Circuits and methods for driving light sources

A circuit for driving a light source, e.g., an LED light source, includes a converter, a sensor, and a controller. The converter converts an input voltage to an output voltage across the LED light source based upon a driving signal. A duty cycle of the driving signal determines an average current flowing through the LED light source. The sensor is selectively coupled to and decoupled from the converter based upon the driving signal. The sensor generates a sense voltage indicative of a current flowing through the LED light source when the sensor is coupled to the converter. The controller is coupled to the converter and sensor. The controller compares the sense voltage to a reference voltage indicative of a predetermined average current through the LED light source to generate a compensation signal and generates the driving signal based upon the compensation signal. The duty cycle of the driving signal is adjusted based upon the compensation signal to adjust the average current flowing through the LED light source to the predetermined average current.

RELATED APPLICATION

This application claims priority to Patent Application No. 201010548415.4, titled “Driving Circuit for Light Source, and Controller and Method for Controlling Luminance of Light Source”, filed on Nov. 15, 2010, with the State Intellectual Property Office of the People's Republic of China.

BACKGROUND

Light sources such as light emitting diodes (LEDs) can be used, e.g., for backlighting liquid crystal displays (LCDs), street lighting, and home appliances. LEDs offer several advantages over alternative light sources. Among these are greater efficiency and increased operating life.

FIG. 1shows a schematic diagram of a conventional circuit100for driving a light source, e.g., an LED string.FIG. 2shows a waveform200of a current flowing through the LED string inFIG. 1. As shown inFIG. 1, the circuit100for driving an LED string108includes a power source102, a rectifier104, a capacitor106, a controller110, and a buck converter111. The power source102provides an input alternating-current (AC) voltage. The rectifier104and the capacitor106converts the input AC voltage to an input direct-current (DC) voltage VIN.

Controlled by the controller110, the buck converter111further converts the input DC voltage VINto an output DC voltage VOUTacross the LED string108. Based on the output DC voltage VOUT, the circuit100produces an LED current ILEDflowing through the LED string108. The buck converter111includes a diode106, an inductor118, and a switch112. The switch112includes an N-channel transistor as shown inFIG. 1. The controller110is coupled to the gate of the switch112via a DRV pin and coupled to the source of the switch112via a CS pin. A resistor114is coupled between the CS pin and ground to produce a sense voltage indicative of the LED current ILED. The switch112controlled by the controller110is turned on and off alternately.

Referring toFIG. 2, when the switch112is in an ON state, the LED current ILEDramps up and flows through the inductor118, the switch112and the resistor114to ground. The controller110receives the sense voltage indicative of the LED current ILEDvia the CS pin and turns off the switch112when the LED current ILEDreaches a peak LED current IPEAK. When the switch112is in an OFF state, the LED current ILEDramps down from the peak LED current IPEAKand flows through the inductor118and the diode106.

The controller110can operate in a constant period mode or a constant off time mode. In the constant period mode, the controller110turns the switch112on and off alternately and maintains a cycle period Ts of the control signal from pin DRV substantially constant. An average value IAVGof the LED current ILEDcan be given by:

IAVG=IPEAK-12·(VIN-VOUT)×VOUTVIN×TSL,(1)
where L is the inductance of the inductor118. In the constant off time mode, the controller110turns the switch112on and off alternately and maintains an off time TOFFof the switch112substantially constant. The average value IAVGof the LED current ILEDcan be given by:

IAVG=IPEAK-12·VOUT×TOFFL.(2)
According to equations (1) and (2), the average LED current IAVGis functionally dependent on the input DC voltage VIN, the output DC voltage VOUTand the inductance of the inductor118. In other words, the average LED current IAVGvaries as the input DC voltage VIN, the output DC voltage VOUTand the inductance of the inductor118change. Therefore, the LED current ILEDmay not be accurately controlled, thereby affecting the stability of LED brightness.

SUMMARY

In one embodiment, a circuit for driving a light source, e.g., an LED light source, includes a converter, a sensor, and a controller. The converter converts an input voltage to an output voltage across the LED light source based upon a driving signal. A duty cycle of the driving signal determines an average current flowing through the LED light source. The sensor is selectively coupled to and decoupled from the converter based upon the driving signal. The sensor generates a sense voltage indicative of a current flowing through the LED light source when the sensor is coupled to the converter. The controller is coupled to the converter and sensor. The controller compares the sense voltage to a reference voltage indicative of a predetermined average current through the LED light source to generate a compensation signal and generates the driving signal based upon the compensation signal. The duty cycle of the driving signal is adjusted based upon the compensation signal to adjust the average current flowing through the LED light source to the predetermined average current.

