Method for detecting a defect in an LED string and electronic circuit with at least one LED string

This application claims the benefit of German Application No. 102018131803.0, filed on Dec. 11, 2018, which application is hereby incorporated herein by reference.

This disclosure in general relates to a method for detecting a defect in an LED (Light Emitting Diode) string and an electronic circuit with at least one LED string.

LED strings, which include a plurality of LEDs connected in series, are widely used for lighting purposes in various kinds of applications such as interior or exterior lighting in automobiles, or lighting in buildings, to name only a view. An LED string may be driven by a drive circuit that generates a drive current received by the LED string. The drive circuit may also be configured to monitor the LED string and detect a defect of one of the LEDs in the LED string. The defect may include a short circuit in one of the LEDs in the string.

Detecting a defect in a system with at least one LED string may include measuring a voltage across the LED string and comparing the measured voltage with a threshold predefined by the manufacturer of the system and stored in a memory of the system. This type of detecting the defect is based on the assumption that the voltage across the LED string (string voltage) is higher than the threshold when each of the LEDs in the string is lit up and that the string voltage falls below the threshold when a short circuit in one of the LEDs occurs. The string voltage is essentially proportional to the number of LEDs in the string and the forward voltage of the LEDs. A reduction of the string voltage when a short circuit in one LED occurs is essentially proportional to a reciprocal of the number n of LEDs in the string. When, for example, the string includes n=2 LEDs and a short circuit in one of the LEDs occurs the string voltage drops for about 50% (=1/n), and when, for example, the string includes n=10 LEDs and a short circuit in one of the LEDs occurs the string voltage drops for about 10% (=1/n). The threshold should be adjusted such that the voltage change caused by a short circuit in one of the LEDs is detected and that variations of the string voltage, which may result from temperature changes, do not result erroneously in the detection of a defect. Suitably selecting the threshold is, in particular, challenging when the number of LEDs increases and a change of the string voltage that has to be reliably detected decreases.

There is therefore a need for a method that reliably detects a defect in an LED string, in particular, an LED string with more than five LEDs.

One example relates to a method. The method includes detecting at least one operating parameter in an electronic circuit that comprises a monitored LED string, adjusting a voltage threshold based on the at least one detected operating parameter, detecting a string voltage across the monitored LED string, comparing the string voltage with the voltage threshold, and detecting a defect in the LED string ( 3 ) based on the comparing.

Another example relates to an electronic circuit. The electronic circuit includes a monitored LED string, and a defect detection circuit. The defect detection circuit is configured to detect at least one operating parameter in the electronic circuit, to adjust a voltage threshold based on the at least one detected operating parameter, to detect a string voltage across the monitored LED string, to compare the string voltage with the voltage threshold, and to detect a defect in the LED string based on the comparing.

Examples are explained below with reference to the drawings. The drawings serve to illustrate certain principles, so that only aspects necessary for understanding these principles are illustrated. The drawings are not to scale. In the drawings the same reference characters denote like features.

FIG. 1 illustrates one example of an electronic circuit that includes an LED string;

FIG. 2 illustrates one example of an LED string in greater detail;

FIG. 3 schematically illustrates a voltage across the LED string (string voltage) in a faultless state and after an LED short has occurred;

FIG. 4 schematically illustrates a dependency of the string voltage on a temperature of the LED string;

FIG. 5 schematically illustrates a dependency of the string voltage on a current through the LED string;

FIG. 6 schematically illustrates a dependency of the string voltage on a forward voltage of LEDs of the LED string;

FIG. 7 shows a flowchart of one example of a method for detecting a defect in an LED string;

FIG. 8 shows a circuit diagram of an electronic circuit with an LED string and an adaptive defect detection circuit;

FIG. 9 shows a modification of the circuit arrangement shown in FIG. 8 ;

FIG. 10 illustrates one example of a defect detection circuit that is configured to adjust a threshold based on a current through the LED string;

FIG. 11 illustrates one example of a defect detection circuit that is configured to adjust a threshold based on a temperature of the LED string;

FIG. 12 illustrates one example of a defect detection circuit that is configured to adjust a threshold based on both a current through the LED string and a temperature of the LED string;

FIG. 13 illustrates another example of a defect detection circuit that is configured to adjust a threshold based on a current through the LED string;

FIG. 14 illustrates another example of a defect detection circuit that is configured to adjust a threshold based on a current through the LED string;

FIG. 15 illustrates one example of a defect detection circuit that is configured to adjust a threshold based on a forward voltage of an LED in the LED string;

FIG. 16 illustrates one example of a defect detection circuit that is configured to adjust a threshold based on a string voltage of another LED string in the electronic circuit; and

FIG. 17 shows a modification of the electronic circuit shown in FIG. 16 .

