermal Regulation		TTR		100		° C.
Onset
Thermal Regulation		ITR		−2		%/° C.
Thermal Shut-down		TOTP		150		° C.
Temperature
PWM Dimming		VPWM	0		1.5	V
Pulse Amplitude
Pull-down Current		IMODE		1		uA
of MODE Pin

According to embodiments of the present invention, input voltage may be up to 400 volts (V). Operating ambient temperature range may be from −40° C. to 85° C. Operating junction temperature may be up to 150° C. Storage temperature may be from −65° C. to 150° C. Lead temperature may be up to 260° C. Thermal resistance junction to ambient may be up to 60° C./W.

FIG. 5 illustrates an exemplary implementation of the LED integrated circuit controller according to embodiments of the present invention. In the example shown in FIG. 5, the LED integrated circuit control includes an input depletion device M1, an output depletion device M2, a band gap voltage reference circuit block 502, a current setting circuit block 504 (also referred to as current regulator circuit), an amplifier circuit 506, an electrostatic discharge (ESD) protection device 508, and metal oxide semiconductor field effect transistors (MOSFETs) M3, M4, M5, and M6 connected as shown in FIG. 5. According to embodiments of the present invention, the depletion devices M1 and M2 can be high voltage devices capable of handling high AC voltages such as 110V AC or 220V AC. On the other hand, the MOSFETs M3, M4, M5, and M6 can be low voltage devices with operating voltage less than 5V. The amplifier 506, together with MOSFETs M3, M4, M5, M6, and depletion device M2 performs the function of current multiplication as shown with the dotted box 510. The depletion device M1 is configured to receive a varying AC voltage and it in turn generates an unregulated voltage, for example about 7V. The depletion device M2 is configured to protect the LED integrated circuit controller from external high voltages. The band gap reference voltage circuit 502 is configured to received the unregulated voltage from the depletion device M1 and it generate a substantially constant direct current (DC) voltage (for example 5V with a range of deviation from 1% to 5% depending on design and manufacturing process variations) to be used by the current setting circuit block 504, which is also referred to as the current regulator circuit. The current setting circuit block 504 is configured to provide a substantially constant direct current, using the substantially constant DC voltage generated by the band gap reference voltage circuit 502, to the current multiplication block 510, which in turn drives a series of light emitting diodes.

FIG. 6 illustrates an exemplary implementation of band gap voltage reference circuit of FIG. 5 according to embodiments of the present invention. In this exemplary implementation, the band gap voltage reference circuit includes MOSFETs, bipolar FETs, invertors, resistors, capacitors, level shifter circuit (LS), and resistors blocks RB1, RB2, RB3, RB4, and RB5 as shown in FIG. 6. According to embodiments of the present invention, the band gap voltage reference circuit 502 is a temperature independent voltage reference circuit implemented in integrated circuits, with an output voltage around 1.25 V, which is close to the theoretical 1.22 eV band gap of silicon at 0 K. According to embodiments of the present invention, voltage difference between two p-n junctions (for example diodes), operated at different current densities, can be used to generate a proportional to absolute temperature (PTAT) current in a first resistor. This current is then used to generate a voltage in a second resistor. This voltage in turn is added to the voltage of one of the junctions. The voltage across a diode operated at constant current, or here with a PTAT current, is complementary to absolute temperature (CTAT), with approx. −2 mV/K. The ratio between the first and second resistor is chosen such that the first order effects of the temperature dependency of the diode and the PTAT current can cancel out. The resulting voltage is about 1.2-1.3 V, depending on the particular technology and circuit design, and is close to the theoretical 1.22 eV band gap of silicon at 0 K. The remaining voltage change over the operating temperature of integrated circuits is on the order of a few millivolts. This temperature dependency can have a parabolic behavior.

Since the output voltage is fixed around 1.25 V for typical band gap reference circuits, the minimum operating voltage can be about 1.4 V, as in a CMOS circuit at least one drain-source voltage of a FET (field effect transistor) has to be added. Therefore, in one approach, currents are summed instead of voltages, resulting in a lower theoretical limit for the operating voltage.

FIG. 7 illustrates an exemplary implementation of current setting circuit of FIG. 5 according to embodiments of the present invention. In this example, the current setting circuit 504 includes circuits for current mode detection 702, which in turn includes circuits for providing programmable control of current 704, and circuits for detecting signal output to the band gap voltage reference circuit 502. At the circuit components level, the current setting circuit 504 includes amplifiers 708 (A1) and 712 (A2), inverters 710 and 714, capacitors, and multiple MOSFETs as shown in FIG. 7. One or more external resistor(s) 720 may be coupled to the current setting circuit 504 to perform the functions of 1) mode detection in the case of adjustable current mode, and 2) programming the current level in accordance with whether the one or more external resistor(s) exists, and the resistance value of the one or more external resistor(s). The circuit block 704 is configured to generate a stable, controllable, and programmable current.

There are numerous benefits with the disclosed LED controller. First, it can operate with either 110V AC or 220V AC power source, which enables the LED controller to be used in a wide range of applications. The LED controller is able to turn on/off a series of LEDs sequentially. It supports user programmable LED current, as well as both a triode alternating current (TRIAC) and pulse width modulation (PWM) dimming architectures. Furthermore, it performs thermal regulation with built-in thermal detection and thermal shut-down capabilities. The LED controller may withstand up to 400 volts input voltage, with input overshoot voltage protection. It operates with low quiescent current and high efficiency.