DETAILED DESCRIPTION

Embodiments in accordance with the present disclosure provide a driving circuit for driving a light source. The driving circuit includes a converter, a sensor, and a controller. The converter converts an input voltage to an output voltage across the light source based upon a driving signal. A duty cycle of the driving signal determines an average current flowing through the light source. The sensor is selectively coupled to and decoupled from the converter based upon the driving signal. The sensor generates a sense voltage indicative of a current flowing through the light source when the sensor is coupled to the converter. The controller is coupled to the converter and sensor. The controller compares the sense voltage to a reference voltage indicative of a predetermined average current through the light source to generate a compensation signal and generates the driving signal based upon the compensation signal. The duty cycle of the driving signal is adjusted based upon the compensation signal to adjust the average current flowing through the light source to the predetermined average current.

FIG. 3illustrates a driving circuit300according to one embodiment of the present invention. In the example ofFIG. 3, the driving circuit300includes a power source302, a rectifier304, a capacitor306, a converter311, a controller310, and a sensor, e.g., a resistor314. The driving circuit300is coupled to one or more light sources, e.g., an LED string308, for controlling the brightness of the light sources. In one embodiment, the power source302provides an AC voltage, and the rectifier304and the capacitor306convert the AC voltage to an input DC voltage VIN. The input DC voltage VINis further converted to an output DC voltage VOUTacross the LED string308by the converter311which includes a diode316, a switch312, and an inductor318, in one embodiment. According to states of the switch312and the diode316, the converter311alternates between coupling the inductor318to the input DC voltage VINto store energy into the inductor318and discharging the inductor318to the LED string308. For a given input DC voltage VIN, the output DC voltage VOUTis determined by a duty cycle D of the switch312, that is, a ratio between a period TONwhen the switch312is on (ON state) and the commutation period TS.

The duty cycle D of the switch312is controlled by the controller310. In one embodiment, the controller310includes a COMP pin, a RT pin, a VDD pin, a GND pin, a DRV pin, and a SOURCE pin. The switch312includes an N-channel transistor, in one embodiment. The gate of the transistor312is coupled to the DRV pin of the controller310. The source of the transistor312is coupled to the SOURCE pin of the controller310. The source of the transistor312together with the SOURCE pin of the controller310is also coupled to ground through the resistor314. The COMP pin of the controller310is coupled to ground through serially connected resistor320and an energy storage element, e.g., a capacitor322. The RT pin is coupled to ground through a resistor324. VDD pin is coupled to ground through a capacitor326, coupled to the input DC voltage VINthrough a resistor336, and coupled to a winding338through a diode332and a resistor334. The winding338is magnetically coupled to the inductor318. A startup voltage is produced at the VDD pin to startup the controller310. Alternatively, a voltage source (now shown) can be coupled to the VDD pin for providing the startup voltage.

In operation, the resistor314is selectively coupled to and decoupled from the converter311based upon the conduction state of the switch312. When the switch312is in the ON state, an LED current ILEDis produced to flow through a first current path including the LED string308, the inductor318, the switch312and the resistor314. The voltage across the resistor314is indicative of the LED current ILEDand received by the controller310via the SOURCE pin as a sense voltage. When the switch312is in an OFF state, the LED current ILEDis produced to flow through a second path including the LED string308, the inductor318and the diode316. No current flows through the switch312and the resistor314. Accordingly, the sense voltage at the SOURCE pin is substantially zero, in one embodiment.

In one embodiment, the controller310compares the sense voltage to a reference voltage VREFindicative of a predetermined average LED current IAVG0to generate a compensation signal328at the COMP pin. Based upon the compensation signal328, the controller310generates a driving signal330at the DRV pin to turn the switch312on and off alternately and adjusts a duty cycle D of the driving signal330. As such, the average LED current IAVGthrough the LED string308is adjusted to the predetermined average LED current IAVG0by adjusting the duty cycle D of the driving signal330. The average LED current IAVGis not functionally dependent on the input DC voltage VIN, the output DC voltage VOUTor the inductance L. Advantageously, by introducing the compensation signal328, the impact of the input DC voltage VIN, the output DC voltage VOUTand the inductance L on the average LED current IAVGis reduced or eliminated, such that the stability of LED brightness is improved.

FIG. 4illustrates a schematic diagram of the controller310inFIG. 3according to one embodiment of the present invention. Elements labeled the same inFIG. 3have similar functions.FIG. 4is described in combination withFIG. 3. In the example ofFIG. 4, the controller310includes a startup circuit402, an oscillator404, a signal generator406, a flip-flop408, a comparator410, an output circuit, e.g., an AND gate412, a protection circuit414, an amplifier, e.g., an operational transconductance amplifier (OTA)416, and a control switch418. The OTA416, the control switch418, and the comparator410constitute a feedback circuit.

