Enhancements for LED lamps for use in luminaires

One example solid state lighting type lamp for a three-way luminaire includes a power source, a controller, an output stage, switching logic circuitry and multiple sets of light emitters. The logic circuitry receives input signals from tip and ring power contacts on a lamp base. The controller provides power from the power source to the output stage which is controlled by the switch logic circuitry to selectively apply power to different ones of the sets of light emitters responsive to the input signals. Each set of light emitters emit light having different color temperatures. In another three-way luminaire example, the control circuitry is configured to control drive current in a sequence to toggle the lamp consecutively between an OFF state and ON state in response to inputs from a three-way socket. Another type of lamp includes circuitry to permanently disable the lamp on detection of an end-of-life condition.

TECHNICAL FIELD

The present subject matter relates to lamps for general lighting applications that utilize solid state light emitting sources and in particular to a solid state lamp for a three-way luminaire. The present subject matter also concerns apparatus and methods for disabling a solid state lamp at the end of its useful lifetime.

BACKGROUND

It has been recognized that incandescent lamps are a relatively inefficient light source. However, after more than a century of development and usage, they are cheap. Also, the public is quite familiar with the form factors and light output characteristics of such lamps. Fluorescent lamps have long been a more efficient alternative to incandescent lamps. For many years, fluorescent lamps were most commonly used in commercial settings. However, recently, compact fluorescent lamps have been developed as replacements for incandescent lamps. While more efficient than incandescent lamps, compact fluorescent lamps also have some drawbacks. For example, compact fluorescent lamps utilize mercury vapor and represent an environmental hazard if broken or at time of disposal. Cheaper versions of compact fluorescent lamps also do not provide as desirable a color characteristic of light output as traditional incandescent lamps and often differ extensively from traditional lamp form factors.

Recent years have seen a rapid expansion in the performance of solid state light emitting sources such as light emitting devices (LEDs). With improved performance, there has been an attendant expansion in the variety of applications for such devices. For example, rapid improvements in semiconductors and related manufacturing technologies are driving a trend in the lighting industry toward the use of light emitting diodes (LEDs), organic light emitting diodes (OLEDs) or other solid state light sources in lamps for general lighting applications. These lamps meet the need for more efficient lighting technologies and address ever increasing costs of energy along with concerns about global warming due to consumption of fossil fuels to generate energy. LED solutions also are more environmentally friendly than competing technologies, such as compact fluorescent lamps, for replacements for traditional incandescent lamps. Hence, there are now a variety of products on the market and a wide range of published proposals for various types of lamps using solid state light emitting sources, as lamp replacement alternatives.

Incandescent lamps are manufactured in many form factors and electrical configurations. For example, the base of an incandescent lamp may be configured as a one-way lamp or a three way lamp.

FIG. 1illustrates an example of a solid state lamp30. The exemplary lamp30may be utilized in a variety of lighting applications analogous to applications for common incandescent lamps and/or compact fluorescent lamps. The lamp30includes solid state light emitters32for producing lamp output light of a desired characteristic, from the emitter outputs and/or from luminescent phosphor emissions driven by the emitter outputs as discussed more fully below. The solid state emitters as well as the other components within the bulb31are visible through the cut-out window view ofFIG. 1.

At a high level, a lamp30, includes solid state light emitters32, a bulb31, an industry standard base35and a housing33. The housing33extends into an interior of the bulb31and supports the bulb, the solid state light emitters32and a circuit board including electronic components of the lamp. In the examples, the orientations of the solid state light emitters32produce emissions through the bulb31that approximate light source emissions from a filament of an incandescent lamp. The illustrated example also uses an optional inner optical processing member34, of a material that is at least partially light transmissive. The member34is positioned radially and longitudinally around the solid state light emitters32supported on the housing33and between an inner surface of the bulb31and the solid state light emitters32. The bulb and/or the inner member may be transparent or diffusely transmissive. If provided, phosphors may be deployed on the inner optical processing member34or on the bulb31. Lamp30also includes heat sink fins36which dissipate heat from the solid state light emitters32.

