Abstract:
The present invention contemplates a variety of improved techniques for the fast switching of current through, among others, LED loads. A current shunting device is utilized to divert current away from a load at high speed when activated, thus enabling the control of the amount current that flows through the load.

Description:
RELATED APPLICATIONS  
       [0001]     The present application claims priority to and is a utility patent application of Nalbant&#39;s U.S. Provisional Application No. 60/819,049, filed Jul. 7, 2006, entitled HIGH CURRENT FAST RISE AND FALL TIME LED DRIVERS, which is hereby incorporated by reference. 
     
    
     BACKGROUND  
       [0002]     1. Field of Invention  
         [0003]     This invention relates to the field of high current LED driver.  
         [0004]     2. Background of the Invention  
         [0005]     High brightness and high current light emitting diodes (LED) are increasingly being used as high intensity light sources. High intensity LEDs provide many benefits over other high intensity light sources, such as longer life, wider color range, less hazardous operating voltages, and higher efficiency. In some rear projection TVs and front projection systems the light from an LED is required to be switched very rapidly as required by the Digital Micromirror Device (DMD).  
         [0006]     The digital micromirror device (DMD) imager is a digital light valve that either reflects or deflects a light source. Color images are formed by sequentially shining the DMD with a Red, Green and Blue light source and by temporal modulation of the intensity of the light reflected from each DMD pixel. Because of this fast modulation the DMD imager requires a red, blue, and green LED to be switched on and off very fast which necessitates the LED current to be switched ON and OFF very fast. The current switching required has been difficult with conventional means. In the past the switching of current to an LED was accomplished by charging and discharging the inductor in a switching regulator. In this case switching regulators with high efficiency are highly desirable to prevent excessive power loss as a result of switching several amperes of current. This suffers from many shortcomings, most importantly the difficulty in switching the current as quickly as needed.  
         [0007]     The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention contemplates a variety of improved techniques for the fast switching of high amplitude current. A current shunting device can be utilized to divert a high amplitude current away from a load at high speed when activated, thus enabling the control of the amount current that flows through the load. These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following descriptions and a study of the several figures of the drawings.  
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0009]     These and other objects, features and characteristics of the present invention will become more apparent to those skilled in the art from a study of the following detailed description in conjunction with the appended claims and drawings, all of which form a part of this specification. In the drawings:  
         [0010]      FIG. 1  is an exemplary block diagram of a high current fast rise and fall time load driver according to one embodiment of the present invention.  
         [0011]      FIG. 2  is an exemplary block diagram of a high current fast rise and fall time load driver according to one embodiment of the present invention.  
         [0012]      FIG. 3  is an exemplary diagram of a high current fast rise and fall time load driver according to one embodiment of the present invention.  
         [0013]      FIG. 4  is an exemplary diagram of a high current fast rise and fall time load driver according to one embodiment of the present invention.  
         [0014]      FIG. 5  is an exemplary diagram of a high current fast rise and fall time load driver according to one embodiment of the present invention.  
         [0015]      FIG. 6  is an exemplary diagram of a ground-referred buck-boost LED driver according to one embodiment of the present invention.  
         [0016]      FIG. 7  is an exemplary block diagram of a method for fast switching of a high amplitude load.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     In the following description, several specific details are presented to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or in combination with other components, etc. In other instances, well-known implementations or operations are not shown or described in detail to avoid obscuring aspects of various embodiments, of the invention.  
         [0018]      FIG. 1  is an exemplary block diagram of a high current fast rise and fall time load driver  100  according to one embodiment of the present invention. The load driver  100  includes current source  102 , and one or more current shunting device  104  which is parallel coupled with a load  106  to a common ground  199 . The current source  102  is a controlled current I C  which may be in parallel with the current shunting device  104  and the load  106 . An output  132  of the current source  102  is a controlled current I C  which may drive the current shunting device  104  and the load  106  with a substantially constant current. The ON and OFF operation (activate or deactivate) of the current shunting device  104  may be controlled by an input signal  130  to the current source  102  from accompanying devices, circuitries and/or systems, e.g., by a video control signal derived from a source such as a video processor or a high speed pulse train. Another input  131  to the current source can be used to adjust the amplitude of the controlled current I C . The controlled current I C  may be switched away from the load  106  at high speed by shunting the controlled current I C  through the current shunting device  104 .  