FIG. 8 illustrates an exemplary implementation of a multi-stage LED driver according to aspects of the present disclosure. As shown in FIG. 8, U₀ may be a full bridge rectifier. LED₁, LED₂ . . . LED_N−1, and LED_N may be a sequence of LEDs connected in series. M₁, M₂ . . . M_N−1, and M_N may be high voltage N-channel MOSFETs. M_L may be a low voltage N-channel MOSFET. According to aspects of the present disclosure, P-channel MOSFETs or other types of transistors may be used in place of the N-channel MOSFETs. U₁, U₂ . . . U_N−1, and U_N may be current sensing comparators. R₁, R₂ . . . R_N−1, and R_N may be one or more current sensing resistors. R_CS may be one or more current setting resistors. U_L may be a current setting amplifier. Ref₁, Ref₂ . . . Ref_N−1, and Ref_N may be reference voltages generated by U_R from a band gap voltage reference circuit. Note that the gate terminal of M_N may be connected to an unregulated voltage as described above in association with FIG. 5. In a particular implementation, Ref₁, Ref₂ . . . Ref_N−1, and Ref_N may be related to each other in a sequentially incrementing manner, such as Ref₁<Ref₂ . . . <Ref_N−1<Ref_N. Us may be a sensing control circuit configured to select one of Ref₁, Ref₂ . . . Ref_N−1, and Ref_N as an output of the sensing control circuit. In one particular implementation, U_S may be a multiplexor whose output may be equal to one of the input Ref₁, Ref₂ . . . Ref_N−1 or Ref_N based on the output of U₁, U₂ . . . U_N−1, and U_N. In the particular implementation, the default output can be set to Ref₁. Note that Ref₁, Ref₂ . . . Ref_N−1, and Ref_N may be generated by the band gap reference circuit as described above in association with FIG. 5 and FIG. 6, which can be independent of voltage and temperature variation. In one particular implementation, after a reference voltage is generated by the band gap reference circuit, once the bandgap is generated, all other reference voltage can be generated by resistor divider. For example, a string of unit resistors can be connected from the band gap reference voltage and the ground and Ref₁ to Ref_N can be generated by tapping to different point along the string of unit resistors.

In this exemplary implementation, when a rectified AC voltage is applied, LED₁ may first be turned on while LED₂ . . . LED_N−1, and LED_N remain off. With LED₁ being turned on, current may pass through LED₁, M₁, M_L, and R_CS, and the current may be approximately equal to Ref₁/R_CS. As current passes through M₁, U₁ can sense the current passing through R₁ and outputs a signal to cause the output of the sensing control circuit U_S to select Ref₁. In some implementations, U₁, and R₁ may be bypassed or removed (as indicated by the dotted lines), when the default reference voltage V_Ref is selected to be Ref₁.

As the rectified AC voltage continues to increase, LED₂ may be turned on while LED₃ . . . LED_N−1, and LED_N remain off. With LED₂ being turned on, current may pass through LED₁, LED₂, M₂, M_L, and R_CS, and the current may be approximately equal to Ref₂/R_CS. As current passes through M₂, U₂ can sense the current passing through R₂ and outputs a signal to turn off M₁ and change the output of the sensing control circuit U_S to select Ref₂.

As the rectified AC voltage continues to increase, the process described above may be repeated, as each of the LED and its corresponding channel may be turned on sequentially. For example, LED_N−1 may be turned on while LED_N remain off. With LED_N−1 being turned on, current may pass through LED₁, LED₂ . . . LED_N−1, M_N−1, M_L, and R_CS, and the current may be approximately equal to Ref_N−1/R_CS. As current passes through M_N−1, U_N−1 can sense the current passing through R_N−1 and outputs a signal to turn off the previous stage (M_N−2 not shown) and change the output of the sensing control circuit U_S to select Ref_N−1. In addition, as the rectified AC voltage continues to increase, LED_N may be turned on. With LED_N being turned on, current may pass through LED₁, LED₂ . . . LED_N−1, LED_N, M_N, M_L, and R_CS, and the current may be approximately equal to Ref_N/R_CS. As current passes through M_N, U_N can sense the current passing through R_N and outputs a signal to turn off M_N−1 and change the output of the sensing control circuit U_S to select Ref_N.

After the rectified AC voltage reaches a peak value, it may start to decrease. As the rectified AC voltage decreases, the current passing through LED_N may be reduced to a certain level and may eventually turn off LED_N. The current sensing comparator U_N can sense the current reduction in R_N and outputs a signal to change the output of the sensing control circuit U_S to select Ref_N−1 and may cause M_N−1 to be turned on. This may cause the current to pass through LED₁, LED₂ . . . LED_N−1, M_N−1, M_L, and R_CS, and the current may be approximately equal to Ref_N−1/R_CS. As the rectified AC voltage continues to decrease, the process described above may repeat, as each of the LED and its corresponding channel may be turned off sequentially until LED₁ turns off due to insufficient voltage applied to LED₁.