FIG. 1Ais a plan view of a screw type lamp base, such as an Edison base or a candelabra base. For many lamp applications, the existing lamp socket provides two electrical connections for AC main power. The lamp base in turn is configured to mate with those electrical connections. As shown, the base60has a center contact tip61for connection to one of the AC main lines. The threaded screw section of the base60is formed of metal and provides a second outer AC contact at62, sometimes referred to as neutral or ground because it is the outer casing element. The tip61and screw thread contact62are separated by an insulator region (shown in gray). When power is applied to the tip connection, a circuit is formed from the tip connection61through the light emitter to the outer AC contact62. This base is for a one-way lamp that is either on or off.

FIG. 2is a plan view of an industry standard three-way dimming screw type lamp base, such as for a three-way mogul lamp base or a three-way medium lamp base. Although other base configurations are possible, the example is that for a screw-in base63as might be used in a three-way mogul lamp or a three-way medium lamp base. As shown, the base63has a center contact tip64for a low power connection to one of the AC main lines. The three-way base63also has a lamp socket ring connector65separated from the tip64by an insulator region (shown in gray). A threaded screw section of the base63is formed of metal and provides a second outer AC contact at66, sometimes referred to as neutral or ground because it is the outer casing element. The socket ring connector65and the screw thread contact66are separated by an insulator region (shown in gray). A conventional incandescent lamp having the base shown inFIG. 2, has two filaments. The first filament is connected between the tip contact64and the outer AC contact66and a second filament is connected between the ring contact65and the outer AC contact66. The luminaire for this type of lamp sequentially applies power to the ring contact65, tip contact64and to both the tip and ring contacts. The filament between the tip contact64and the outer AC contact66typically produces light having a higher lumen level than the filament between the ring contact65and the outer AC contact66. Thus, as the luminaire is cycled, light having three different lumen levels is produced.

Another attribute of incandescent lamps is their lifetime. At the end of its life, an incandescent lamp typically “burns out” when its filament breaks. A solid-state lamp, however, typically does not fail abruptly but exhibits increasingly degraded performance as it ages.

To be accepted by the public, it is desirable that LED lamps to conform to the form factors, electrical configurations and/or the end of life performance of incandescent lamps.

SUMMARY

The teachings herein provide further improvements over existing lamp technologies. A three-way lamp example is configured to produce light from different sets of light emitters, one for each of the three electrical connections made by the luminaire. In another example, a lamp is configured to operate as a one-way lamp even when inserted in a three-way socket. In yet another example, a lamp is monitored for indications that it is approaching the end of its useful life and, when one or more of these indications crosses a threshold, the lamp is disabled, simulating an abrupt failure.

In the first example, a three-way lamp includes a power source, a controller, an output stage, switching logic circuitry and at least one set of light emitters. The logic circuitry is coupled to the power source to receive signals from the tip and ring contacts. The controller is coupled to provide power from the power source to the output stage and the output stage is coupled to the switch logic circuitry to selectively apply power to the light emitters responsive to the signals from the tip and ring contacts.

According to one aspect of this example, the at least one set of light emitters includes three sets of light emitters that are configured to emit light having respectively different color temperatures and the logic circuitry is configured to activate respectively different ones of the three sets of light emitters for each of three active states of the signals provided by the tip and ring contacts.

According to another aspect of this first example, the three sets of light emitters each has a respectively different number of light emitters.

According to yet another aspect of this first example, at least one of the three sets of light emitters is configured to produce light in a different color than the other two sets of light emitters.

According to still another aspect of this example, the at least one set of light emitters includes a single set of light emitters and the logic circuitry is configured to cause the controller to apply power to the light emitters responsive to a signal on the ring contact and on the tip and ring contacts and not to apply power to the light emitters responsive to a signal only on the tip contact so that the single set of light emitters cycles on and off responsive to changing switch positions of a three-way switch.

According to another example, a lamp includes a power source, a controller, an output stage, at least one set of light emitters and status monitoring circuitry, coupled to the controller, that monitors the status of the light emitters. The controller is coupled to provide power from the power source to the output stage and is coupled to the status monitoring circuitry to apply power to the light emitters as long as the status monitoring circuitry determines that the light emitters are within their useful lifetime. The status monitoring circuitry provides a non-volatile signal enabling the controller. When the status monitoring circuitry determines that the light emitters are no longer within their useful lifetime, it switches the non-volatile signal to disable the controller.