The startup circuit402receives the startup voltage via the VDD pin. When the startup voltage at the VDD pin reaches a predetermined startup voltage level of the controller310, the startup circuit420provides power to other components in the controller310to enable operation of the controller310. The oscillator404generates a pulse signal420which has a preset frequency determined by the resistor324, in one embodiment. The flip-flop408receives the pulse signal420via a set pin S. The pulse signal420is further provided to the signal generator406which generates a ramp signal422having the same frequency as the pulse signal420. In one embodiment, the ramp signal422has a sawtooth wave. As mentioned in relation toFIG. 3, the SOURCE pin of the controller310is coupled to the resistor314to receive the sense voltage indicating the LED current ILED. The sense voltage is provided to the protection circuit414which outputs a protection signal424to the AND gate412to indicate whether the driving circuit300is in a normal condition or an abnormal condition, e.g., a short circuit condition or an over current condition.

Moreover, the sense voltage is provided to an input terminal, e.g., an inverting terminal, of the OTA416. The other input terminal, e.g., a non-inverting terminal of the OTA416receives the reference voltage VREFindicative of the predetermined average LED current IAVG0. The OTA416outputs a current which is a function of the differential input voltage. In one embodiment, the output current is proportional to the voltage difference between the sense voltage and the reference voltage VREF. The output current charges the capacitor322via a charging path including the control switch418and the resistor320to produce the compensation signal328at the COMP pin. The compensation signal328is provided to an input terminal, e.g., an inverting terminal, of the comparator410. The comparator410compares the compensation signal328to the ramp signal422to output a reset signal428to a reset pin R of the flip-flop408. In one embodiment, the reset signal428comprises a pulse-width modulation signal (PWM) signal. Triggered by the pulse signal420and the reset signal428, the flip-flop408outputs a control signal430via an output pin Q. The control signal430is further provided to both the AND gate412and the control switch418, in one embodiment.

Thus, the AND gate412receives the control signal430and the protection signal424. As such, when an abnormal condition occurs as indicated by the protection signal424, the driving signal330from the AND gate412switches the switch312off to prevent the driving circuit300from undergoing abnormal conditions. When the driving circuit300operates in the normal condition, the driving signal330is determined by the control signal430to alternate the switch312between the ON state and OFF state. In other words, the waveform of the driving signal300follows that of the control signal430when the driving circuit300operates in the normal condition, in one embodiment. As such, the state of the control switch418is synchronized with the state of the switch312. Referring toFIG. 3, when the switch312is off, the charging path of the capacitor322is cut off accordingly such that the compensation signal328is clamped to a non-zero value. When the switch312is on, the charging path of the capacitor322is conductive and the controller310senses the sense voltage via the SOURCE pin to produce the compensation signal328. Based on the compensation signal328, the driving signal330at DRV pin drives the switch312such that the average LED current IAVGthrough the LED string308is adjusted to the predetermined average LED current IAVG0.

Advantageously, in one embodiment, the predetermined average LED current IAVG0is determined by the predetermined reference voltage VREFindependent of various circuit conditions, such as the input DC voltage VIN, the load condition, and the inductor318. As such, brightness stability of the light sources is improved.

FIG. 5illustrates a timing diagram500of the driving circuit300FIG. 3according to one embodiment of the present invention.FIG. 5is described in combination withFIGS. 3 and 4. The waveform502represents the pulse signal420. The waveform504represents the ramp signal422, the waveform506represents the sense voltage at the SOURCE pin, the waveform508represents the compensation signal328at the COMP pin, the waveform510represents the reset signal428, and the waveform512represents the driving signal330at the DRV pin.

In the example ofFIG. 5, when the pulse signal420steps from a low level (logic 0) to a high level (logic 1) and the ramp signal422begins to ramp up at time T0, the driving signal330is set to logic 1 to switch on the switch312. The sense voltage at the SOURCE pin increases as the LED current ILEDflowing through the resistor314increases. With the increase of the sense voltage, the output current of the OTA416decrease, so does the compensation signal328. The compensation signal328decreases until the compensation signal328intersects with the ramp signal422at time T1. Due to the intersection of compensation signal328with the ramp signal422at time T1, the reset signal428output from the comparator410steps from logic 0 to logic 1 and the driving signal330is set to logic 0 to switch off the switch312.