In the exemplary implementation of FIG. 8, as the magnitude of rectified AC voltage goes through the cycles of increasing and decreasing, the multi-stage LED driver circuit of FIG. 8 may be configured to turn on the plurality of LEDs sequentially from LED₁ to LED_N as the rectified AC voltage increases, and to turn off the plurality of LEDs sequentially from LED_N to LED₁ as the rectified AC voltage decreases. As a result, the current consumption of the plurality of LEDs may be matched to the profile of the rectified AC voltage applied, and the energy usage may be optimized.

FIG. 9 illustrates an exemplary implementation of a multi-stage LED driver of FIG. 8 with a TRIAC dimmer circuit according to aspects of the present disclosure. In the example shown in FIG. 9, majority of the components are the same as that of FIG. 8 except M_T is added to the circuit of FIG. 8. The operation of this circuit is also substantially the same as that of FIG. 8.

The TRIAC dimmer needs some current in order to maintain a conduction mode. A minimum current needed for the TRIAC dimmer to maintain the conduction mode is call the holding current. According to aspects of the present disclosure, M_T is added so that holding current condition is met when LED₁, LED₂ . . . LED_N−1, and LED_N are turned off.

FIG. 10 illustrates another exemplary implementation of a multi-stage LED driver according to aspects of the present disclosure. In the exemplary implementation shown in FIG. 10, U₀ may be an external full wave bridge rectifier. LED₁, LED₂ . . . LED_N−1, and LED_N may be an external sequence of LEDs connected in series and being driven by a multistage LED driver circuit. M₁, M₂ . . . M_N−1, and M_N may be high voltage MOSFETs. U₁, U₂ . . . U_N−1, and U_N may operate as error amplifiers. R_CS may be one or more current setting resistors. Ref₁, Ref₂ . . . Ref_N−1, and Ref_N may be reference voltages generated by U_R from a band gap reference voltage. In some implementations, Ref₁, Ref₂ . . . Ref_N−1, and Ref_N may be related to each other in a sequentially incrementing manner, such as Ref₁<Ref₂ . . . <Ref_N−1<Ref_N, with Ref₁ being the lowest reference voltage and Ref_N being the highest reference voltage.

In this exemplary implementation, when an AC voltage is applied to the input of the rectifier U₀, the voltage at the output of the rectifier starts to increase. At some point, the voltage across LED₁ is sufficient to turn on LED₁ and a current through the LED₁ flows through M₁ and R_CS. LED₂ . . . LED_N−1 and LED_N are off at this point since the rectified voltage is not sufficient to turn them on. U₁, M₁ and R_CS together with a reference voltage Ref₁ form a current setting look that sets the current to V_CS/R_CS, which may be approximately equal to Ref₁/R_CS at this point.

As the rectified AC voltage continues to increase, LED₂ may gain enough voltage across it and LED₂ may be turned on. As a result, current may flow through M₂ and R_CS. The error amplifier U₂ may force V_CS to increase from Ref₁ to Ref₂. As V_CS increases, it may be greater than Ref₁, since Ref₂ is greater than Ref₁. Since the negative input of the error amplifier U₁ is higher than its positive input, the output of error amplifier U₁ swing low and causing M₁ to be turned off. Current may then flow through LED₁, LED₂, M₂ and R_CS.

The process may be repeated for the rest of the LEDs. For example, when LED_N−1 gains enough voltage to turn on LED_N−1, M_N−1 may be turned on and the previous stage (M_N−2, not shown) may be turned off. Similarly, when M_N is turned on, M_N−1 may be turned off.

When the rectified AC voltage reaches the peak, it may start to decrease. When the rectified AC voltage decreases to certain point, where LED_N may not have enough voltage to be fully turned on, and current in M_N may reduce and V_CS may also be reduced due to insufficient current flowing through R_CS. At some point, LED_N would be turned off, and V_CS would be reduced to Ref_N−1 level. When V_CS drops below Ref_N−1, M_N would be turned on and V_CS would be regulated to Ref_N−1 by U_N−1. Current may then flow through LED₁, LED₂ . . . LED_N−1, M_N−1 and R_CS.

As the rectified AC voltage continues to decrease, LED_N−1 may start to turn off and V_CS may be further reduced due to insufficient current flowing through R_CS. When LED_N−1 is off, V_CS may be dropped to Ref₂ level, and when V_CS drops below Ref₂, M₂ may start to conduct current at a level of Ref₂/R_CS, current path may then be flowing through LED₁, LED₂, M₂ and R_CS. As the rectified AC voltage continues to decrease, LED₂ and LED₁ may also be turned off, for the similar reasons described above between the stage N and N−1.

In the exemplary implementation of FIG. 8, as the magnitude of rectified AC voltage goes through the cycles of increasing and decreasing, the multi-stage LED driver circuit of FIG. 10 may be configured to turn on the plurality of LEDs sequentially from LED₁ to LED_N as the rectified AC voltage increases, and to turn off the plurality of LEDs sequentially from LED_N to LED₁ as the rectified AC voltage decreases. As a result, the current consumption of the plurality of LEDs may be matched to the profile of the rectified AC voltage applied, and the energy usage may be optimized.

