Patent Publication Number: US-8525423-B2

Title: Circuitry for driving light emitting diodes and associated methods

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority to Chinese Patent Application No. 201010130329.1, filed on Mar. 23, 2010, the disclosure of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present technology relates generally to circuitry for driving light emitting diodes (“LEDs”). 
     BACKGROUND 
     White light LEDs (“WLEDs”) are increasingly used as backlight source in liquid crystal displays (“LCDs”) instead of cold cathode fluorescent lamps (“CCFLs”). A plurality of WLEDs may be connected in series to form a WLED string. Conventionally, circuitry for driving WLEDs may control a plurality of WLED strings synchronously. Such circuitry may comprise a voltage converter configured to provide a direct current (“DC”) driving voltage for each WLED string and a current balance circuit configured to regulate a current flowing through each WLED string. 
     The voltage converter (e.g., a PWM control circuit) and the current balance circuit may be integrated in a WLED application-specific integrated circuit (“ASIC”) chip. For example,  FIG. 1  illustrates conventional circuitry for driving 2n WLED strings. The parameter “n” hereinafter is a random positive integer. As shown in  FIG. 1 , the conventional circuitry comprises two operation-up circuits  101 - 1  and  101 - 2  and two ASIC chips  102 - 1  and  102 - 2 . Each of the operation-up circuits  101 - 1  and  101 - 2  provides a DC driving voltage V dc  to n WLED strings. Each of the ASIC chips  102 - 1  and  102 - 2  controls one of the corresponding operation-up circuits and regulates a current flowing through the corresponding n WLED strings. 
     If the circuitry shown in  FIG. 1  needs to drive 3n WLED strings, an additional voltage converter and ASIC chip are required for the additional LED string. Thus, the conventional circuitry is inconvenient to expand to control additional WLED strings. Also, the conventional circuitry requires a large number of components with corresponding high costs and low efficiencies. Accordingly, certain improvements of circuitry for driving LED strings may be desirable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates prior art circuitry for driving 2n WLED strings. 
         FIG. 2  illustrates circuitry for driving a plurality of LED strings according to embodiments of the present technology. 
         FIG. 3  illustrates circuitry for driving a plurality of LED strings according to embodiments of the present technology. 
         FIG. 4  illustrates a schematic circuit of the error amplifier shown in  FIG. 3  according to embodiments of the present technology. 
         FIG. 5  illustrates a schematic circuit of the voltage converter shown in  FIG. 3  according to embodiments of the present technology. 
         FIG. 6  illustrates block diagram circuitry for driving a plurality of LED strings according to embodiments of the present technology. 
         FIG. 7  illustrates block diagram circuitry for driving a plurality of LED strings according to embodiments of the present technology. 
         FIG. 8  illustrates a processing flow diagram of driving a plurality of LED strings according to embodiments of the present technology. 
     
    
    
     DETAILED DESCRIPTION 
     In the present disclosure, numerous specific details are provided, such as examples of circuits, components, and methods, to provide a thorough understanding of embodiments of the technology. Persons of ordinary skill in the art will recognize, however, that the technology can be practiced without one or more of the specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the technology. In the following description, the same reference labels in different drawings indicate similar or like components in structure and function. 
     Certain embodiments of the present technology relates to circuitry, apparatus, and methods for driving a plurality of LED strings. The circuitry may comprise a voltage converter, a plurality of current regulating circuits, and at least two control circuits. Each control circuit generates a control signal according, at least in part, to the output voltage of the current regulating circuits. The voltage converter regulates the DC driving voltage according to the control signals. 
     When additional LED strings are required, additional control circuits may be added. Thus the expansion of LED strings can be achieved conveniently. For the purposes of illustration, the following description is directed to circuitry for driving two groups (2n) of LED strings with two control circuits. However, one of ordinary skill in the relevant art will understand that the number of LED strings is not limited to 2n. In other embodiments, the circuitry may include three, four, or any other suitable number of control circuits each being responsible for a number of current regulating circuits. 
     In a particular embodiment, circuitry for driving a plurality of LED strings comprises a plurality of current regulating circuits, a plurality of control circuits, and a voltage converter. The current regulating circuits are separated into a plurality of groups with each current regulating circuit coupled to a LED string. The current regulating circuit is configured to regulate a current flowing through a corresponding LED string. Each control circuit is configured to generate a control signal based on output from a corresponding group of current regulating circuits. The voltage converter is electrically coupled to the LED strings and the control circuits, and is configured to provide a DC driving voltage for driving the LED strings. The voltage converter is configured to regulate the DC driving voltage based on the control signals from the control circuits. 