According to one aspect of this example, the status monitoring circuitry measures an amount of time that the light emitters emit light and disables the controller when this amount of time exceeds a threshold value.

According to another aspect of this example, the status monitoring circuitry measures a lumen level of the light provided by the light emitters and disables the controller when the measured lumen level is less than a threshold value.

According to yet another aspect of this example, the status monitoring circuitry measures a temperature of the light emitters and disables the controller when the measured temperature is greater than a threshold value.

DETAILED DESCRIPTION

The various examples disclosed herein relate to solid state lamp assemblies that mimic and extend the functionality of corresponding incandescent lamp assemblies. Each of the embodiments described below concerns the electronic components of the lamp assembly. In addition to the described electronic components, each lamp includes a bulb and a housing on which the bulb and the electronic components are mounted and a base, such as shown inFIGS. 1 and 2through which power signals are provided to the electronic components. The lamp may also include a heat sink to dissipate heat generated by the LEDs, as represented for example by the fins36.

Reference now is made in detail to the examples illustrated in the accompanying drawings and discussed below.FIG. 3illustrates an first example three-way lamp300. The lamp includes tip, ring and neutral (N) lines that connect to the respective tip, ring and outer AC contacts, as shown inFIG. 1. The tip and ring lines are connected to provide AC power to respective power supplies314and316through fuses310and312. In one implementation, the power supply314is a half-bridge rectifier and the power supply316is a full-bridge rectifier. The half-bridge rectifier314is connected to the full-bridge rectifier316, as shown inFIG. 3, such the two diodes of the bridge rectifier316that are connected to the neutral line are shared between the rectifiers314and316. Thus, both rectifiers provide a full-wave rectified power signal. It is contemplated that other types of power supplies may be used.

The positive output terminals of the power supplies314and316are connected to each other as are the negative output terminals. The combined positive and negative output terminals of the power supplies are connected to a filter circuit318. The combined positive terminals of the power supplies are connected to provide operational power to switch logic circuitry320. The operational power signal provided by the filter318is applied to the controller stage322, which converts the filtered DC power signal into a power signal having a voltage and current suitable for the LED sets. Controller stage322applies this power signal to the output stage324. The output stage324includes driver circuits that provide the power signal to the three sets of light emitters: set A including LEDs1-N of color A; set B including LEDs1-N of color B and set C including LEDs1-N of color C. The output stage includes an output select matrix326which switches among the three sets of light emitters under control of the switch logic circuitry320.

Switch logic circuitry320is coupled to receive signal inputs from the tip and ring lines, via the fuses310and312. The switch logic circuitry is also coupled to a source of reference potential (e.g. ground). The circuitry320converts the alternating current (AC) power signals provided by the tip and ring lines into logic signals that are applied to the output select matrix326of the output stage324to control which set of LEDs is activated. In one implementation, the switch logic may employ two opto-isolators that receive the respective AC signals provided by the tip and ring lines as input signals, and produce output signals suitable for driving digital logic circuits in the output select matrix326. An example opto-isolator circuit is described with reference to FIG. 16 in U.S. Pat. No. 8,212,469 entitled LAMP USING SOLID STATE SOURCE AND DOPED SEMICONDUCTOR NANOPHOSPHOR which is incorporated herein by reference. Alternatively, electromechanical relays may be used in place of the opto-isolators in the switch logic. Other applications describing the operation of lamps having solid-state light emitters include U.S. pub. nos. 2011/0176291, 2011/0176316 and 2011/0175528, which are incorporated herein by reference

Table 1 is a truth table showing the logic signals327and328produced by the switch logic responsive to the tip and ring signals and the resulting light emitter set selected by the output select matrix326.

The logic signals output by the switch logic circuitry are described as logic-high (H) and logic-low (L). These designations do not indicate signal levels. For example, if the output select matrix uses negative logic, the voltage value of the H signal may be less than that of the L signal. The output select matrix326may be an analog 1 by 4 multiplexer. The operational power signal generated by the output stage324may be switched among the sets of LEDs as shown in Table 1.