Since the switch312is turned off, no current flows through the resistor314such that the sense voltage at the SOURCE pin drops to substantially zero at time T1. As discussed in relation toFIG. 4, the control switch418is turned off together with the switch312, such that the charging path of the capacitor322is cut off and the compensation signal328is clamped to the non-zero value at time T1. In a commutation period TSof the pulse signal420after time T0, e.g., at time T2, the pulse signal420steps from logic 0 to logic 1 to assert a new pulse while the ramp signal422having the same frequency as the pulse signal420drops sharply and becomes lower than the compensation signal328which is clamped to a non-zero value. The reset signal428is set to logic 0 and the drive signal330is set to logic 1 again at time T2. As such, a commutation cycle from time T0to time T2completes. A new commutation cycle starts from time T2.

As shown inFIG. 5, the duty cycle D of the driving signal330is determined by the compensation signal328indicative of the difference between the sense voltage at the SOURCE pin and the reference voltage VREF. The duty cycle D of the driving signal330is used to regulate the average LED current IAVGto the predetermined average LED current IAVG0indicated by the reference voltage VREF. In other words, a feedback loop is formed where the sense voltage is fed back to the controller310and compared to the reference voltage VREFand the difference between the sense voltage and the reference voltage is used to generate the compensation signal328to regulate the average LED current IAVGto the predetermined average LED current IAVG0. As such, even if the circuit condition of the circuit300changes, the duty cycle D of the driving signal330changes dynamically due to the feedback loop to keep the average LED current IAVGsubstantially equal to the predetermined average LED current IAVG0.

For example, when the input DC voltage VINincreases, the instant LED current ILEDincreases and the instant sense voltage at the SOURCE pin increases accordingly. With the increased sense voltage, the compensation signal328decreases such that the duty cycle D of the driving signal330is reduced. As the duty cycle D of the driving signal330decreases, the LED current ILEDdecreases accordingly such that the effect of the increased input DC voltage VINis canceled out by the reduced duty cycle D of the driving signal330to maintain the average LED current IAVGsubstantially equal to the predetermined average LED current IAVG0. Similarly, when other circuit condition changes, e.g., the load condition and the inductor318, the average LED current IAVGis kept substantially equal to the predetermined average LED current IAVG0due to the dynamic adjustment of the duty cycle D of the driving signal330.

FIG. 6illustrates a schematic diagram of a driving circuit600according to another embodiment of the present invention. Elements labeled the same inFIG. 3have similar functions. Besides the power source302, the rectifier304, the capacitor306, the diode316and the inductor318, the driving circuit600further includes a controller610having a VDD pin, a DRAIN pin, a SOURCE pin, a GND pin, a HV_GATE pin, a COMP pin, a CLK pin and a RT pin. The HV_GATE pin is coupled to the input DC voltage VINthrough a resistor606and coupled to ground through a capacitor608. The COMP pin is coupled to ground through serially connected resistor618and an energy storage element, e.g., a capacitor620. The CLK pin is coupled to ground through parallel connected resistor614and capacitor616. The CLK pin is also coupled to input DC voltage VINthrough a resistor612. The RT pin is coupled to ground through a resistor628. The VDD pin is coupled to the HV_GATE pin through serially connected resistor604, switch602and diode622. In one embodiment, the switch602includes an N-channel transistor, with gate coupled to the resistor604, source coupled to anode of the diode622, and drain coupled to the inductor318. The VDD pin is also coupled to ground through a capacitor624. The DRAIN pin is coupled to source of the switch602. The SOURCE pin is coupled to ground through a resistor626. The GND pin is coupled to ground.

Different from the driving circuit300where the switch312for alternating the inductor318between charging and discharging is located outside the controller310, the controller610in the driving circuit600has the function of alternating the inductor318between charging and discharging.

FIG. 7illustrates a schematic diagram of the controller610according to one embodiment of the present invention. Elements labeled the same inFIG. 4have similar functions.FIG. 7is described in combination withFIGS. 4 and 6. In the example ofFIG. 7, the controller610includes the startup circuit402, the oscillator404, the signal generator406, the flip-flop408, the comparator410, the AND gate412, the protection circuit414, the OTA416, the switch418, a switch702, a zener diode704, and an enbable HV_GATE block706. The switch702alternates the inductor318between charging and discharging. When the switch702is in the ON state, the LED current ILEDflows through the LED string308, the inductor318, the switch602, the switch702and the resistor626to ground. When the switch702is in the OFF state, the LED current flows through the LED string308, the inductor318and the diode316. As such, the SOURCE pin produces the sense voltage indicative of the LED current ILEDwhen the switch702is in the ON state.