       FIG. 2  illustrates a circuitry for driving a plurality of LED strings according to embodiments of the present technology. The circuitry comprises a voltage converter circuit  201 , 2n current regulating circuits and two control circuits  203 - 1  and  203 - 2 . Even though only 2n current regulating circuits are shown in  FIG. 2 , in other embodiments, the circuitry may include 3n, 4n, or any other suitable number of current regulating circuits and/or other suitable components. 
     As shown in  FIG. 2 , the voltage converter circuit  201  is coupled to 2n LED strings, and is configured to receive and convert an input voltage V in  into a DC driving voltage V dc  for driving the LED strings. The voltage converter circuit  201  may include a boost converter circuit, a buck converter circuit, a fly-back converter circuit, and/or other suitable types of DC/DC converter circuit or AC/DC converter circuit. The voltage converter circuit  201  may be controlled using pulse width modulation (PWM), pulse frequency modulation (“PFM”), and/or other suitable control techniques. Suitable feedback control technique for the voltage converter circuit  201  may include peak current control, average current control, hysteresis current control, and/or other suitable feedback control techniques. 
     Each LED string is coupled to a current regulating circuit. Each current regulating circuit comprises a switch Q k , an amplifier OP k , and a sensing resistor R sk , and is configured to regulate a current flowing through the corresponding LED string based on a target value 
                 V   refk       R   sk       ,         
where k=1, 2, 3 . . . 2n.
 
     In the illustrated embodiment, the switch Q k  includes a metal oxide semiconductor field effect transistor (“MOSFET”) the drain of which is coupled to a cathode of the corresponding LED string. The sensing resistor R sk  is electrically coupled between the source of switch Q k  and ground. The non-inverting input node of amplifier OP k  receives a current reference value V refk  that represents the predetermined luminance of the LED string. The inverting input node of amplifier OP k  is electrically coupled between sensing resistor R sk  and the source of switch Q k . The output end of amplifier OP k  is coupled to the gate of switch Q k . The current reference value V refk  for one particular LED string may be different from others. In other embodiments, switch Q k  may include a bipolar junction transistor (“BJT”). 
     In operation, when a current flowing through the LED string becomes larger than the target value 
                 V   refk       R   sk       ,         
the voltage developed across sensing resistor R sk  becomes higher than the current reference value V refk . Output voltage of amplifier OP k  decreases to increase the on-state resistance R ds(on)  of the switch Q k . Accordingly the current of the LED string decreases. when the current flowing through the LED string becomes smaller than the target value
 
                 V   refk       R   sk       ,         
the voltage developed across sensing resistor R sk  becomes lower than the current reference value V refk . Output voltage of amplifier OP k  increases to decrease the on-state resistance R ds(on)  of the switch Q k . Accordingly the current of the LED string increases.
 
     The control circuits  203 - 1  and  203 - 2  are individually coupled to a group of n current regulating circuits. Based on the output voltages of the corresponding current regulating circuits, each of the control circuits  203 - 1  or  203 - 2  generates a control signal. The voltage converter circuit  201  receives the generated control signals from the control circuits  203 - 1  and  203 - 2 , and regulates the DC driving voltage V dc  accordingly. 
     When the current regulating circuits operate in normal status, the switch Q k  is in a saturated region (for MOSFETs), so that the current through the switch Q k  is in proportion to the drain-source voltage V ds . The individual LEDs have different on-state voltage drops from one another. The DC driving voltage V dc  may be regulated according to a minimum voltage among the output voltages of the current regulating circuits. The smaller the minimum voltage is, the higher the DC driving voltage may be. 
     Each of the control circuit  203 - 1  and  203 - 2  may comprise a voltage selecting circuit and an error amplifying circuit. The voltage selecting circuit is electrically coupled to n channels of the current regulating circuits, and configured to output a minimum voltage V minj  (j=1,2) among the n output voltages of the current regulating circuits. The error amplifying circuits AMP 1  and AMP  2  are each electrically coupled to the voltage selecting circuit, configured to amplify a difference between the reference voltage V ref  and the minimum voltage V minj , and to generate a control signal. The control signal may be a source current or a sink current in the error amplifying circuit. 