In one implementation, the three sets of LEDs are of any type rated to emit energy of wavelengths from the blue/green region around 460 nm down into the UV range below 380 nm. In an example lamp, the light emitted by the LEDs is converted into white light by nanophosphors that have absorption spectra with upper limits around 430 nm, although other doped semiconductor nanophosphors may have somewhat higher limits on the wavelength absorption spectra and therefore may be used with LEDs or other solid state devices rated for emitting wavelengths as high as say 460 nm. In the specific examples, particularly those for white light lamp applications, the LEDs are near UV LEDs rated for emission somewhere in the 380-420 nm range, although UV LEDs could be used alone or in combination with near UV LEDs even with the exemplary nanophosphors. A specific example of a near UV LED, used in several of the specific white lamp examples, is rated for 405 nm emission.

The structure of a LED includes a semiconductor light emitting diode chip, within a package or enclosure. A transparent cover (typically formed of glass, plastic or the like), of the package that encloses the chip, allows for emission of the electromagnetic energy in the desired direction. In this implementation, the transparent cover also encloses semiconductor nanophosphors that convert the near UV light emitted by the LEDs into white light.

One or more doped semiconductor nanophosphors are used in the LEDs to convert energy from the source into visible light of one or more wavelengths to produce a desired characteristic of the visible light output of the lamp. In one example, the nanophosphors are selected such that the LEDs in set A produce white light with a color temperature of 2700K, the LEDs in set B set produce white light with a color temperature of 3500K and the LEDs in set C set produce white light with a color temperature of 5000K. The nanophosphors used to produce light in different color temperatures are a blend of single wavelength nanophosphors that produce white light having the desired color temperature.

The nanophosphor materials may be a solid, although liquid or gaseous materials may help to improve the florescent emissions by the nanophosphors in the material. For example, alcohol, oils (synthetic, vegetable, silicon or other oils) or other liquid media may be used. A silicone material, however, may be cured to form a hardened material, at least along the exterior (to possibly serve as an integral container), or to form a solid throughout the intended volume. If hardened silicone is used, however, a glass container still may be used to provide an oxygen barrier to reduce nanophosphor degradation due to exposure to oxygen. If a gas is used, the gaseous material, for example, may be hydrogen gas, any of the inert gases, and possibly some hydrocarbon based gases. Combinations of one or more such types of gases might be used.

While the example implementation uses LEDs providing white light at three different color temperatures, it is contemplated that LEDs providing light of a single color may be used for one or more of the light emitter sets. For example, the three-way lamp may provide a red light, to act as a night-light, if only the ring line is active and provide white light having a first different color temperature when only the tip line is active and having a second color temperature when both the tip and ring lines are active. In this instance, the nanophosphors in the LEDs in set A are selected to emit red light and the nanophosphors in the LEDs in sets B and C are selected to emit white light at the respective color temperatures.

For some lighting applications where a single color is desirable rather than white, the lamp might use a single type of nanophosphor in the material. For a red lamp type application the one nanophosphor would be of a type that produces predominantly red light emission in response to pumping energy from the LEDs. The upper limits of the absorption spectra of the exemplary nanophosphors are all at or around 430 nm, therefore, the LEDs used in such a monochromatic lamp would emit energy in a wavelength range of 430 nm and below.

Alternatively, conventional red LEDs may be used in place of the near UV LEDs and the red nanophosphors. If a red LED is used, however, it may be desirable to use one that produces a relatively bright light, for example a superluminescent LED (SLED). It is contemplated that the LED sets A, B and C, may all be single color sets using either near UV LEDs with a single color phosphor or single color LEDs or SLEDs.

FIGS. 3A and 3Bare perspective drawings illustrating examples of how the LED sets A, B and C may be mounted in the lamp300. The mounting posts shown in both of these figures have triangular cross-sections. The LEDs in each of the sets A, B and C are mounted on all three sides of the post. For convenience, only one side is shown, the other two sides have the same arrangement although it is contemplated that they may have different arrangements.