FIG. 11 illustrates an exemplary implementation of a multi-stage LED driver of FIG. 10 with a TRIAC dimmer circuit according to aspects of the present disclosure. As shown in the exemplary implementation of FIG. 11, majority of the components are the same as that of FIG. 10 except M_T and U_T have been added to the circuit of FIG. 10. The operation of this circuit may also be substantially similar to that of FIG. 10.

The TRIAC dimmer may require some current in order to maintain a conduction mode. A minimum current needed for the TRIAC dimmer to maintain the conduction mode is call the holding current. When the rectified AC voltage may be low, all LEDs may be turned off and no holing current can be provided to the TRIAC dimmer by the LEDs. According to aspects of the present disclosure, M_T and U_T can be added so that holding current condition can be met with Ref₀ when LED₁, LED₂ . . . LED_N−1, and LED_N are turned off.

FIG. 12 illustrates an exemplary implementation of a LED driver according to aspects of the present disclosure. In the exemplary implementation shown in FIG. 12, U₀ may be an external full wave bridge rectifier. LED_L represents an external LED connected in series and being driven by the LED driver circuit. M_L represents a high voltage MOSFET. U_L may operate as an error amplifier. R_CS may be one or more current setting resistors. Ref_L is a reference voltage generated by a band gap reference voltage circuit U_R.

According to aspects of the present disclosure, the resistance value of R_CS may be fixed. In addition, the current setting resistor R_CS, the voltage regulator circuit U₀, the band gap voltage reference circuit U_R and the current setting circuit (U_L and M_L) may reside within an integrated circuit.

In this exemplary implementation, when an AC voltage is applied to the input of the rectifier U₀, the voltage at the output of the rectifier starts to increase. At some point, the voltage across LED_L is sufficient to turn on LED_L and a current through the LED_L flows through M_L and R_CS. U_L, M_L and R_CS together with a reference voltage Ref_L form a current setting loop that sets the current to V_CS/R_CS, which may be approximately equal to Ref_L/R_CS at this point.

When the rectified AC voltage reaches the peak, it may start to decrease. When the rectified AC voltage decreases to certain point, where LED_L may not have enough voltage to be fully turned on, and current in M_L may reduce and V_CS may also be reduced due to insufficient current flowing through R_CS. At some point, LED_L would be turned off.

In the exemplary implementation of FIG. 12, as the magnitude of rectified AC voltage goes through the cycles of increasing and decreasing, the LED driver circuit may be configured to turn on LED_L as the rectified AC voltage increases, and to turn off the LED_L as the rectified AC voltage decreases. As a result, the current consumption of the plurality of LEDs may be matched to the profile of the rectified AC voltage applied, and the energy usage may be optimized.

FIG. 12B illustrates an exemplary implementation of a band gap reference voltage circuit according to aspects of the present disclosure. As shown in FIG. 12B, the band gap reference voltage circuit U_R includes a first adjustable load L₁ and a second adjustable load L₂. The reference voltage Ref_L may be adjusted by the first adjustable load L₁ and the second adjustable load L₂. In some implementations, referring to FIG. 12A, the voltage at the source terminal of M_L, the positive differential input of U_L, the output of the band gap reference voltage circuit U_R, and the input of band gap reference voltage circuit U_R (also referred to as the unregulated voltage) can be measured. Based on the above measured values, the first adjustable load L₁ and the second adjustable load L₂ may be modified to produce a desired output voltage at the band gap reference voltage circuit U_R, which in turn may produce a desired voltage at the positive differential input of U_L, and at the source terminal of M_L. The reference voltage Ref_L may be computed using the unregulated voltage multiplied by a ratio of the first adjustable load and the sum of the first adjustable load and the second adjustable load. As a result, a desired V_CS may be generated. Along with a predetermined R_CS, a desired current may be controlled to flow through the LED_L. The adjustment of is further described in association of FIGS. 13A-13B and FIGS. 14A-14B in the following sections.

FIG. 12C illustrates another exemplary bandgap circuit reference voltage circuit according to aspects of the present disclosure. In the example shown in FIG. 12C, the band gap reference voltage circuit U_R includes a plurality of adjustable loads, including a first adjustable load L₁, a second adjustable load L₂, so on to the N+1th adjustable load L_N+1. The corresponding output reference voltages from the bandgap circuit reference voltage circuit, including Ref₁, Ref₂ . . . to Ref_N, may be adjusted by the first adjustable load L₁, the second adjustable load L₂ . . . , all the way to the N+1th adjustable load L_N+1. In some implementations, referring to FIG. 10, the voltage at the source terminals V_CS, the positive differential inputs of error amplifiers U₁, U₂ . . . U_N−1, and U_N, the output of the band gap reference voltage circuit U_R, and the input of band gap reference voltage circuit U_R can be measured. Based on the above measured values, the first adjustable load L₁, the second adjustable load L₂ . . . , and the N+1th adjustable load L_N+1 may be modified to produce a desired output voltage at the band gap reference voltage circuit U_R, which in turn may produce a desired voltage at the positive differential input of error amplifiers U₁, U₂ . . . U_N−1, and U_N, and at the source terminals V_CS. The reference voltages Ref₁, Ref₂ . . . to Ref_N may be computed using the unregulated voltage adjustable loads L₁, L₂ . . . and L_N+1. As a result, a desired V_CS may be generated. Along with a predetermined R_CS, a desired current may be controlled to flow through the series of LEDs. The adjustment of is further described in association of FIGS. 13A-13B and FIGS. 14A-14B in the following sections.