     The circuitry for driving LED shown in  FIG. 2  further comprises two compensation networks  204 - 1  and  204 - 2  and a compensation signal selecting circuit  205 . The compensation networks  204 - 1  and  204 - 2  are coupled between the output ends of the corresponding error amplifying circuits and ground, respectively, and are configured to generate a first compensation signal COMP 1  and a second compensation signal COMP 2  based on the control signals. The compensation signal selecting circuit  205  is electrically coupled to the compensation network  204 - 1  and  204 - 2 , and is configured to select either the first compensation signal COMP 1  or the second compensation signal COMP 2  as a compensation signal COMP. Compensation signal COMP is provided to the voltage converter circuit  201  to regulate the DC driving voltage V dc . The DC driving voltage V dc  follows the variation of the minimum voltage among the output voltages of the 2n current regulating circuits. 
     In the illustrated embodiment in  FIG. 2 , each voltage selecting circuit comprises n diodes. The voltage selecting circuit of control circuit  203 - 1  comprises n diodes labeled as Ds1-Dsn and the voltage selecting circuit of control circuit  203 - 2  comprises another n diodes labeled as Ds(n+1)-Ds2n. For one voltage selecting circuit, the cathodes of the n diodes are electrically coupled to the n corresponding current regulating circuits in one-to-one correspondence, and the anodes of the n diodes are coupled together to output a minimum voltage V minj  of the output voltages of the group of n current regulating circuits. 
     Each of the error amplifying circuits comprises an error amplifier AMP j . The non-inverting input node of the error amplifier AMP j  receives a reference voltage V ref  while the inverting input end is electrically coupled to the voltage selecting circuit to receive the minimum voltage V minj . The output end of error amplifier AMP j  provides a control signal which can be a source current or a sink current in the error amplifier. When the minimum voltage V minj  is smaller than the reference voltage V ref , the error amplifier AMP j  sources a current. When the minimum voltage V minj  is larger than the reference voltage V ref , the error amplifier AMP j  sinks a current. The value of the reference voltage V ref  may be selected based on a threshold voltage of switch Q k , the current reference value V refk  and the resistance of the sensing resistor R sk . 
     In the illustrated embodiment, each of the compensation networks  204 - 1  and  204 - 2  comprises a compensation capacitor respectively labeled as C itg1  and C itg2 . The voltages developed between the first and the second end of the compensation capacitors are applied as the first and second compensation signal COMP 1  and COMP 2 , respectively. Compensation signal selecting circuit  205  comprises a pair of diodes D 1  and D 2 . The anode of diode D 1  is electrically coupled to compensation capacitor C itg1  and the output end of error amplifier AMP 2 . The cathodes of the two diodes D 1  and D 2  are coupled together, configured to provide a larger value between COMP 1  and COMP 2  as the COMP signal to the voltage converter  201  for regulating the DC driving voltage V dc . 
       FIG. 3  illustrates circuitry for driving a plurality of LED strings according to additional embodiments of the present technology. Compared with the circuitry shown in  FIG. 2 , in the circuitry shown in  FIG. 3 , the output ends of the two error amplifying circuits are connected together and are also coupled to the voltage converter circuit  301  configured to regulate the DC driving voltage V dc . 
     A compensation network  304  is electrically coupled between the output end of the error amplifier and ground. The gain of the two error amplifiers is variable. The gain g m2  which relates to when the minimum voltage V minj  is larger than the reference voltage V ref  is different than the gain g m1  which relates to when the minimum voltage V minj  is smaller than the reference voltage V ref . In one embodiment, the gain g m1  is more than two times larger than g m2 . In other embodiments, the gain g m1  is more than three times larger than the gain g m2  or may have other relative relations to the gain g m2 . 
     As shown in  FIG. 3 , the error amplifying circuit comprises two error amplifiers AMP 3  and AMP 4 . The control signal is a source current or a sink current generated from the error amplifiers AMP 3  and AMP 4 . Setting the maximum source current larger than the maximum sink current in the error amplifiers AMP 3  and AMP 4  can enable the gain g m1  larger than g m2 . 
     In the illustrated embodiment, the compensation network  304  comprises compensation capacitor G itg . The voltage developed between the first and second end of compensation capacitor C itg  forms the compensation signal COMP. If V min1 &lt;V ref  and V min2 &gt;V ref , the error amplifier AMP 3  sources a current, and the error amplifier AMP 4  sinks a current. Because the maximum source current is larger than the sink current, a voltage across the compensation capacitor C itg , or the compensation signal COMP primarily, depends primarily on the source current generated from error amplifier AMP 3 . The increase of compensation signal COMP leads to the increase of DC driving voltages V dc  and V min1 , such that all switches operate in a saturated region to properly regulate currents flowing through the LED strings. 
