Patent Grant
7622871
The technology described herein is generally related to the field of integrated circuits and, more particularly, to driver circuits for light emitting diodes (“LEDs”).
LEDs are known to act as a source of emitted light for a wide variety of applications. LEDs are known to provide many advantages over incandescent and fluorescent illumination because of their long operating life, high efficiency, lightweight, and low profile.
LEDs have improved in light emitting efficiency (i.e., conversion of electricity to light) by several orders of magnitude over the past decade. Newer LEDs provide an advantage over early LEDs in that, when provided sufficient power, they emit enough light to be seen in direct daylight. In contrast, early LEDs (i.e., those produced in the mid to late 1990s) appeared to be OFF when operated in direct daylight, no matter what level of current was applied to the LED. The efficiency improvement of newer LEDs has made possible their use in efficient outdoor video billboards.
However, with the increased light emitting efficiency of current LEDs, a new problem arises in that they can emit sufficient light to be visible with only a few microamps of current flowing through them. This low on-current can have negative repercussions in real life applications if the driver circuit driving the LEDs (e.g., driver circuit 50, described above with reference to
The leakage/emission problem summarized above most affects LED drivers designed around LDO and switching regulator topologies, but can be can also be found in linear or DC drive topologies as well. In these products small leakage currents passing through the switching or control transistor are considered to be inconsequential and in many cases may be unavoidable due to the characteristics of the semiconductor process and the applied voltages. One solution to the low on-current characteristic of LEDs would be to produce LED driver circuits that do not generate any appreciable leakage current. However, this goal would require special semiconductor processes or device designs that would increase production costs over LED driver circuits designed and produced using conventional processing methods.
Further, even if special fabrication processes were used to generate a “perfect” LED driver circuit (i.e., an LED driver circuit exhibiting zero leakage current), undesirable current may still be caused, for example, by impurities on the PC board supporting the LED chain. That is, even if a perfect, non-leaking driver IC is produced, if the user's assembly process leaves residue on the display board that allows a leakage current to flow through the LEDS, the LEDs can appear to be turned on when they are intended to be turned off.
What is needed is a LED driver circuit that avoids the current/emission problems associated with conventional LED driver circuits.
The present invention is directed to a LED driver circuit including a shunt circuit that is connected between output and feedback terminals (i.e., to opposite ends of an externally connected LED or strings of LEDs), wherein the shunt circuit is selectively controlled to shunt leakage current around the LED/LEDs when a user applied control signal is disabled, thereby maintaining comparatively low voltages across the LEDs that preclude undesirable light emission. That is, when an applied drive current is turned off to the LED(s), the shunt circuit is activated to cause the two voltages applied to the respective terminals of the LED(s) (i.e., VOUT and VLIMIT) to have same voltage level, which guarantees that the voltage across the LED(s) is close to zero, and much less than the voltage required to turn the LED on. By utilizing a standard transistor (and/or other circuit elements) to facilitate the desired shunting function, the present invention overcomes problems associated with conventional LED driver circuits without requiring special semiconductor processing techniques, and regardless of any impurities that may exist on the display PC board. The shunt switch has the added benefit of rapidly discharging any external capacitance that may be present across the LED(s), thereby turning off the LED(s) in a shorter amount of time than is possible using conventional methods.
These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings, where:
The present invention relates to an improvement in LED displays. The following description is presented to enable one of ordinary skill in the art to make and use the invention as provided in the context of a particular application and its requirements. The terms “coupled” and “connected”, which are utilized herein, are defined as follows. The term “connected” is used to describe a direct connection between two circuit elements, for example, by way of a metal line formed in accordance with normal integrated circuit fabrication techniques. In contrast, the term “coupled” is used to describe either a direct connection or an indirect connection between two circuit elements. For example, two coupled elements may be directly connected by way of a metal line, or indirectly connected by way of an intervening circuit element (e.g., a capacitor, resistor, inductor, or by way of the source/drain terminals of a transistor). Various modifications to the preferred embodiment will be apparent to those with skill in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the particular embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features herein disclosed.
In accordance with the present invention, LED driver circuit 100 further includes a shunt circuit 120 that functions to couple output terminal 101 and feedback terminal 104 when control signal VREF has the second predetermined voltage level (i.e., when the intent of control signal VREF is to turn off LEDs 103-1 and 103-2). By fabricating output control circuit 110 and shunt circuit 120 utilizing standard electronic components (e.g., CMOS transistors and/or other circuit elements) to facilitate the desired output control and shunting functions, LED driver circuit 100 overcomes problems associated with conventional LED driver circuits without requiring special semiconductor processing techniques, and regardless of any impurities that may exist on a display PC board hosting LEDs 103-1 and 103-2.
In accordance with the specific embodiment shown in
During operation, while the control signal VREF remains relatively high, inverter 122 generates a low output signal that turns off (opens) FET 124, thereby preventing any current from flowing between output terminal 101 and feedback terminal 104. In this operating state, the relatively high control voltage VREF causes error amplifier 115, PWM 118, internal capacitor CINT, transistor T1 and diode S1 to generate output voltage VOUT across LEDs 103-1 and 103-2 such that LEDs 103-1 and 103-2 generate visible light.
Conversely, when control signal VREF drops below a predetermined threshold voltage level, inverter 122 generates a high output signal that turns on (closes) FET 124, thereby shunting output voltage VOUT from output terminal 101 to feedback terminal 104 (i.e., such that output voltage VOUT substantially equals feedback voltage VLIMIT, or differs by an amount that is less than that required to cause LEDs 103-1 and 103-2 to generate visible light). That is, any leakage current generated at output terminal 101 of driver circuit 100A is shunted around LEDs 103-1 and 103-2, preventing them from generating visible light.
Those skilled in the art will recognize that many different switch circuits may be used to implement shunt circuit 120A in accordance with the present invention to achieve the shunting function described herein. As such, the present invention is not intended to be limited by the specific circuit arrangement of LED driver circuit 100A unless specified in the appended claim.
LED driver circuit 100A provides several advantages over conventional LED driver circuits. First, LED driver circuit 100A solves the low on-current characteristic of newer LEDs using circuitry that can be produced using standard processing methods (e.g., inverter 122 and FET 124 are “standard” CMOS devices), thereby avoiding the need for special semiconductor processes or device designs that would increase production costs. The shunting operation also provides the added benefit of rapidly discharging any external capacitors (e.g., capacitor CEXT, see
Although the present invention has been described with respect to certain specific embodiments, it will be clear to those skilled in the art that the inventive features of the present invention are applicable to other embodiments as well, all of which are intended to fall within the scope of the present invention. For example, it will be recognized by those skilled in the art that the methodology can be used to drive a single LED, or extended to a plurality of banks of LEDs of a variety of commercially available types and sizes.
| Number | Name | Date | Kind |
|---|---|---|---|
| 6586890 | Min et al. | Jul 2003 | B2 |
| 6867757 | Nakamura | Mar 2005 | B1 |
| 7317403 | Grootes et al. | Jan 2008 | B2 |
| 20070229001 | McIntosh et al. | Oct 2007 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20090085490 A1 | Apr 2009 | US |