The post330shown inFIG. 3Ahas different numbers of LEDs in each of the three sets. In this example, there is one LED332from set A, two LEDs334from set B and three LEDs336from set C. In this configuration, the light intensity of color A will be less than that of color B which, in turn, will be less than that of color C. The relative numbers of LEDs shown inFIG. 3Aare illustrative only. It is contemplated that each set may have different numbers of LEDs on each side of the post and that the ratios of the numbers of LEDs in the various sets may be different.

Alternatively, the post340shown inFIG. 3Bhas equal numbers of LEDs from each set. In this example, each side of the post340has two LEDs332from set A, two LEDs334from set B and two LEDs336from set C. Again, it is contemplated that the lamp may use more of fewer LEDs in each set.

FIG. 4illustrates another implementation of a solid-state lamp for a three-way luminaire. This lamp has only a single set of LEDs and is controlled by the control logic to make the three-way luminaire operate in the same way that a one-way luminaire would operate with a one-way lamp. In a three-way luminaire, the sequence of power signals is 1) ring, 2) tip, 3) ring+tip and 4) off. When a one-way lamp is used in a three-way luminaire, this sequence translates to 1) Off, 2) On, 3) On, 4) Off. This is because the one-way lamp does not have a ring contact and, thus, only turns on when power is applied to the tip contact.

The example lamp shown inFIG. 4includes both a ring contact and a tip contact. Switch logic in the lamp, however, causes it to operate according to the sequence 1) On, 2) Off, 3) On, 4) Off. Thus, the lamp in the three-way luminaire operates in the same way as a one-way lamp in a one-way luminaire, alternating between On and Off states as the three-way switch in the luminaire is actuated.

The lamp shown inFIG. 4includes many of the same elements as the lamp inFIG. 3(i.e. fuses310and312, power supplies314and316and filter318). For the sake of brevity, the operation of these elements, is not described herein. The lamp shown inFIG. 4uses different switch logic410that receives the input signals, tip and ring, via the fuses310and312. The output signal of the switch logic410is a signal, Enable, which is applied to the controller stage412. When this signal is logic-high (H), controller412is enabled, providing operational power to the output stage414to turn on the LEDs416. When the controller is disabled, no power is provided to the output stage414and the LEDs416are turned off.

Table 2 describes the function implemented by the switch logic410.

From this table, it may be seen that the logic function may be performed using an opto-isolator (not shown) to convert the ring signal to the Enable logic signal.

As previously described, it may be desirable for both one-way and three-way solid state lamps to include circuitry that disables the lamp when a condition is detected indicating that the lamp has reached the end of its useful life. An incandescent lamp provides an essentially constant same lumen output over its lifetime. The lumen output of solid state lamps gradually decreases over the lifetime of the lamp. This may be hazardous if a lamp is used in an environment requiring a predetermined minimum lumen level. Because the luminosity of the solid state lamps decreases gradually, a person using the lamp may not notice that it has been degraded. In addition, as solid state lamps age, they become less efficient, producing more heat as they produce less light. This may be undesirable in applications where the efficiency of the lamp is important, such as lighting systems run from battery power.

The example lamps described below with reference toFIGS. 5 and 6address these problems by disabling the solid state lamp when it is determined that the lamp has reached the end of its useful lifetime. The example lamps inFIG. 5make this determination based on an operational characteristic of the LEDs, for example an amount of heat or light emitted by the LEDs. In the example lamps inFIG. 6this characteristic is an amount of time that the lamp has been on or on a number of times that it has been cycled on and off.

FIG. 5shows an example lamp500having an input power stage510coupled to a filter512. The input power stage may include one or more power supplies and, if the lamp is a three-way lamp, switching logic of the type described above with reference toFIGS. 3and/or4. The output signal provided by the power stage510is one or more voltage signals. The filter512provides a filtered output voltage signal to controller stage514which, in turn, provides operational power to the output stage516to drive the LEDs517. As described above, the controller stage514reduces the voltage of the signal provided by the filter512to generate an operational power signal having voltage and current levels that are appropriate for the LEDs517. The output stage applies this power signal to the LEDs517. The combination of the input power stage510, filter512, controller stage514and output stage516are collectively referred to as the driver circuitry of the solid state lamp.