       FIG. 4  illustrates a circuit of the error amplifier shown in  FIG. 3  according to embodiments of the present technology. Switches MP 1 , MP 2  and MP 3  form a current mirror, while switches MN 3  and MN 4  form another current mirror. Current flowing through the switch MP 1  is labeled as I source . Current flowing through switch MP 2  equals to the summed current following through switches MP 5  and MP 6 . The current difference between the switch MP 6  and a switch MN 4  sinks to the gate of a switch MN 5 . The gates of the switches MP 5  and MP 6  serve as the inverting and non-inverting input nodes of the error amplifier configured to receive the minimum voltage V minj  and the reference voltage V ref  respectively. The drains of switches MP 3  and MN 5  are coupled together to form an output end. 
     In one embodiment, a negative-feedback network is designed outside the error amplifier. For instance, the output end and the inverting input node of the error amplifier are electrically connected together. Setting the width-to-length ratio of the switches MP 3  and MN 5  can be used to control the maximum current flowing through switches MP 3  and MN 5 , i.e., the maximum source current and the maximum sink current of the error amplifier. In one embodiment, the maximum source current is 2000 uA and the maximum sink current is 500 uA. In other embodiments, the maximum source current and/or the maximum sink current may have other suitable values. 
     In another embodiment, the error amplifying circuit further comprises a limiting circuit configured to limit the range of the control signal. When the control signal is larger than a preselected threshold, the limiting circuit operates to limit the control signal to this threshold. A second threshold I th2  which relates to when the minimum voltage V minj  is larger than the reference voltage V ref  is different than a first threshold I th1  which relates to when the minimum voltage V minj  is smaller than the reference voltage V ref . The first threshold I th1  may be larger than the second threshold I th2 . (e.g. I th1 &gt;2I th2 ) In one embodiment, the first threshold is about 400 uA while the second threshold is about 100 uA. In other embodiments, the first and/or second thresholds may have other suitable values. 
     If V minj &lt;V ref  and V min2 &gt;V ref , the error amplifier AMP 3  sources a current and the error amplifier AMP 4  sinks a current. The source current and the sink current are limited to the first threshold I th1  and the second threshold I th2  respectively. Since I th1 &gt;I th2 , the voltage developed across the compensation capacitor C itg , or the compensation signal COMP, primarily depends on the source current I th1  generated from the error amplifier AMP 3 . The increase of the compensation signal COMP leads to the increase of the DC driving voltage V dc  and V min1 , such that all switches can operate in a saturated region to properly regulate currents flowing through the LED strings. 
       FIG. 5  is a schematic circuitry diagram of the voltage converter shown in  FIG. 3  according to embodiments of the present technology. The voltage converter is shown as a boost circuit and the control mode is shown in peak current control. The voltage converter comprises an input capacitor C in , an inductor L, a switch S 1 , a diode D, an output capacitor C out , a sensing resistor R sense , a comparator COM and an RS flip-flop FF. Input capacitor C in  is parallelly coupled to the input end of the voltage converter. The first end of inductor L is coupled to input capacitor C in  configured to receive the input voltage V in , and the second end is coupled to the drain of switch S 1  and the anode of diode D. The cathode of diode D is electrically coupled to output capacitor C out  configured to output DC driving voltage V dc . Sensing resistor R sense  is electrically coupled between the source of switch S 1  and ground, configured to sense current flowing through switch S 1  and to generate a sensed-current signal I sense  that represents the current. The inverting input node of comparator COM receives the compensation signal COMP and the non-inverting input node is coupled to sensing resistor R sense  configured to receive the sensed-current signal. The S (Setting) input end of the RS flip-flop FF receives a clock signal CLK, and the R (Resetting) input end is coupled to the output end of comparator COM. The output end of RS flip-flop FF is electrically coupled to the gate of switch S 1  configured to control the on and off of switch S 1 . 
     An adder SUM is electrically coupled between sensing resistor R sense  and comparator COM to maintain system stability when the duty cycle of the switch S 1  is larger than 0.5. The first input end of adder SUM is electrically coupled to sensing resistor R sense  to receive the sensed-current signal I sense , and the second input end receives a slope-compensation signal (e.g., a saw-tooth wave signal synchronized with the clock signal CLK). The output end of adder SUM is electrically coupled to the non-inverting input node of the comparator COM. 