In the following detailed description, reference is made to the accompanying drawings. The drawings form a part of the description and for the purpose of illustration show examples of how the invention may be used and implemented. It is to be understood that the features of the various embodiments described herein may be combined with each other, unless specifically noted otherwise.

FIG. 1 illustrates one example of an electronic circuit that includes an LED string 3 and a defect detection circuit 4 configured to detect a defect in the LED string 3 . Referring to FIG. 1 , the LED string 3 includes a first circuit node 31 and a second circuit node 32 , which are also referred to as first string node 31 and second string node 32 in the following. A voltage V 3 between the first string node 31 and the second string node 32 is referred to as string voltage in the following. The defect detection circuit 4 is configured to detect the string voltage V 3 in order to detect a defect in the LED string. For detecting the string voltage V 3 , the defect detection circuit 4 may be connected to the first and second string nodes 31 , 32 , as illustrated in FIG. 1 .

The LED string 3 includes a plurality (two or more) of LEDs. One example of the LED string 3 is illustrated in FIG. 2 . Referring to FIG. 2 , the LED string 3 may include a plurality of LEDs 3 1 - 3 m connected in series between the first string node 31 and the second string node 32 .

When the electronic circuit is in operation, the LED string 3 may receive a string current I 3 , which is a current flowing between the first string node 31 and the second string node 32 . Dependent on a current level of the string current I 3 the LEDs 3 1 - 3 m of the string 3 light up or do not light up. In the following, “on” and “off” are used interchangeably in place of “light up” and “do/does not light up”, respectively.

The overall number of LEDs in the LED string 3 can range from 2 to 50, in particular from 2 to 30. Just for the purpose of illustration, the LED string 3 shown in FIG. 2 includes m=6 LEDs 3 1 - 3 m connected in series.

According to one example, the LEDs 3 1 - 3 m connected in series in the LED string 3 are LEDs of the same type, so that, at a given string current I 3 , the LEDs 3 1 - 3 m light with essentially the same intensity, when the string current I 3 is above a threshold that causes the LEDs 3 1 - 3 m to light up.

During operation of the LED string 3 , a defect may occur. One type of defect that may occur is a short circuit of one single LED. This type of defect is briefly referred to as LED short in the following. In the case of an LED short, the defect (shorted) LED is off, while the remainder of the LEDs in the LED string 3 is still on. An example of an LED short in one 3 3 of the LEDs 3 1 - 3 m of the LED string 3 is illustrated in bold dashed lines in FIG. 2 . In this example, LED 3 3 is off and the remainder 3 1 - 3 m , 3 4 - 3 m of the LEDs are on when a string current I 3 higher than a certain current threshold flows through the LED string 3 .

Referring to FIG. 2 , the string voltage V 3 is essentially given by a sum of voltages V 3 1 -V 3 m across the individual LEDs 3 1 - 3 m in the string 3 . When a string current I 3 is driven into the string 3 that causes the LEDs 3 1 - 3 m to light up and when the LEDs 3 1 - 3 m are of the same type, the LED voltages V 3 1 -V 3 m are essentially the same. Thus, when there is no defect in the string 3 , the string voltage V 3 is essentially proportional to the number m of LEDs 3 1 - 3 m connected in series. When an LED short occurs and the string current I 3 does not change, the string voltage V 3 drops because the voltage (V 3 3 in the example shown in FIG. 2 ) across the shorted LED ( 3 3 in the example shown in FIG. 2 ) essentially drops to zero.

Referring to FIG. 1 , the string current I 3 may be generated by a current source 2 connected in series with the LED string 3 , wherein a series circuit including the LED string 3 and the current source 2 is connected between input nodes 11 , 12 where an input voltage V IN is available. The current source 2 may be a switched current source that switches on or off dependent on an input signal S 2 . According to one example, the current source 2 is configured to generate a string current I 3 that causes the LEDs in the LED string 3 to light up when the input signal S 2 has an on-level and to generate a current level of the string current I 3 such that the LEDs 3 1 - 3 m are off when the input signal S 2 has an off-level.