FIG. 13A illustrates an exemplary implementation of a first adjustable load of a bandgap voltage reference circuit according to aspects of the present disclosure. As shown in FIG. 13A, the first adjustable load of a bandgap voltage reference circuit includes a first fixed resistor R_{1_fixed} and a first set of resistance units. In this exemplary implementation, each resistance unit in the first set of resistance units includes a fuse and a resistor, for example F_1a and R_1a, F_1b and R_1b . . . F_1h and R_1h. The fuse and the resistor are connected in parallel.

According to aspects of the present disclosure, each resistance unit in the first set of resistance units has a resistance value in a format of power of two, with a first resistance unit adjacent to the first fixed resistor having a highest resistance value, and a last resistance unit furthest from the first fixed resistor having a value of one resistance unit. For example, R_1a has a value of 1 resistance unit; R_1b has a value of 2 resistance units; R_1c has a value of 4 resistance units; R_1d has a value of 8 resistance units, and so on. The value of a resistance unit may be set based on a particular application, such as μΩ, mΩ, Ω (Ohm), KΩ, MΩ, or other values. By keeping or “cutting” the fuse, the corresponding resister in a particular resistance unit may be bypassed or may be enabled.

The first adjustable load of a bandgap voltage reference circuit further includes a first set of resistance fractions. In this example, the first set of resistance fractions are connected in series. In this exemplary implementation, each resistance fraction in the first set of resistance fractions includes a fuse and one or more resistors. For example F_1n and two parallel resistors that give a resistance value of R_1n, F_1m and three parallel resistors that give a resistance value of R_1m . . . F_1r and a number of parallel resistors that give a resistance value R_1r. Note that within each resistance fraction in the first set of resistance fractions, the fuse and the one or more resistors are connected in parallel. The first fixed resistor, the first set of resistance units, and the first set of resistance fractions are connected in series.

According to aspects of the present disclosure, each resistance fraction in the first set of resistance fractions has a resistance value in a format of negative power of two, with a first resistance fraction adjacent to the first set of resistance units having a value of a half of one resistance unit, and a last resistance fraction furthest from the first set of resistance units having a lowest resistance value. For example, R_1n has a value of ½ resistance unit; R_1m has a value of ¼ resistance units, and so on. Based on a particular application, the value of a resistance fraction may be set similar to the corresponding resistance unit, such as μΩ, mΩ, Ω, KΩ, MΩ, or other values. By keeping or “cutting” the fuse, the corresponding resister(s) in a particular resistance fraction may be bypassed or may be enabled.

With the above technique, the first adjustable load may be modified to attain a particular resistance value, which equals to R_{1_fixed} plus a programmable number of resistance units from the first set of resistance units and plus a programmable number of resistance fractions from the first set of resistance fractions. This approach enables the bandgap circuit reference voltage circuit to set the value of the first adjustable load L₁ to be a desired value within a predetermined percentage of error.

FIG. 13B illustrates an exemplary implementation of a second adjustable load of the bandgap circuit according to aspects of the present disclosure. Similar to the approach shown in FIG. 13a, the second adjustable load of a bandgap voltage reference circuit includes a second fixed resistor R_{2_fixed} and a second set of resistance units. In this exemplary implementation, each resistance unit in the second set of resistance unit includes a fuse and a resistor, for example F_2a and R_2a, F_2b and R_2b . . . F_2j and R_2j. The fuse and the resistor are connected in parallel.

According to aspects of the present disclosure, each resistance unit in the second set of resistance units has a resistance value in a format of power of two, with the unit adjacent to the second fixed resistor having a highest resistance value, and a last resistance unit furthest from the second fixed resistor having a value of one resistance unit. With this implementation, R_2a has a value of 1 resistance unit; R_2b has a value of 2 resistance units; R_2c has a value of 4 resistance units; R_2d has a value of 8 resistance units, and so on. The value of a resistance unit may be set based on a particular application, such as μΩ, mΩ, Ω (Ohm), KΩ, MΩ, or other values. By keeping or “cutting” the fuse, the corresponding resister in a particular resistance unit may be bypassed or may be enabled.

The second adjustable load of a bandgap voltage reference circuit further includes a second set of resistance fractions. In this example, the second set of resistance fractions are connected in series. In this exemplary implementation, each resistance fraction in the second set of resistance fractions includes a fuse and one or more resistors. For example F_2n and two parallel resistors that give a resistance value of R_2n, F_2m and three parallel resistors that give a resistance value of R_2m . . . F_2t and a number of parallel resistors that give a resistance value R_2t. Note that within each resistance fraction in the second set of resistance fractions, the fuse and the one or more resistors are connected in parallel. The second fixed resistor, the second set of resistance units, and the second set of resistance fractions are connected in series.

