The present disclosure is directed to control of light emitting diode (LED) arrays and other loads, for example, to current regulation of such loads.
Light emitting diode (LED) arrays are commonly employed in a wide range of applications. For example, LED arrays are now employed to provide backlighting for liquid crystal display (LCD) televisions, LCD monitors, LED displays, lighting devices, and/or the like. In systems where numerous LEDs are employed, the LEDs are commonly arranged in multiple strings of LEDs (e.g., to simplify drive and control circuitry while still enabling selective control of portions of the LED array).
Non-limiting and non-exhaustive embodiments are described with reference to the following drawings. In the drawings, like reference numerals refer to like parts throughout the various figures unless otherwise specified. These drawings are not necessarily drawn to scale. Likewise, the relative sizes of elements illustrated by the drawings may differ from the relative size depicted.
The following description provides a description for exemplary embodiments of the technology. One skilled in the art will understand that the technology may be practiced without many or all of the features described herein. In some instances, well-known structures and functions have not been shown or described in detail to avoid unnecessarily obscuring the description of the embodiments of the technology. It is intended that the terminology used in the description presented below be interpreted in its broadest reasonable manner, even though it is being used in conjunction with a detailed description of certain embodiments of the technology. Although certain terms may be emphasized below, any terminology intended to be interpreted in any restricted manner will be overtly and specifically defined as such in this Detailed Description section. The term “based on” or “based upon” is not exclusive and is equivalent to the term “based, at least in part, on” and includes being based on additional factors, some of which are not described herein. The term “coupled” means at least either a direct electrical connection between the items connected, or an indirect connection through one or more passive or active intermediary devices or mediums. The term “circuit” means at least either a single component or a multiplicity of components, either active and/or passive, that are coupled together to provide a desired function or functions. The term “signal” means at least one current, voltage, charge, temperature, data, or other signal. A “signal” may be used to communicate using active high, active low, time multiplexed, synchronous, asynchronous, differential, single-ended, or any other digital or analog signaling or modulation techniques. References in the singular are made merely for clarity of reading and include plural references unless plural references are specifically excluded. Further, references to groups of elements (e.g., loads 111-11n, current limiter circuits 121-12n, current limiter circuits 141-14n, etc.) in collective relation to other groups of elements are made merely for clarity of reading. Such references refer to the relationships of each element of the first group to each respective element of a second group unless specifically indicated otherwise. For example, “loads 111-11n are coupled to current limiter circuits 121-12n” means that load 111 is coupled to current limiter circuit 121, load 112 is coupled to current limiter circuit 122, and load 11n is coupled to current limiter circuit 12n. Likewise, references directly to a group may also include individual reference to each element of the group. For example, “loads 111-11n” may mean “each of load 111, load 112, and load 11n.” The term “or” is an inclusive “or” operator and is equivalent to the term “and/or” unless specifically indicated otherwise. In the description that follows, the scope of the term “some embodiments” is not to be so limited as to mean more than one embodiment, but rather, the scope may include one embodiment, more than one embodiment, or perhaps all embodiments.
Some embodiments of apparatus, systems, and methods for controlling multiple strings of light emitting diodes (LEDs) are disclosed. An apparatus may include internal current limiter circuits that are each coupled in series with an associated string of LEDs and are configured to at least partially regulate the current through the associated LED string. The apparatus may also be configured to control external current limiter circuits that are each coupled in series with a corresponding internal current limiter circuit and the LED string associated with the corresponding internal current limiter circuit. The external current limiter circuits may all be configured to be controlled with the same drive signal or several drive signals.
Some embodiments of the technology described herein may be employed to address thermal issues related to regulating currents through multiple LED strings with system controller integrated circuit (IC) having a relatively low pin-count. In an example system, internal current limiter circuits (e.g., internal to the system controller IC) are employed to at least partially regulate current through each of the multiple LED strings. In addition, external current limiter circuits (e.g., current limiter circuits external to the system controller IC) may also be employed to partially regulate currents through each of the multiple LED strings, for example, based on whether the internal current limiter circuits are operating within a regulation range (e.g., with head-room, within a linear operating region, within a middle portion of an operating region, etc.). For example, partial regulation of a current may include controlled blocking of any portion of the voltage dropped in regulating a current, controlled dissipation of any portion of the power lost in regulating a current, and/or the like. The technology may be employed to regulate the current through LED arrays having any number of LED strings.