Basically, detecting an LED short may include detecting the string voltage V 3 and comparing the string voltage V 3 with a voltage threshold V 3 TH . This is illustrated in FIG. 3 which schematically illustrates a timing diagram of the string voltage V 3 . In this example, an LED short in one of the LEDs 3 1 - 3 m occurs at a first time instance t 1 , so that at the first time instance t 1 the string voltage V 3 drops from a first voltage level V 3 N , which is referred to as faultless level in the following, to a second voltage level V 3 D , which is referred to as defect level in the following. The faultless level V 3 N is essentially given by V 3 N =m V 3 F , wherein V 3 F is the voltage across each of the LEDs 3 1 - 3 m when the LEDs are switched on (lights up) and when the LEDs 3 1 - 3 m are of the same type. This voltage V F is also referred to as forward voltage in the following. The defect level is essentially given by V 3 D =(m−1)·V 3 F , so that the difference V 3 N −V 3 D between the faultless level and the defect level is essentially given by V 3 F . In order to detect this type of defect, the voltage threshold V 3 TH is selected such that it is between the faultless level V 3 N and the defect level V 3 D , that is, V 3 N >V 3 TH >V 3 D . For several reasons, however, the string voltage V 3 may vary so that situations may occur in which comparing the string voltage V 3 with a fixed voltage threshold V 3 TH does not result in reliably detecting an LED short. Some examples are explained in the following.

Referring to FIG. 4 , the string voltage V 3 is dependent on a temperature of the LED string 3 , wherein the string voltage V 3 decreases as the temperature T decreases. FIG. 4 schematically illustrates the faultless level V 3 N and the defect level V 3 D dependent on the temperature T. In this example, when the temperature T is below a temperature threshold T 1 each of the faultless level V 3 N and the defect level V 3 D is above the threshold V 3 TH , so that detecting an LED short would not be possible at temperatures below the voltage threshold T 1 . Moreover, at high temperatures, the faultless level V 3 N may fall below the threshold V 3 TH so that there is only a certain temperature range in which detecting an LED short by comparing the string voltage V 3 with a fixed threshold V 3 TH works reliably.

Referring to FIG. 5 , the string voltage V 3 is further dependent on the string current I 3 , wherein the string voltage V 3 increases as the string current I 3 increases. FIG. 5 shows the faultless level V 3 N and the defect level V 3 D dependent on the string current I 3 . Referring to FIG. 5 , the defect level V 3 D becomes higher than the fixed threshold V 3 TH when the string current I 3 is higher than a current threshold I 3 1 . Thus, reliably detecting an LED short might no longer be possible when, by virtue of current variations, the string current I 3 rises above the current threshold I 3 1 .

Further, referring to FIG. 6 , the string voltage V 3 is also dependent on the forward voltage V F of the LEDs 3 1 - 3 m in the LED string 3 . The forward voltage V F is the voltage across a single LED at a certain string current that causes the LED to light up. Due to variations in the manufacturing process of the LEDs, different LEDs may have different forward voltages V F . In some cases, when assembling an LED string, LEDs with essentially the same forward voltage are selected so that different LED strings with the same number of LEDs and receiving a string current with the same level may have significantly different string voltages V 3 . Thus, in some LED strings, the defect level, at a given string current I 3 , may be higher than the threshold voltage V 3 TH , so that a reliable LED short detection is not possible in these LED strings.

FIG. 7 illustrates a flowchart of a method which, in spite of string voltage V 3 variations of the type explained with reference to FIGS. 4 to 6 , is capable of reliably detecting an LED short in an LED string 3 . Referring to FIG. 7 , the method includes detecting at least one operating parameter in the electronic circuit that includes the LED string 3 ( 101 ). This LED string 3 is also referred to as monitored LED string in the following. The method further includes adjusting a voltage threshold based on the at least one detected operating parameter ( 102 ), measuring the string voltage across the monitored LED string ( 103 ), comparing the string voltage with the voltage threshold ( 104 ), and detecting a defect in the LED string based on the comparing ( 105 ). By detecting the at least one operating parameter and adjusting the voltage threshold based on the at least one detected operating parameter, the method is robust against changes of the string voltage that are not due to an LED short, such as changes of the type explained with reference to FIGS. 4 to 6 .

FIG. 8 shows one example of a defect detection circuit 4 that is configured to operate in accordance with the method illustrated in FIG. 7 . Referring to FIG. 8 , the defect detection circuit 4 includes a comparator 41 and a voltage threshold generation circuit 42 , wherein the comparator 41 receives a signal S 3 representing the string voltage V 3 and a threshold signal S 3 TH representing the voltage threshold V 3 TH and is configured to output a defect signal S DEF dependent on comparing the string voltage signal S 3 with the voltage threshold signal S 3 TH . According to one example, the comparator 41 is configured to generate the defect signal S DEF such that the defect signal S DEF has a normal level or a defect level, wherein the defect level indicates that a defect has been detected. According to one example, the comparator 41 is configured to generate the defect level each time the string voltage V 3 as represented by the string voltage signal S 3 falls below the voltage threshold V 3 TH as represented by the voltage threshold signal S 3 TH .