         [0019]     In some example embodiments, the current shunting device  104  may shunt substantially all of current I C  when the current shunting device is activated, making I S  substantially equal to I C  and I LOAD  substantially equal to zero. When the current shunting device  104  is not activated the current shunting device  104  shunts substantially none of the current I C , making I C  substantially equal to I LOAD . In an example embodiment, the current shunting device  104 , when activated, may shunt only a portion of I C . The current shunting device  104  may vary in resistance and the resistance may be controlled by accompanying devices, circuitries and/or systems, e.g., by a video control signal derived from a source such as a video processor or a high speed pulse train. Depending on the resistance value of the current shunting device  104 , Is and I LOAD  may both be greater than zero, so long as I C  is greater than zero.  
         [0020]     In some example embodiments, the current source  102  includes an inductor. The inductor and its associated switching circuitry may be kept in a charged state, and may supply the substantially stable current, I C . The inductor may also be charged and discharged while in operation, which may result in a varying current source, I C , rather than a substantially stable current. Discharging the inductor may be used in combination with shunting the current I C .  
         [0021]     In some example embodiments, the shunting device  104  includes a switch, which can be but is not limited to, a low impedance metal oxide semiconductor field-effect transistor (MOSFET), an insulated-gate field-effect transistor (IGFET), or a bipolar junction transistor (BJT). In the case of MOSFET, for a non-limiting example, the use of a MOSFET in the current shunting device  104  may require a voltage difference to be applied across the source and gate on the MOSFET. The voltage difference may be varied, and may result in the impedance of the MOSFET being varied. The MOSFET may also be used digitally where the voltage difference is varied between two states, one to divert substantially all of a current, and a second to divert substantially none of the current.  
         [0022]     In some example embodiments, the load  106  is any device and/or system known or convenient. The load  106  may have substantially constant or varying impedance. In some exemplary embodiments the load  106  is coupled to a ground source such as ground  199 . An example load  106  includes a light emitting diode (LED) or a string of LEDs. The load driver  100  may switch the LED or LEDs rapidly and may allow high amplitude current to be switched in sub-microseconds time. In some example embodiments, a LED may be switched in less than 2 μsecs.  
         [0023]     In some example embodiments, the high current fast rise and fall time load driver  100  may have synchronous rectification. Synchronous rectification may be achieved by including a diode and a transistor in parallel. In an exemplary operation, synchronous rectification may reduce voltage drop because when the diode is forward-biased, the transistor is closed and thereby reduces the voltage drop. When the diode is reverse-biased, the transistor is open. In some example embodiments, the transistor used may be a MOSFET. Synchronous rectification is not required but may be advantageous in some embodiments.  
         [0024]     In some example embodiments, a freewheeling diode can be used to provide a path for the release of energy stored in the load when the load voltage drops to zero. The freewheeling diode helps to prevent damage to circuit components caused by the energy stored in the load in case such energy arcs across the contacts of the switch when the switch is opened.  
         [0025]      FIG. 2  is an exemplary block diagram of a high current fast rise and fall time load driver  200  according to one embodiment of the present invention. The load driver  200  includes a controller (a controlling circuit/power circuit)  201 , a current source  202 , and a current shunting device  204  which is parallel coupled with a load  206  to a common ground  299 . The controller  201  may be an integrated circuit (IC) including both the current source  202  and the current shunting device  204 . An output  232  of the current source  202  is a controlled current I C  which may drive the parallel coupled low impedance current shunting device  204  and the load  206  with a substantially constant current. The ON/OFF operation of the current shunting device  204  may be controlled by an input signal  230  to the controller  201  accompanying devices, circuitries or systems, for example, by a video control signal derived from a source such as a video processor or a high speed pulse train. Another input  231  to the controller  201  can be used to adjust the amplitude of the controlled current I C . The current I C  may be applied to the load  106  or switched away from the load  106  by shunting the controlled current I C  through the current shunting device  104 . The load  206  may be external to the controller  201 . In some example embodiments the load  206  and controller  201  are on the same IC or printed circuit board (PCB). In other example embodiments the load  206  is not on the same IC or PCB as the controller  201  and may be coupled to the controller  201  in any manner known and convenient (i.e. wires, etc.).  