In one embodiment, the switch702includes an N-channel transistor, with gate coupled to the AND gate412, drain coupled to the DRAIN pin, and source coupled to the SOURCE pin. The zener diode704is coupled between the HV_GATE pin and ground. The enable HV_GATE block706is coupled between the CLK pin and the HV_GATE pin. When the driving circuit600is powered on, an enable signal is produced at the CLK pin in response to the input DC voltage VIN. In response to the enable signal, the enable HV_GATE block706activates the HV_GATE pin to produces a constant DC voltage, e.g., 15V, determined by the zener diode704. Driven by the constant DC voltage at the HV_GATE pin, the switch602is switched on. The VDD pin obtains a startup voltage derived from a source voltage at the source of the switch602. The startup voltage enables the operation of the controller610. The sense voltage at the SOURCE pin is fed back and compared to the reference voltage VREFindicative of the predetermined average LED current IAVG0to generate the compensation signal328. Based on the compensation signal328, the duty cycle D of the driving signal330is determined. The driving signal330having the determined duty cycle D switches the switch702on and off alternately to adjust the average LED current IAVGto the predetermined average LED current IAVG0.

With the configuration ofFIGS. 6 and 7, the controller610operates automatically due to the enable signal at the CLK pin, the constant DC voltage at the HV_GATE pin, and the startup voltage at the VDD pin, when the driving circuit600is powered on. In normal operation, the DRAIN pin receives the LED current ILED, the SOURCE pin alternates between coupling to and decoupling from the DRAIN pin based upon the driving signal330. The duty cycle D of the driving signal330determines the average LED current IAVG. The COMP pin generates the compensation signal328based upon the voltage difference between the sense voltage and the reference voltage VREF. Based upon the compensation signal328, the duty cycle D of the driving signal330is adjusted to the predetermined average LED current IAVG0.

The embodiments ofFIGS. 3,4,6and7are for the purposes of illustration but not limitation. The exemplary circuits can have numerous variations within the spirit of the invention. For example, the OTA416can be replaced by an error amplifier or other similar elements as long as the compensation signal328can be produced to represent the voltage difference between the sense voltage and the reference voltage VREF. Also, the inductor318can be placed between the input DC voltage VINand the LED string308.

FIG. 8illustrates a flowchart800of a method for controlling brightness of a light source according to one embodiment of the present invention.FIG. 8is described in combination withFIGS. 3 and 4. Although specific steps are disclosed inFIG. 8, such steps are examples. That is, the present invention is well suited to performing various other steps or variations of the steps recited inFIG. 8.

In block802, an input voltage is converted to an output voltage across a light source, e.g., an LED light source, based upon a driving signal by a converter. In one embodiment, the converter311converts the input DC voltage VINto the output DC voltage VOUTacross the LED string308based upon the driving signal330from the DRV pin of the controller310.

In block804, an average LED current is determined by a duty cycle of the driving signal. In one embodiment, the duty cycle D of the driving signal330determines the conduction state of the switch312so as to adjust the average LED current IAVG. In other words, the average LED current IAVGis determined by the duty cycle of the driving signal330.

In block806, a sense voltage indicative of the LED current is generated across a sensor when the sensor is coupled to the converter. The sensor is selectively coupled to and decoupled from the converter based upon the driving signal. In one embodiment, the voltage across a sensor, e.g., the resistor314, indicates the LED current ILEDwhen the switch312is in the ON state. The voltage across the resistor314is received by the controller310via the SOURCE pin as the sense voltage indicative of the LED current ILED. When the switch312is in the OFF state, the resistor314is decoupled from the converter311. The conduction state of the switch312is determined by the driving signal330.

In block808, the sense voltage is compared to a reference voltage indicative of a predetermined average LED current to generate a compensation signal. In one embodiment, the sense voltage is compared to the reference voltage indicative of the predetermined average LED current IAVG0by the OTA416to generate the compensation signal328at the COMP pin.

In block810, the duty cycle of the driving signal is adjusted based upon the compensation signal to adjust the average LED current IAVGto the predetermined average LED current IAVG0. In one embodiment, the compensation signal328is compared to a ramp signal422by the comparator410. Output of the comparator410adjusts the duty cycle D of the driving signal330to adjust the average LED current IAVGto the predetermined average LED current IAVG0.

While the foregoing description and drawings represent embodiments of the present invention, it will be understood that various additions, modifications and substitutions may be made therein without departing from the spirit and scope of the principles of the present invention. One skilled in the art will appreciate that the invention may be used with many modifications of form, structure, arrangement, proportions, materials, elements, and components and otherwise, used in the practice of the invention, which are particularly adapted to specific environments and operative requirements without departing from the principles of the present invention. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, and not limited to the foregoing description.