According to aspects of the present disclosure, each resistance fraction in the second set of resistance fractions has a resistance value in a format of negative power of two, with the resistance fraction adjacent to the second set of resistance units having a value of a half of one resistance unit, and a last resistance fraction furthest from the first set of resistance units having a lowest resistance value. For example, R_2n has a value of ½ resistance unit; R_2m has a value of ¼ resistance units, and so on. Based on a particular application, the value of a resistance fraction may be set similar to the corresponding resistance unit, such as μΩ, mΩ, Ω, KΩ, MΩ, or other values. By keeping or “cutting” the fuse, the corresponding resister(s) in a particular resistance fraction may be bypassed or may be enabled.

With the above technique, the second adjustable load may be modified to attain a particular resistance value, which equals to R_{2_fixed} plus a programmable number of resistance units from the second set of resistance units and plus a programmable number of resistance fractions from the second set of resistance fractions. This approach enables the bandgap circuit reference voltage circuit to set the value of the second adjustable load L₂ to be a desired value within a predetermined percentage of error.

According to aspects of the present disclosure, using the techniques described above in adjusted the first adjustable load and the second adjustable load, the band gap reference voltage circuit can be configured to produce a reference output voltage at a desired value within a predetermined percentage of error, which can be used in turn to produce a current to drive the one or more LEDs with a desired magnitude and within a predetermined percentage of error.

According to aspects of the present disclosure, the techniques described above in adjust the first adjustable load and the second adjustable load may be employed to adjust the other adjustable loads as shown in FIG. 12C. As a result, a plurality of reference voltages, such as Ref₁, Ref₂ . . . , and Ref_N, may be generated by the band gap reference voltage circuit, and which in turn can be used to generate a corresponding plurality of currents to drive the plurality of LEDs as shown in FIG. 10, for example.

FIG. 14A illustrates an exemplary implementation of fuse units of the first adjustable load of FIG. 13A according to aspects of the present disclosure. As shown in FIG. 14a, the fuses from the first adjustable load of FIG. 13A can be placed in a package that they are separated from their corresponding resistors, although these fuses are still electrically connected to their corresponding resistors in the same manner as in FIG. 13A. For example, for the fuse F1a, one end is connected to a first end of R1a, and the other end is connected to a second end of R1a. The other fuses are connected in the same manner. As another example, for the fuse F1r, one end is connected to a first end of R1r, and the other end is connected to a second end of R1r.

It is beneficial to place the fuses of the first adjustable load together as shown in FIG. 14A. This placement enables the manufacturing process to be more efficient. For example, after determinations have been made with regards to whether to “keep” or “cut” each of the fuse, such operations can be performed from one fuse to the next fuse efficiently, without unnecessary searching for the next fuse in the case when the fuses are placed in different locations, for example being placed next to their corresponding resistors, on a printed circuit board.

FIG. 14B illustrates an exemplary implementation of fuse units of the second adjustable load of FIG. 13B according to aspects of the present disclosure. Similar to the example shown in FIG. 14A, the fuses from the second adjustable load of FIG. 13B can be placed in a package that they are separated from their corresponding resistors, although these fuses are still electrically connected to their corresponding resistors in the same manner as in FIG. 13B. For example, for the fuse F2a, one end is connected to a first end of R2a, and the other end is connected to a second end of R2a. The other fuses are connected in the same manner. As another example, for the fuse F2t, one end is connected to a first end of R2t, and the other end is connected to a second end of R2t.

There are benefits to place the fuses of the second adjustable load together as shown in FIG. 14B. This placement enables the manufacturing process to be more efficient. For example, after determinations have been made with regards to whether to “keep” or “cut” each of the fuse, such operations can be performed from one fuse to the next fuse efficiently, without unnecessary searching for the next fuse in the case when the fuses are placed in different locations, for example being placed next to their corresponding resistors, on a printed circuit board.

FIG. 15A illustrates a method of controlling one or more serially connected LEDs according to aspects of the present disclosure. In the example shown in FIG. 15A, in block 1502, the method receives a rectified AC voltage by a voltage regulator circuit, where the voltage regulator circuit includes a depletion device configured to generate an unregulated voltage using the rectified AC voltage. In block 1504, the method generates one or more reference voltages by a band gap voltage reference circuit using the unregulated voltage. In block 1506, the method connects a current setting circuit to a circuit ground through a current setting resistor having a fixed resistance value. In block 1508, the method controls the one or more serially connected LEDs by a current setting circuit using the one or more reference voltages. In some implementations, the current setting resistor, the voltage regulator circuit, the band gap voltage reference circuit and the current setting circuit reside within an integrated circuit.

FIG. 15B illustrates a method of generating one or more reference voltages according to aspects of the present disclosure. As shown in FIG. 15B, in block 1512, the method provides a first adjustable load in the band gap voltage reference circuit. In block 1514, the method provides a second adjustable load in the band gap voltage reference circuit. In block 1516, the method computes a reference voltage using the unregulated voltage multiplied by a ratio of the first adjustable load and the sum of the first adjustable load and the second adjustable load. In block 1518, the method adjusts the reference voltage in the one or more reference voltages by the first adjustable load and the second adjustable load.