Both one-way and three-way lamps may benefit from lifetime monitoring. If the lamp500is a three-way lamp, there may be control signals generated by control logic (not shown) implemented in the input power stage510. These optional control signals are shown by the dashed line from the input power stage510to the controller stage514and output stage510as described above with reference toFIGS. 3 and 4, for example.

The controller stage514in the lamp500receives an Enable signal from sensor logic and conditioning circuitry518. The circuitry518is coupled to a sensor520. In one implementation, the sensor520includes a thermal sensor which is coupled to the LEDs517. In another implementation, it includes an optical sensor that is configured to measure the light provided by the LEDs517. In yet another implementation, the sensor520includes both optical and temperature sensors. In the example lamps shown inFIG. 5, the Enable signal applied to the controller state514is a non-volatile signal indicating that the lamp has reached its end of life (EOL). The Enable signal may be, for example, a logic-high signal while the lamp is performing within its specifications and a logic-low signal otherwise. The logic-low signal may be generated by elements of the sensor logic and conditioning circuitry that short the signal to ground, causing the Enable signal to transition from logic high to ground potential (e.g. logic low), and remains at ground potential.

As described in the above-referenced published patent application, solid state lamps typically include heat dissipation elements that prevent the solid state light emitters from being damaged by excessive heat. In addition, as described above, the solid state emitters may become less efficient as they age, generating more heat and less light. One implementation of a thermal sensor may thermally couple a temperature sensor, for example a thermocouple or thermistor, to one or more of the LEDs517. This implementation may generate the signal disabling the controller514when the sensed temperature is greater than a threshold value. This type of sensor may also be useful for preventing the LEDs from being damaged in normal operation when the lamp is used in an environment when the heat dissipation elements are not effective at removing heat. In this usage, however, the disable signal may not be permanent but may re-enable the lamp when the measured temperature falls below the threshold value.

Because the lamp may be operated in environments having different heat profiles, absolute temperature may not be a good measure of lamp lifetime. One alternative may be to measure differential temperature, for example when the LEDs517are cycled between Off and On states. An LED at the beginning of its lifetime has a different temperature profile than an LED near the end of its lifetime. For example, as it approaches the end of its useful life, the LED may heat up quickly to a higher temperature. The sensor logic and conditioning circuitry518may include differentiating circuitry that measures the rate of increase of the temperature and disables the controller stage514when the measured rate exceeds a threshold.

In a three-way lamp, it may be desirable to include multiple thermal sensors520, one for each set of LEDs. In this implementation, the Enable signal provided to the controller stage may be a two-bit signal indicating which set of LEDs should be disabled. The operation of this implementation would be similar to an incandescent three-way lamp in which one filament can fail but the lamp continues to provide light from another filament.

In an alternative implementation, the sensor520may be an optical sensor rather than a thermal sensor. The optical sensor may be positioned in the lamp to receive light from the LEDs517. In one implementation, the lamp may include an extra LED that is not used for light generation but, instead, is coupled directly to the light sensor520. In another implementation, the light sensor520may be positioned in the lamp to measure the light emitted by one or more of the LEDs in their normal operation.

In this implementation, the sensor logic and conditioning circuitry518may compare the measured light level to a threshold value and generate the EOL output signal to disable the controller stage514when the measured light level is less than the threshold value.

In yet another implementation, the lamp may include both thermal and optical sensors. In this implementation, the signals provided by the two sensors may be combined to determine whether the lamp has reached its end of life. This combination may include disabling the lamp if either sensor indicates an end of life condition or only if both sensors indicate the end of life condition.

The threshold values of the operational characteristic indicating an end of life condition may be empirically derived from test data for a statistically significant number of lamps. Alternatively, the temperature and luminosity thresholds may be based on manufacturer's specifications for the LEDs517. Determination of the threshold values may also take into account changes in the sensors due to time and environmental conditions. Also, because there may be some variation in the sensed values from sensor to sensor, it may be desirable for the sensor logic and conditioning circuitry to initially calibrate the sensor or to take predictable sensor variation into account when comparing the sensor values to the threshold values.