       FIG. 6  illustrates circuitry for driving a plurality of LED strings according to further embodiments of the present technology. The circuitry comprises a voltage converter, two current balance circuits and two control circuits. The general operation principle of the circuitry is similar to that of the circuitry shown in  FIG. 2 . Each control circuit is integrated with a current balance circuit into an ASIC chip similar to the scheme shown in  FIG. 1 . The current balance circuit may comprise n channels of current regulating circuits as described with reference to  FIG. 2  and  FIG. 3 . The voltage converter  601  is shown as a boost circuit. 
     A first ASIC chip  602 - 1  is electrically coupled to number 1−n LED strings. The integrated current balance circuit comprising n current regulating circuits regulates current flowing through each LED string. The voltage selecting circuit receives the output voltages of the current regulating circuits, and outputs the minimum voltage to the error amplifier. The error amplifier generates a first control signal according to the minimum voltage and the reference voltage V ref . Compensation network  604 - 1  converts the first control signal into the first compensation signal COMP 1 . 
     A second ASIC chip  602 - 2  is electrically coupled to number n+1−2n LED strings, configured to regulate current flowing through each LED string and to generate a second control signal. The second control signal is converted into the second compensation signal COMP 2  through compensation network  604 - 2 . Compensation signal selecting circuit  605  receives the first and second compensation signals and selects the larger one as the compensation signal COMP, and output the compensation signal COMP to the first ASIC chip  602 - 1 . 
     The first ASIC chip  602 - 1  serves as a master chip. The PWM control circuit controls on and off of the switch in voltage converter  601  according to the compensation signal COMP, so that the DC driving voltage V dc  is regulated. The second ASIC chip  602 - 2  severs as a slave chip, and its PWM control circuit is in idle status. In one embodiment, the chips  602 - 1  and  602 - 2  may further comprise an over-voltage protection circuit enable circuit, a dimming and current setting circuit, and/or other suitable circuits. Compared with the conventional circuitry shown in  FIG. 1 , the circuitry in  FIG. 6  includes only one voltage converter instead of a plurality of voltage converters. 
       FIG. 7  illustrates circuitry for driving a plurality of LED strings according to yet further embodiments of the present technology. Compared with the circuitry shown in  FIG. 6 , in the circuitry of  FIG. 7 , no PWM control circuit is integrated in the ASIC chip  702 - 2 . In other embodiments, the ASIC chip  702 - 1  may not comprise the PWM controller, and the PWM controller may be integrated into another discrete IC chip (not shown). 
       FIG. 8  illustrates a flow diagram of a process of driving a plurality of LED strings according to embodiments of the present technology. The process includes:
         Operation A: providing a DC driving voltage V dc  to a plurality of LED strings;   Operation B: regulating a current flowing through the LED strings by a plurality of current regulating circuits. The plurality of current regulating circuits are separated into a plurality of groups;   Operation C: generating at least two control signals. Each control signal is generated according the output voltages of one group of the current regulating circuits. in one embodiment, first and second control signals are generated according to a reference voltage and the minimum voltage detected among the output voltages of the group of the current regulating circuits. The control signal can then be generated by selecting to output the minimum voltage and amplifying the difference between the minimum voltage and the reference voltage through the error amplifying circuit;   Operation D: regulating the DC driving voltage V dc  according to the at least two control signals.       

     In one embodiment, the method for driving a plurality of LED strings further comprises generating compensation signals according to the at least two control signals, and using one of the at least two compensation signals (e.g. the maximum value) to regulate the DC driving voltage. In another embodiment, the gain g m2  of the error amplifier which relates to when the minimum voltage is larger than the reference voltage is different than the gain g m1  which relates to when the minimum voltage is smaller than the reference voltage. The gain g m1  may be larger than g m2 , For instance, g m1 &gt;2 g m2 , or is otherwise different than g m2 . 
     In other embodiments, the error amplifying circuits may limit the control signals to a threshold when the control signals are larger than the threshold. The threshold may be related to instances when the minimum voltage is smaller than the reference voltage, and is different than the threshold that is related to instances when the minimum voltage is larger than the reference voltage. The threshold I th1  may be larger than I th2 , for instance: I th1 &gt;2I th2 , or is otherwise different than I th2 . 
     From the foregoing, it will be appreciated that specific embodiments of the technology have been described herein for purposes of illustration, but that various modifications may be made without deviating from the disclosed technology. Elements of one embodiment may be combined with other embodiments in addition to or in lieu of the elements of the other embodiments. Accordingly, the technology is not limited except as by the appended claims.