FIG. 16A illustrates an exemplary implementation of providing a first adjustable load according to aspects of the present disclosure. In the example shown in FIG. 16A, in block 1602, the method provide a first fixed resistor and a first set of resistance units. In block 1604, the method connects the first set of resistance units in series, wherein each resistance unit in the first set of resistance units comprises a fuse and a resistor, wherein the fuse and the resistor are connected in parallel. In block 1606, the method provides a first set of resistance fractions. In block 1608, the method connects the first set of resistance fractions in series, wherein each resistance fraction in the first set of resistance fractions comprises a fuse and one or more resistors, wherein the fuse and the one or more resistors are connected in parallel. In block 1610, the method connects the first fixed resistor, the first set of resistance units, and the first set of resistance fractions in series.

According to aspects of the present disclosure, each resistance unit in the first set of resistance units has a resistance value in a format of power of two, with a first resistance unit adjacent to the first fixed resistor having a highest resistance value, and a last resistance unit furthest from the first fixed resistor having a value of one resistance unit. Each resistance fraction in the first set of resistance fractions produces a resistance value of in a format of negative power of two, with a first resistance fraction adjacent to the first set of resistance units having a value of a half of one resistance unit, and a last resistance fraction furthest from the first set of resistance units having a lowest resistance value.

FIG. 16B illustrates an exemplary implementation of providing a second adjustable load according to aspects of the present disclosure. In the exemplary implementation shown in FIG. 16B, in block 1612, the method provides a second fixed resistor and a second set of resistance units. In block 1614, the method connects the second set of resistance units in series, wherein each resistance unit in the second set of resistance units comprises a fuse and a resistor, wherein the fuse and the resistor are connected in parallel. In block 1616, the method provides a second set of resistance fractions. In block 1618, the method connects the second set of resistance fractions in series, wherein each resistance fraction in the second set of resistance fractions comprises a fuse and one or more resistors, wherein the fuse and the one or more resistors are connected in parallel. In block 1620, the method connects the second fixed resistor, the second set of resistance units, and the second set of resistance fractions in series.

According to aspects of the present disclosure, each resistance unit in the second set of resistance units has a resistance value in a format of power of two, with a second resistance unit adjacent to the second fixed resistor having a highest resistance value, and a last resistance unit furthest from the second fixed resistor having a value of one resistance unit. Each resistance fraction in the second set of resistance fractions produces a resistance value of in a format of negative power of two, with a second resistance fraction adjacent to the first set of resistance units having a value of a half of one resistance unit, and a last resistance fraction furthest from the second set of resistance units having a lowest resistance value.

The invention can be implemented in any suitable form, including hardware, software, and firmware. The invention may optionally be implemented partly as computer software running on one or more data processors and/or digital signal processors. The elements and components of an embodiment of the invention may be physically, functionally, and logically implemented in any suitable way. Indeed, the functionality may be implemented in a single unit, in a plurality of units, or as part of other functional units. As such, the invention may be implemented in a single unit or may be physically and functionally distributed between different units and processors.

One skilled in the relevant art will recognize that many possible modifications and combinations of the disclosed embodiments may be used, while still employing the same basic underlying mechanisms and methodologies. The foregoing description, for purposes of explanation, has been written with references to specific embodiments. However, the illustrative discussions above are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described to explain the principles of the invention and their practical applications, and to enable others skilled in the art to best utilize the invention and various embodiments with various modifications as suited to the particular use contemplated.

Claims

1. A light emitting diode (LED) driver for driving one or more serially connected LEDs, comprising: a voltage regulator circuit configured to receive a rectified AC voltage, wherein the voltage regulator circuit includes a depletion device configured to generate an unregulated voltage using the rectified AC voltage;a band gap voltage reference circuit configured to generate one or more reference voltages using the unregulated voltage, wherein the band gap voltage reference circuit comprises a first adjustable load and a second adjustable load, wherein a reference voltage in the one or more reference voltages is adjusted by the first adjustable load and the second adjustable load, and wherein the reference voltage is computed using the unregulated voltage multiplied by a ratio of the first adjustable load and the sum of the first adjustable load and the second adjustable load; anda current setting circuit configured to control the one or more serially connected LEDs using the one or more reference voltages, wherein the current setting circuit is connected to a circuit ground through a current setting resistor having a fixed resistance value.

2. The LED driver of claim 1, wherein the current setting resistor, the voltage regulator circuit, the band gap voltage reference circuit and the current setting circuit reside within an integrated circuit.

3. The LED driver of claim 1, wherein the first adjustable load comprises: a first fixed resistor;a first set of resistance units, wherein the first set of resistance units are connected in series and wherein each resistance unit in the first set of resistance units comprises a fuse and a resistor, wherein the fuse and the resistor are connected in parallel; anda first set of resistance fractions, wherein the first set of resistance fractions are connected in series and wherein each resistance fraction in the first set of resistance fractions comprises a fuse and one or more resistors, wherein the fuse and the one or more resistors are connected in parallel;wherein the first fixed resistor, the first set of resistance units, and the first set of resistance fractions are connected in series.

4. The LED driver of claim 3, wherein each resistance unit in the first set of resistance units has a resistance value in a format of power of two, with a first resistance unit adjacent to the first fixed resistor having a highest resistance value, and a last resistance unit furthest from the first fixed resistor having a value of one resistance unit.