According to one example, the comparator 41 receives the string voltage signal S 3 at an inverting input and the voltage threshold signal S 3 TH at a non-inverting input, so that the defect signal S DEF has a low signal level whenever the string voltage signal S 3 falls below the voltage threshold signal S 3 TH . This, however, is only an example. The inverting and non-inverting input or the comparator 41 could be changed, so that a high signal level of the defect signal S DEF represents a defect.

Just as an example, the string voltage signal S 3 is identical with the string voltage V 3 in the example illustrated in FIG. 8 . Equivalently, the voltage threshold signal S 3 TH may be identical with the voltage threshold V 3 TH . This, however, is only an example. According to another example, the string voltage signal S 3 is not identical with the string voltage V 3 but in any way represents the string voltage V 3 . Equivalently, the voltage threshold signal S 3 TH may not be identical with the threshold voltage V 3 TH but in any way represent the voltage threshold V 3 TH . In each case, however, comparing the string voltage signal S 3 with the voltage threshold signal S 3 TH is equivalent to comparing the string voltage V 3 with the voltage threshold V 3 TH .

In the example illustrated in FIG. 8 , the string voltage V 3 is referenced to a second input node 12 of the electronic circuit, wherein the second input node 12 may be a ground node of the electronic circuit. In this example, the voltage threshold generation circuit 42 is also connected to the second input node 12 and the voltage threshold signal S 3 TH is a signal referenced to the second input node 12 . This, however, is only an example. According to another example illustrated in FIG. 9 , the string voltage V 3 is a voltage between the first input node 11 and the second string node 32 . In this example, the voltage threshold generation circuit 42 generates the voltage threshold signal S 3 TH such that this signal is referenced to the first input node 11 .

Referring to FIGS. 8 and 9 , the voltage threshold generation circuit 42 receives an operating parameter signal Sop, wherein the operating parameter signal Sop represents at least one operating parameter of the electronic circuit.

According to one example, the detected operating parameter is the string current I 3 . In this example, the operating parameter signal Sop represents the string current I 3 . A defect detection circuit 4 configured to detect the string current I 3 and generate the voltage threshold signal S 3 TH based on the detected string current I 3 is illustrated in FIG. 10 . In this example, the voltage threshold generation circuit 42 includes a current sensor 45 that is configured to sense the string current I 3 and provide a sense current I SENSE that represents the string current I 3 . The current sensor 45 can be any kind of current sensor configured to sense the string current I 3 and provide a current I SENSE that represents the string current I 3 . According to one example, the sense current I SENSE is proportional to the string current I 3 . In this example, the voltage threshold generation circuit 42 is configured to generate the voltage threshold signal S 3 TH (which is identical with the voltage threshold V 3 TH ) such that the voltage threshold signal S 3 T H increases as the sense current I SENSE increases and, therefore, as the string current I 3 increases. In this way, variations of the string voltage V 3 that are due to variations of the string current I 3 can be compensated in the detection of a defect such as an LED short.

In the example shown in FIG. 1 o , the voltage threshold V 3 TH is a voltage across a resistor 44 , wherein a constant current I 43 provided by a current source 43 and the sense current I SENSE are driven through this resistor 44 . Thus, the voltage threshold V 3 TH is proportional to an overall current given by the constant current I 43 and the sense current I SENSE and the voltage threshold V 3 TH increases as the sense current I SENSE , which represents the string current I 3 , increases.

According to another example, the operating parameter is a temperature of the LED string 3 so that the operating parameter signal Sop represents the temperature of the LED string 3 . A defect detection circuit 4 configured to generate the voltage threshold signal S 3 TH dependent on the temperature of the LED string 3 is illustrated in FIG. 11 . In this example, the voltage threshold generation circuit 42 includes a temperature sensor 51 configured to sense a temperature of the LED string 3 and output a temperature signal S TEMP that represents the string temperature. This temperature signal S TEMP is the operating parameter signal Sop in this example. A control circuit 52 receives the temperature signal S TEMP and drives a variable current source 53 dependent on the temperature signal S TEMP . According to one example, the control circuit 52 is configured to drive the current source 53 such that a current I TEMP provided by the current source 53 increases as the string temperature represented by the temperature signal S TEMP increases. The defect detection circuit includes the constant current source 43 and the resistor 44 already explained with reference to FIG. 10 , wherein the variable current source 53 is connected in parallel with the resistor 44 . In this way, the current I 44 through the resistor is proportional to a difference between the constant current I 44 and the temperature dependent current I TEMP and decreases as the temperature dependent current I TEMP increases. In this way, the voltage threshold signal S 3 TH decreases as the temperature dependent current I TEMP increases, that is, as the string temperature increases. In this way, the voltage threshold V 3 TH represented by the voltage threshold signal S 3 TH can be generated such that it is between the faultless level V 3 N and the defect level V 3 D even when these voltage levels decrease with increasing temperature, as illustrated in FIG. 3 . Thus, variations of the string voltage V 3 that are due to variations of the string temperature can be compensated in the detection of a defect such as an LED short.