By employing external current limiter circuits, the heat generated in the system controller IC may be less than if internal current limiter circuits were employed to regulate the currents through the multiple LED strings without also employing external current limiter circuits. In addition, a relatively low pin-count for the system controller IC may be maintained by employing a common/shared drive signal for driving each of the external current limiter circuits.
In one embodiment, loads 111-11n may include any number of LEDs, electroluminescent devices, or other illumination devices, and/or the like, configured as single devices, in strings of devices, in arrays of LEDs, and/or the like. Loads 111-11n may also be controlled to provide illumination at any of multiple intensity levels by current limiter circuits 121-12n, system controller 130, and power converter 190. As one example, loads 111-11n may be controlled to provide any of multiple intensity settings for all loads 111-11n or for individual loads. For example, such control over intensity levels may be employed to provide dynamic contrast, to optimize between brightness and power consumption, and/or the like.
While loads 111-11n are generally referred to in this Detailed Description section as being illumination devices, loads 111-11n may include non-illumination device loads. As one example, non-illumination device loads may include any electrical load through which electrical current may flow. For example, loads 111-11n may include electronic devices or circuits such as motors, sensors, transmitters, ICs, batteries, battery chargers, and/or the like.
In one embodiment, current limiter circuits 121-12n are coupled in series with loads 111-11n and are configured to partially regulate currents through loads 111-11n. As illustrated in
As one example, current limiter circuits 121-12n may include electronically controllable switches having electronically controllable impedances. For example, devices having linear active regions may be employed as suitable electronically controllable switches. Such devices may include insulated-gate bipolar transistors (IGBTs), junction field effect transistors (JFETs), bipolar junction transistors (BJTs), metal oxide semiconductor field effect transistors (MOSFETs), metal semiconductor field effect transistors (MESFETs), and/or the like. Other devices such as linear current regulators and other current regulators may also be suitably employed.
As one example, system 100 may be configured such that current limiter circuits 121-12n provide a majority of the power dissipation and/or voltage dropping as compared to current limiter circuits 141-14n. With such an example, devices having relatively high-power handling characteristics may be employed as current limiter circuits 121-12n. In this manner, a portion of the overall heat generated in system 100 may be generated by current limiter circuits 121-12n rather than by system controller 130.
System controller 130 may be configured to regulate the currents through loads 111-11n by (1) employing internal current limiter circuits to perform at least partial current regulation, (2) controlling current limiter circuits 121-12n to perform additional partial current regulation, and/or (3) by controlling the power conversion operations of power converter 190. As shown, system controller 130 includes current limiter circuits 141-14n and sense and control unit 150.
System controller 130 may be embodied in a monolithic IC, in an application specific integrated circuit (ASIC), and/or the like. System controller 130 may also be fully or partially embodied as discrete components, as a circuit board assembly, and/or the like. In these and other embodiments, system controller 130 may have a relatively low pin-count and/or a relatively simple interface with the rest of system 100.
As shown in
Further, system controller 130 may also include sense and control unit 150, which may be configured to control current limiter circuits 121-12n via common drive signal V_DRV, and to control power converter 190 via power converter control signal PWR_CTL. Sense and control unit 150 may also be configured to provide both common drive signal V_DRV and power converter control signal PWR_CTL based on a voltage differential across at least one of current limiter circuits 141-14n, for example, as received via one of signals FB1-FBn.
In addition, sense and control unit 150, current limiter circuits 141-14n, or other elements may further include protection circuitry or logic to disable currents through any of loads 111-11n if an error condition occurs. Potential error conditions may include open or short circuit conditions in any of loads 111-11n or other circuitry, over or under temperature conditions of current limiter circuits 121-12n or other circuitry, and/or any other conditions.
As shown in
As one example, power converter 190 may include a switched mode power supply configured to provide a direct current (DC) voltage of a suitable value to loads 111-11n. To provide some examples, power converter 190 may include a boost converter, a buck converter, a buck/boost converter, a fly-back converter, an inverting converter, a push-pull converter, and/or the like.
Further, system controller 130 and power converter 190 may be interfaced via additional power converter control signals. For example, system controller 130 may provide control signals for a boost regulator's synchronous switch, asynchronous switch, safety disconnect switch, to configure and/or compensate for frequency characteristics, and/or the like. Power converter 190 may also be configured to provide a current sense signal, an over-voltage protection sensing signal, other feedback signals, and/or the like, to system controller 130.