         [0026]      FIG. 3  is an exemplary diagram of a high current fast rise and fall time load driver  300  according to one embodiment of the present invention. The load driver  300  includes a controller  301 , an inductor  302 , and a switching transistor  304  which is parallel coupled with a light emitting diode (LED)  306  and a common ground  399 . The controller  301  includes a DD pin, which is coupled to the switching transistor  304  and may activate and de-activate the switching transistor  304 , thereby diverting the current supplied from the inductor  302  away from the LED  306 . The DD pin may control activation of the switching transistor  304  by varying the DD pin voltage value. The controller  301  may be implemented in any manner known or convenient, for example as an integrated circuit (IC), and in some example embodiments will include additional pins for increased functionality. The inductor  302  may be any inductor known or convenient. The inductor  302  is charged by a voltage source through the switching transistor  304 . It controls the ripple current and opposes changes in currents when charged, and thus provides a substantially stable current so long as the inductor is charged.  
         [0027]     In some example embodiments, the required and/or preferred properties of the inductor  302  will vary the operating requirements of the load driver  300 . For example, switching frequency, peak inductor current and allowable ripple at the output may determine the inductance value and size of the inductor  302 . In general, selecting higher switching frequencies reduces the inductance requirement of the inductor  302  but will result in a lower efficiency. Also, the charging and discharging cycle of the inductor  302  and the drain capacities in the switching transistor  304  may create switching losses. In some example embodiments, lower switching frequencies should be used to reduce switching losses.  
         [0028]     The switching transistor  304  may be any transistor known or convenient. In some example embodiments, a MOSFET may be used. The MOSFET may operate as a gate or shunting device, allowing substantially zero current across the source and drain terminals when inactive. If a MOSFET is used as the switching transistor  304 , an input pin named LEDPWM or DIM or PWM to controller  301  is operable to control the ON and OFF sequence of  304  via the DD pin on controller  301 , where DD may activate the MOSFET by the voltage applied on the gate terminal. Alternatively, the control signal may come directly from a control system without first being applied to the controller  301 . A MOSFET may be chosen by the total gate charge (RDS(ON)), power dissipation, package thermal impedance, cost, etc. A MOSFET optimized for high-frequency switching applications may be advantageous in some embodiments.  
         [0029]     The LED  306  may be any LED known or convenient. In operation, the LED  306  may require high amplitude current to operate and may require and/or benefit from fast switching of the current. In some example embodiments, the LED  306  may be a string of LEDs. An input pin named ICOM to controller  301  is operable to adjust the amplitude of the current required to operate the LED.  
         [0030]      FIG. 4  is an exemplary diagram of a high current fast rise and fall time load driver  400  according to one embodiment of the present invention. The load driver  400  includes a controller  401 , an inductor  402 , switching transistors—Q 1   404 - 1 , Q 2   404 - 2 , and Q 3   404 - 3 , a light emitting diode (LED)  406 , resistors—R 1   407 - 1 , R 2   407 - 2 , R 3   407 - 3 , capacitors  408 —C 1   408 - 1 , C 2   408 - 2 , C 3   408 - 3 , C 4   408 - 4 , C 5   408 - 5 , C 6   408 - 6 , a diode  409  and a ground  499 .  
         [0031]     The controller  401  includes at least the following pins PGN, GND, RTCT, CSS, COMP, SYNC, ICOM, PWM, EN, IN, REG5, BST, DH, LX, DL, CSP, CSN and DD. The DD pin is coupled to the switching transistor  404  and may activate the switching transistor  404 , thereby controlling the switching of current from the inductor  402  away from the LED  406 . The DD pin may control activation of the switching transistor  404  by the voltage value applied to the pin. The controller  401  may be implemented in any manner known or convenient, for example as an integrated circuit (IC), and in some example embodiments will include additional pins for increased functionality, while in others some pins may be omitted.  