FIG. 6shows another lamp configuration to handle LED end of life issues. The embodiments shown inFIG. 6disable the lamp based on a total amount of time that the LEDs have been in the on state or number of on-off cycles. Although these are shown as separate embodiments, it is contemplated that they may be combined with the end-of-life detection circuitry described above with reference toFIG. 5.

The implementations shown inFIG. 6share many of the elements of the implementations shown inFIG. 5. Accordingly, these elements are not discussed here. The key difference between the embodiments shown inFIG. 6and that shown inFIG. 5is the timer or counter logic610. In a first example, the circuitry610includes a timer responsive to a clock signal. In one implementation, the clock signal may be derived from the AC line frequency. In another embodiment it may be controlled by a tuned circuit such as an RC, RL or LC tank circuit. In yet another embodiment, it may be a generated by an resonant crystal oscillator.

The timer includes a non-volatile register that is reset when the lamp is manufactured and is incremented at a predetermine rate, for example, once per second or once per minute, while the lamp is turned on. This register may, for example, employ a sufficient number of flash memory cells to hold a Boolean value that is greater than the expected lifetime of the lamp. The circuitry610may also include control circuitry that writes new values into the flash memory cells. The circuitry610may be configured to use the flash memory cells as the timer register or to use a separate timer register that is loaded from the flash memory when the lamp is turned on and stored back into the flash memory when the lamp is turned off. The circuitry610may include a small capacitor to store sufficient power to complete the storage operation after the lamp has been turned off.

In this example, the circuitry610may also include logic that generates the EOL disable signal when the timer reaches a predetermined value. This logic may be a digital comparator that compares the timer value to an EOL time value or it may be logic circuitry, such as a multi-input AND gate, that generates the EOL disable signal when the value in the timer register is a predetermined EOL value. As in the embodiment shown inFIG. 5, the EOL disable signal is a non-volatile signal such that, once the lamp has been disabled, it cannot be re-enabled.

The circuitry610determines when the LEDs are turned on responsive to a monitor input from the output stage518. This value may be a voltage drop measured across the LEDs when they are active. The circuitry610may also determine when the LEDs are turned on based on output signals provided by control logic (not shown) internal to the input power stage512. As described above with reference toFIG. 5, in a lamp configuration having multiple LED sets, such as shown inFIG. 3, it may be desirable to maintain separate timers for each LED set and selectively disable each set based on its usage time. Alternatively, the circuitry610may maintain a single timer that records the amount of time that any LED set is turned on and disables the lamp when a predetermined time value is exceeded.

The predetermined time value(s) are generated based on empirical lifetime data collected from a statistically significant number of lamps.

In another example implementation, the circuitry610does not measure an amount of time that the LEDs have been turned on but the number of times that they have been cycled from an off state to an on state. To implement this function, the circuitry may be configured to generate a delayed pulse signal when the lamp is turned on. This signal may be generated, for example, using an RC ramp circuit and a threshold comparator. When the lamp is turned on, the counter is powered up in time to count the delayed pulse signal. The count value is stored in a non-volatile register which may include a number of flash memory cells sufficient to hole a count of off-on cycles greater than the expected lifetime of the LEDs. When this count value is greater than a predetermined maximum count value, the circuitry610generates an EOL disable signal to disable the lamp.

In the examples described above, with reference toFIGS. 5 and 6the lamp may be a three-way lamp and power is applied to different sets of LEDs based on the AC power being detected on the ring line, the tip line and the tip and ring lines. In these implementations, when one of the sets of LEDs is permanently disabled, the switch logic of the driver circuitry, for example the switch logic320, shown inFIG. 3, may detect the disabled set of LEDs and change its operation to be the operation of the switch logic410, shown inFIG. 4. Thus, the remaining sets of LEDs are turned on when power is detected on the ring line but not on the tip line and when power is detected on both the tip and ring lines and are turned off when power is detected on the tip line but not on the ring line. This effectively converts the lamp to a one-way lamp alternating between ON and OFF states as the three-way switch is operated.