In one embodiment, power converter 190 is a boost converter configured to provide a DC voltage of between approximately 30 volts and 100 volts to drive a multi-string LED array of an LCD television or LCD monitor.
As shown, current limiter circuits 141-14n include sense resistors RS1-RSn, error amplifiers EA1-EAn, and internal switches Si1-Sin. Current limiter circuits 141-14n may also be configured to perform closed loop regulation of the currents through internal switches Si1-Sin based on the value of sense resistors RS1-RSn and the value of reference signals IR1-IRn.
Sense resistors RS1-RSn are configured to provide current sense signals CS1-CSn to the inverting inputs of error amplifiers EA1-EAn based on the currents through sense resistors RS1-RSn. Sense resistors RS1-RSn may be of any suitable type and/or value and may be selected based on expected or designed ranges of currents through loads 111-11n.
Error amplifiers EA1-EAn may be configured to receive reference signals IR1-IRn and current sense signals CS1-CSn, and to provide pass transistor control signals DR1-DRn based on a comparison of reference signals IR1-IRn and current sense signals CS1-CSn. Error amplifiers EA1-EAn may also include operational amplifiers, instrumentation amplifiers, differential amplifiers, and/or the like and circuits thereof.
Internal switches Si1-Sin may be configured as pass transistors coupled in series with loads 111-11n to partially regulate the currents through loads 111-11n based on pass transistor control signals DR1-DRn from error amplifiers EA1-EAn. While internal switches Si1-Sin are illustrated as being N-Channel MOSFET switches, any suitable types of switches may be employed.
Although illustrated as linear current regulators, current limiter circuits 141-14n may include other types of current limiter circuits. For example, switches (such as the switches discussed above with respect to current limiter circuits 121-12n), current mirrors, and/or the like, may be employed in other embodiments.
In operation, current limiter circuits 121-12n and current limiter circuits 141-14n may function together to regulate the currents though loads 111-11n. As an example of the combined operation of these circuits, when a given one of signals FB1-FBn is low, the corresponding external switch may be fully on and the corresponding internal linear regulator may fully and/or primarily regulate the current for the associated load. As the given one of signals FB1-FBn increases, the gate-to-source voltage of the corresponding external switch may decrease such that it enters a linear region and drops more voltage while the corresponding internal linear regulator begins to only partially regulate the associated load. For a gate-to-source threshold voltage equaling Vth, the maximum voltage drop of any of the internal linear regulators may be V_DRV-Vth, which may be significantly less than if only internal linear regulators were employed to regulate the currents to loads 111-11n.
Although not shown, system 200 may also include circuitry and/or functionality to provide selective dimming of each of loads 111-11n independent of each of the other loads. For example, system 200 may include additional control circuitry to selectively open and close internal switches Si1-Sin as controlled by, for example, a pulse width modulation (PWM) or other controller. Likewise, system 200 may include circuitry and/or functionality to provide selective black-outs or blanking. As one example, common drive signal V_DRV may be pulled low to disable current through all of loads 111-11n at the same time. Such circuitry or functionalities may be controlled from within system controller 130, via an external signal, within sense and control unit 150, and/or the like.
As discussed above, sense and control unit 150 may be configured to control current limiter circuits 121-12n via common drive signal V_DRV based on a voltage differential across at least one of current limiter circuits 141-14n, for example, as received via one of signals FB1-FBn.
To provide this functionality, current limiter drive control unit 351 may be configured to sense voltage differentials across each of current limiter circuits 141-14n by monitoring signals FB1-FBn with maximum selector 352. Alternately, current limiter drive control unit 351 may be configured to sense voltage differentials across each of current limiter circuits 121-12n or to sense voltage differentials across each combination of current limiter circuits 141-14n and corresponding ones of current limiter circuits 121-12n (e.g., the sum of the voltage across current limiter circuit 121 and the voltage across current limiter circuit 141, the sum of the voltage across current limiter circuit 122 and the voltage across current limiter circuit 142, etc.).
Maximum selector 352 may also be configured to provide the largest of these differentials/signals to error amplifier EA_DRV for comparison to reference signal VR1. Based on this comparison, error amplifier EA_DRV may provide common drive signal V_DRV. In this manner, the voltages across each of current limiter circuits 141-14n may be less than the voltage of reference signal VR1, and may thus limit the power dissipated within system controller 130. In addition, use of the closed-loop feedback system of current limiter drive control unit 351 may enable sense and control unit 150 to adjust for current limiter circuits 121-12n having different threshold voltages, temperature-related characteristics, manufacturing characteristics, operational characteristics, and/or the like.