         [0032]     The inductor  402  may be any inductor known or convenient. The inductor  402  may control the ripple current and may oppose changes in current when charged, and thereby may provide a substantially stable current. The switching frequency, peak inductor current and allowable ripple at the output may determine the suitable inductance value and size of the inductor  402 . In general, selecting higher switching frequencies reduces the inductance requirement of the inductor  402  but will result in a lower efficiency. The charging and discharging cycle of the inductor  402  and the drain capacities in the switching transistor  404  may create switching losses. Using lower switching frequencies may reduce switching losses.  
         [0033]     The switching transistors  404  may be any combination of transistors known or convenient. In some exemplary embodiments, MOSFETs may be used for Q 1   404 - 1 , Q 2   404 - 2 , and Q 3   404 - 3 . The switching transistors  404  may operate as gates, allowing substantially zero current across the source and drain terminals when inactivate. If a MOSFET is used as Q 1   404 - 3 , input PWM from a control system to controller  401  is operable to control the ON and OFF sequence of  404 - 3  via the DD pin on controller  401 , where DD may activate the MOSFET by the voltage applied on the gate terminal. Alternatively, the signal may come directly from the control system without first being applied to  401 . Input ICOM to controller  401  is operable to adjust the amplitude of the current required to operate the LED. In some example embodiments, a MOSFET may be chosen by the total gate charge (RDS(ON)), power dissipation and package thermal impedance. In some example embodiments, it may be advantageous to choose a MOSFET optimized for high-frequency switching applications. The Q 1   404 - 1  and Q 2   404 - 2  may be controlled respectively by the voltages of the DH and DL pins of the controller  401 .  
         [0034]     The resistors  407  may be any combination of resistors known or convenient. The resistors  407  may be of any combination of resistance value, tolerance, and operating parameters as required for the driver and may depend on the values of the other components. Alternatively, this resistor can be placed between the common connection of the source of Q 3  and LED cathode and the ground. This just makes it more convenient to sense the current flow and it is electrically equivalent to the connection method of  FIG. 4 . In some cases there maybe some capacitance added across the output to reduce the current ripple that flows through the LED.  
         [0035]     The capacitors  408  may be any combination of capacitors known or convenient. The capacitors  408  may be of any combination of capacitance value, tolerance, and operating parameters as required for the driver  400  and may depend on the values of the other components.  
         [0036]     The diode  409  may be any diode known or convenient. For example, in some example embodiments the diode  409  may be a zener or schottky diode. The diode  409  may be of any combination of operating parameters as required for the driver  400  and may depend on the values of the other components.  
         [0037]      FIG. 5  is an exemplary diagram of a high current fast rise and fall time load driver  500  according to one embodiment of the present invention. The load driver  500  includes an integrated circuit (IC)  501 , a (buck) inductor  502 , switching transistors—Q 1   504 - 1 , Q 2   504 - 2 , and Q 3   504 - 3 , a high amp load  506 , a resistor  507 , capacitors  508 —C 1   508 - 1 , C 2   508 - 2 , and a ground  599 . A control signal such as a high-frequency pulse train  530  can be used to control the switching transistor Q 3   504 - 3 .  