5. The LED driver of claim 3, wherein each resistance fraction in the first set of resistance fractions produces a resistance value of in a format of negative power of two, with a first resistance fraction adjacent to the first set of resistance units having a value of a half of one resistance unit, and a last resistance fraction furthest from the first set of resistance units having a lowest resistance value.

6. The LED driver of claim 1, wherein the second adjustable load comprises: a second fixed resistor;a second set of resistance units, wherein the second set of resistance units are connected in series and wherein each resistance unit in the set of second set of resistance units comprises a fuse and a resistor, wherein the fuse and the resistor are connected in parallel; anda second set of resistance fractions, wherein the second set of resistance fractions are connected in series and wherein each resistance fraction in the second set of resistance fractions comprises a fuse and one or more resistors, wherein the fuse and the one or more resistors are connected in parallel;wherein the second fixed resistor, the second set of resistance units, and the second set of resistance fractions are connected in series.

7. The LED driver of claim 6, wherein each resistance unit in the second set of resistance units has a resistance value in a format of power of two, with a second resistance unit adjacent to the second fixed resistor having a highest resistance value, and a last resistance unit furthest from the second fixed resistor having a value of one resistance unit.

8. The LED driver of claim 7, wherein each resistance fraction in the second set of resistance fractions produces a resistance value of in a format of negative power of two, with a second resistance fraction adjacent to the first set of resistance units having a value of a half of one resistance unit, and a last resistance fraction furthest from the second set of resistance units having a lowest resistance value.

9. The LED driver of claim 1, further comprising: one or more additional adjustable loads, wherein the first adjustable load, the second adjustable load, and the one or more additional adjustable loads are configured to generate the corresponding one or more reference voltages.

10. A method for controlling one or more serially connected LEDs, comprising: receiving a rectified AC voltage by a voltage regulator circuit, wherein the voltage regulator circuit includes a depletion device configured to generate an unregulated voltage using the rectified AC voltage;generating one or more reference voltages by a band gap voltage reference circuit using the unregulated voltage, comprising providing a first adjustable load in the band gap voltage reference circuit, providing a second adjustable load in the band gap voltage reference circuit, computing a reference voltage using the unregulated voltage multiplied by a ratio of the first adjustable load and the sum of the first adjustable load and the second adjustable load, and adjusting the reference voltage in the one or more reference voltages by the first adjustable load and the second adjustable load;connecting a current setting circuit to a circuit ground through a current setting resistor having a fixed resistance value; andcontrolling the one or more serially connected LEDs by the current setting circuit using the one or more reference voltages.

11. The method of claim 10, wherein the current setting resistor, the voltage regulator circuit, the band gap voltage reference circuit and the current setting circuit reside within an integrated circuit.

12. The method of claim 10, wherein providing the first adjustable load comprises: providing a first fixed resistor;providing a first set of resistance units;connecting the first set of resistance units in series, wherein each resistance unit in the first set of resistance units comprises a fuse and a resistor, wherein the fuse and the resistor are connected in parallel;providing a first set of resistance fractions;connecting the first set of resistance fractions in series, wherein each resistance fraction in the first set of resistance fractions comprises a fuse and one or more resistors, wherein the fuse and the one or more resistors are connected in parallel; andconnecting the first fixed resistor, the first set of resistance units, and the first set of resistance fractions in series.

13. The method of claim 12, wherein each resistance unit in the first set of resistance units has a resistance value in a format of power of two, with a first resistance unit adjacent to the first fixed resistor having a highest resistance value, and a last resistance unit furthest from the first fixed resistor having a value of one resistance unit.

14. The method of claim 12, wherein each resistance fraction in the first set of resistance fractions produces a resistance value of in a format of negative power of two, with a first resistance fraction adjacent to the first set of resistance units having a value of a half of one resistance unit, and a last resistance fraction furthest from the first set of resistance units having a lowest resistance value.

15. The method of claim 10, wherein providing the second adjustable load comprises: providing a second fixed resistor;providing a second set of resistance units;connecting the second set of resistance units in series, wherein each resistance unit in the second set of resistance units comprises a fuse and a resistor, wherein the fuse and the resistor are connected in parallel;providing a second set of resistance fractions;connecting the second set of resistance fractions in series, wherein each resistance fraction in the second set of resistance fractions comprises a fuse and one or more resistors, wherein the fuse and the one or more resistors are connected in parallel; andconnecting the second fixed resistor, the second set of resistance units, and the second set of resistance fractions in series.

16. The method of claim 15, wherein each resistance unit in the second set of resistance units has a resistance value in a format of power of two, with a second resistance unit adjacent to the second fixed resistor having a highest resistance value, and a last resistance unit furthest from the second fixed resistor having a value of one resistance unit.

17. The method of claim 16, wherein each resistance fraction in the second set of resistance fractions produces a resistance value of in a format of negative power of two, with a second resistance fraction adjacent to the first set of resistance units having a value of a half of one resistance unit, and a last resistance fraction furthest from the second set of resistance units having a lowest resistance value.

18. The method of claim 10, further comprising: providing one or more additional adjustable loads; andgenerating the corresponding one or more reference voltages using the first adjustable load, the second adjustable load, and the one or more additional adjustable loads.