Any suitable circuits or devices may be employed as maximum selector 352 or error amplifier EA_DRV. As one example, a common cathode voltage follower circuit may be employed as maximum selector 352.
Although not shown, current limiter drive control unit 351 may also be configured to provide common drive signal V_DRV from a programmable value, as a fixed value, and/or the like. In such embodiments, common drive signal V_DRV may be provided based on, for example, threshold voltages of current limiter circuits 121-12n, based on information received via a Serial peripheral Interface (SPI) or Inter-Integrated Circuit (I2C) interface, and/or the like. In addition, impedances (e.g., resistors, inductors, capacitors, other passive and/or active intermediary devices, etc.) may be provided between current limiter drive control unit 351 and current limiter circuits 121-12n.
As also discussed above, sense and control unit 150 may be configured to control power converter 190 via power converter control signal PWR_CTL based on a voltage differential across at least one of current limiter circuits 141-14n, for example, as received via one of signals FB1-FBn.
To provide this functionality, power converter controller 355 may be configured to sense voltage differentials across each of current limiter circuits 141-14n by monitoring signals FB1-FBn with minimum selector 356. Alternately, power converter controller 355 may be configured to sense voltage differentials across each of current limiter circuits 121-12n or to sense voltage differentials across each combination of current limiter circuits 141-14n and corresponding ones of current limiter circuits 121-12n. Minimum selector 356 may also be configured to provide the smallest of these differentials/signals to error amplifier EA_PWR for comparison to reference signal VR2. Based on this comparison, error amplifier EA_PWR may then provide power converter control signal PWR_CTL. In operation, power converter controller 355 may drive supply voltage Vout to a closed-loop level sufficient or just sufficient enough to provide full operating voltage for all of loads 111-11n.
Any suitable circuit or device may be employed as minimum selector 356 or error amplifier EA_PWR. As one example, a common anode voltage follower circuit may be employed as minimum selector 356.
In addition, sense and control unit 450 may be configured to sense the power dissipated by each of current limiter circuits 421-42n, and to disable the current through the load coupled to a given current limiter circuit if the sensed power dissipation is greater than a threshold value. Likewise, sense and control unit 450 may alternatively be configured to sense the voltage across each of current limiter circuits 421-42n and to disable the current through the load coupled to a given current limiter circuit if the sensed voltage across the given current limiter circuit is less than a threshold value.
In operation, this additional feature may increase the ability of the circuitry to detect excessive current or other faults in an illumination system and may function as a safety mechanism to prevent the burnout of components (e.g., loads 111-11n, current limiter circuits 421-42n, system controller 430, power converter 190, etc.), or to prevent a fire risk or other safety hazard.
In an embodiment of system 500, drive signal generator 580 may provide common drive signal V_DRV as a programmable or fixed value signal from a voltage divider, a digital to analog converter (DAC), a reference voltage source, and/or the like. If, for example, drive signal generator 580 provides common drive signal V_DRV from a DAC, drive signal generator 580 may be further configured to receive a digital control signal from a microprocessor, microcontroller, digital signal processor, and/or the like. As some examples, a digital control signal may be an I2C signal, a SPI signal, and/or the like.
Further, a value of common drive signal V_DRV may be selected based on a threshold value characteristic of switches of current limiter circuits 121-12n or be selected to define the power dissipated, or voltage dropped, by system controller 530 versus current limiter circuits 121-12n while providing at least a threshold level of current flow through loads 111-11n.
While the above Detailed Description describes certain embodiments, and describes the best mode contemplated, the present invention is not limited to the features described and may be practice in many ways. Details of the system may vary in implementation, while still being encompassed by the present invention disclosed herein. As noted above, particular terminology used when describing certain features or aspects of the present invention should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the present invention with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the present invention to the specific embodiments disclosed in the specification, unless the above Detailed Description explicitly defines such terms. Accordingly, the scope of the present invention encompasses not only the disclosed embodiments, but also all equivalent ways of practicing or implementing the present invention under the claims. Further, the claims below are incorporated herein as additional exemplary embodiments of the present invention.