         [0038]     The IC  501  includes the following pins PGN, CLP, OVI, ILIM, EN, IN, DH, DL, and CSP. The PGN pin may operate as a power-supply ground or as substantially equivalent to ground. The CLP pin may operate as a current-error amplifier output. The CLP pin may compensate the current loop by connecting an RC network to ground. The OVI pin may operate as an overvoltage protection. The OVI pin may be coupled to a difference amplifier coupled to the input and output terminals of the load  506 , and if the difference output by the difference amplifier exceeds a predetermined value the DH and DL pin values are changed. The ILIM pin may operate as a current-limit setting input. The ILIM pin may be connected to ground through a resistor, and the resistance value of the resistor sets the “hiccup” current-limit threshold. The ILIM may be connected to the ground  599  through a capacitor to ignore output overcurrent pulses. The EN pin may operate as an output enable. The EN pin may be driven high or unconnected for normal operation mode. The EN pin may also be driven low to shut down the power drivers. The EN pin may also be connected ground through a capacitor to program a hiccup-mode duty cycle. The IN pin may operate as a supply voltage connection. The DH pin is coupled to the gate terminal on the Q 1   504 - 1  and may operate as a high-side gate driver output for Q 1   504 - 1 . The DL pin is coupled to the gate terminal on the Q 2   504 - 2  and may operate as a low-side gate driver output for Q 2   504 - 2 . The CSP pin may operate as a current-sense differential amplifier positive input. The differential voltage between the CSP and a negative voltage input may be amplified internally to measure the current from the inductor  502 .  
         [0039]     The inductor  502  may be any inductor known or convenient. The inductor  502  controls the ripple current and may oppose changes in currents when charged and thereby may provide a substantially stable current when charged. The switching frequency, peak inductor current and allowable ripple at the output of the inductor  502  may determine the inductance value and size of inductor  502 . In general, selecting higher switching frequencies reduces the inductance requirement of the inductor  502  but will result in a lower efficiency. The charging and discharging cycle of the inductor  502  and the drain capacities in the Q 3   504 - 3  may create switching losses. Lower switching frequencies may be used to reduce switching losses.  
         [0040]     The switching transistors  504  may be any combination of transistors known or convenient. In some exemplary embodiments, a combination of MOSFETs and/or IGFETs may be used for Q 1   504 - 1 , Q 2   504 - 2 , and Q 3   504 - 3 . The MOSFETs may operate as gates, allowing substantially zero current across the source and drain terminals when inactivate and allowing substantially all current across the source and drain terminals when activated. If a MOSFET is used as Q 3   504 - 3 , the coupled pulse train  530  may activate the Q 3   504 - 3  by changing a voltage on the gate terminal of Q 3   504 - 3 . A MOSFET may be chosen by the total gate charge (RDS(ON)), power dissipation and package thermal impedance. It may be advantageous to choose a MOSFET optimized for high-frequency switching applications. The Q 1   504 - 1  and Q 2   504 - 2  may be controlled by the voltages of the DH and DL pins, respectively, of the IC  501 .  
         [0041]     The resistor  507  may be any resistor known or convenient. The resistor  507  may be of any combination of resistance value, tolerance, and operating parameters as required for the driver  500  and may depend on the values of the other components. In some example embodiments resistor  507  operates so VI is not shorted to the ground  599 .  
         [0042]     The capacitors  508  may be any combination of capacitors known or convenient. The capacitors  508  may be of any combination of capacitance value, tolerance, and operating parameters as required for the driver  500  and may depend on the values of the other components.  
         [0043]     In some example embodiments, the load driver  500  is in a basic buck topography where the inductor  502  is always connected to the high amp load  506 . This design may minimize the current ripple by oversizing the inductor  502  and may allow for a very small output capacitor (C 2   508 - 2 ). The Q 3   504 - 3  may be activated and divert the current around the high amp load  506  at a very fast rate. The Q 3   504 - 3  may also discharge an output capacitor (C 2   508 - 2 ) and because the capacitance is so small the capacitor (C 2   508 - 2 ) will not be stressed. In some example embodiments, the resistor  507  may sense the current and there is no reaction to the short that Q 3   504 - 3  places the across the high amp load  506 . The Q 3   504 - 3  may need to dissipate the high amp load  506  current applied on the Q 3   504 - 3  RDS(ON) at some maximum duty cycle. If the driver  500  needs to control very high currents switching transistors in parallel may be used.  
         [0044]      FIG. 6  is an exemplary diagram of a ground-referred buck-boost driver  600  according to one embodiment of the present invention. The LED driver  600  includes an integrated circuit (IC)  601 , inductors  602 , switching transistors  604 —Q 1   604 - 1 , Q 2   604 - 2 , Q 3   604 - 3 , a light emitting diode (LED) string  606 , resistors—R 1   607 - 1 , R 2   607 - 2 , R 3   607 - 3 , R 4   607 - 4 , R 5   607 - 5 , R 6   607 - 6 , R 7   607 - 7 , R 8   607 - 8 , R 9   607 - 9 , R 10   607 - 10 , R 11   607 - 11 , R 12   607 - 12 , capacitors  608 —C 1   608 - 1 , C 2   608 - 2 , C 3   608 - 3 , C 4   608 - 4 , C 5   608 - 5 , C 6   608 - 6 , C 7   608 - 7 , C 8   608 - 8 , C 9   608 - 9 , C 10   608 - 10 , C 11   608 - 11 , a diode  609  and a ground  699 .  
         [0045]     The inductor  602  may be any inductor known or convenient. The inductor  602  controls the ripple current and may oppose changes in currents when charged and thereby may provide a substantially stable current when charged. The switching frequency, peak inductor current and allowable ripple at the output may determine the inductance value and size of inductor  602 . In general, selecting higher switching frequencies reduces the inductance requirement of the inductor  602  but will result in a lower efficiency. The charging and discharging cycle of the inductor  602  and the drain capacities in the switching transistor  604  may create switching losses. Using lower switching frequencies may be used to reduce switching losses.  
         [0046]     The switching transistors  604  may be any combination of transistors known or convenient. In some example embodiments, a MOSFET or IGFET may be used for Q 3   604 - 3 . The MOSFET will operate as gate, allowing substantially zero current across the source and drain terminals when inactivate. In some example embodiments, a MOSFET may be chosen by the total gate charge (RDS(ON)), power dissipation and package thermal impedance. In some example embodiments it may be advantageous to choose a MOSFET optimized for high-frequency switching applications. The Q 1   604 - 1  and Q 2   604 - 2  may be controlled respectively by the voltages of the DH and DL pins of the controller  601 .  
         [0047]     The resistors  607  may be any combination of resistors known or convenient. The resistors  607  may be of any combination of resistance value, tolerance, and operating parameters as required for the driver and may depend on the values of the other components.  
         [0048]     The capacitors  608  may be any combination of capacitors known or convenient. The capacitors  608  may be of any combination of capacitance value, tolerance, and operating parameters as required for the driver  600  and may depend on the values of the other components.  
         [0049]     In some example embodiments, the driver  600  may be in a buck/boost topography. During the on-time the current may flow from the input capacitor (C 2   608 - 2 ), through the Q 1   604 - 1 , the L 1   602 - 1 , and the Q 3   604 - 3  and back to the input capacitor. During the off-time current may flow up through the Q 2   604 - 2 , the inductor  602  and the diode  609  and to the output capacitor (C 1   608 - 1 ). The driver  600  may allow the inductor  602  to reside between input and ground during the on-time and during the off-time and may allow the inductor  602 - 1  to reside between the ground  699  and the output capacitor (C 1   608 - 1 ). This may allow the driver  600  to output voltage which may be any voltage less than, equal to, or greater than the input voltage.  
         [0050]      FIG. 7  is an exemplary block diagram of a method for fast switching of a high amplitude load. Block  702  depicts providing a substantially constant high amplitude current source. Block  704  depicts providing a load. Block  706  depicts providing a shunting circuit. Block  708  depicts applying a high amplitude current to the load from the current source. Block  710  depicts activating the shunting circuitry. Block  712  depicts diverting the current away from the load by the shunting circuitry creating a low impedance connection.  
         [0051]     As used herein, the term “embodiment” means an embodiment that serves to illustrate by way of example but not limitation.  
         [0052]     It will be appreciated to those skilled in the art that the preceding examples and embodiments are exemplary and not limiting to the scope of the present invention. It is intended that all permutations, enhancements, equivalents, and improvements thereto that are apparent to those skilled in the art upon a reading of the specification and a study of the drawings are included within the true spirit and scope of the present invention. It is therefore intended that the following appended claims include all such modifications, permutations and equivalents as fall within the true spirit and scope of the present invention.