FIG. 12 shows one example of a defect detection circuit 4 that is configured to generate the voltage threshold signal S 3 TH dependent on both, the string current I 3 and the string temperature. This defect detection circuit 4 includes the current sensor 54 illustrated in FIG. 10 and the temperature sensor 51 , the control circuit 52 and the variable current source 43 illustrated in FIG. 11 . In this example, the current I 44 through the resistor 44 is given by the constant current I 43 provided by the current source 43 plus the sense current I SENSE minus the temperature dependent current I TEMP . In this way, the voltage threshold signal S 3 TH increases as the sensor current I SENSE increases and decreases as the temperature dependent current I TEMP increases. In this way, the voltage threshold signal S 3 TH can be generated such that—despite variations of the string temperature and the string current I 3 —it is between the faultless level V 3 N and the defect level V 3 D . In this way, variations of the string voltage V 3 that are due to variations of the string current I 3 and variations of the string temperature can be compensated in the detection of a defect such as an LED short.

In the example illustrated in FIG. 10 , the voltage threshold signal S 3 TH continuously changes as the string current I 3 and, therefore, the sense current I SENSE changes. Another example of the defect detection circuit 4 that is configured to generate the voltage threshold signal S 3 TH such that the voltage threshold signal S 3 TH increases step-wise as the string current I 3 increases is illustrated in FIG. 13 . In this example, a switch controller 47 receives a sense voltage V SENSE , wherein the sense voltage V SENSE represents the string current I 3 . According to one example, the sense voltage V SENSE is proportional to the string current I 3 . The sense voltage V SENSE may be generated by a current sensor 45 of the type explained with reference to FIG. 10 and a resistor 46 connected in series with the current sensor 45 and receiving the sense current I SENSE so that a voltage across the resistor 46 is proportional to the sense current I SENSE and forms the sense voltage V SENSE .

The switch controller 47 , which may include an analog-to-digital converter (ADC), receives the sense voltage V SENSE and drives one of a plurality of switches 47 2 - 47 5 dependent on the sense voltage V SENSE . Each of these switches 47 2 - 47 5 is connected to a respective tap of a resistive voltage divider 44 1 - 44 5 , wherein this voltage divider 44 1 - 44 5 is connected in series to the current source 43 and wherein the voltage threshold signal S 3 TH is available across the resistive voltage divider 44 1 - 44 5 . The resistive voltage divider 44 1 - 44 5 includes a plurality of resistors 44 1 - 44 5 connected in series between the current source 43 and a reference node, which is the second input node 12 in this example. A circuit node between the current source 43 and the resistive voltage divider 44 1 - 44 5 is connected to that input of the comparator 41 that receives the voltage threshold signal S 3 TH (this comparator input is the non-inverting input in this example). Two of the plurality of resistors 44 1 - 44 5 are connected to each tap, wherein each of the switches 47 2 - 47 5 controlled by the controller is connected between a respective tap and the reference node. The “reference node” is the circuit node to which the voltage threshold signal S 3 TH is referenced to. In the on-state, each of these switches 47 2 - 47 5 bypasses at least one of the resistors 44 1 - 44 5 . Switch 47 2 , for example, bypasses resistors 44 2 - 44 5 , switch 47 3 bypasses resistors 44 3 - 44 5 , and so on. Thus, a resistance of the resistive voltage divider between the input of the comparator 41 and the reference node and, therefore, the voltage threshold signal S 3 TH can be varied by switching on a respective one of the switches 47 2 - 47 5 . In the example shown in FIG. 13 , the resistive voltage divider 44 1 - 44 5 has the highest resistance when each of the switches 47 2 - 47 5 is switched off and has the lowest resistance when switch 47 2 is switched on. Basically, the controller 47 is configured to drive the switches 47 2 - 47 5 such that the number of resistors that are bypassed by switching on a respective one of the switches 47 2 - 47 5 decreases as the current sense signal V SENSE increases. In this way, the voltage threshold signal S 3 TH increases in discrete steps as the current sense signal V SENSE increases.

In the example illustrated in FIG. 13 , the switch controller 47 receives a signal V SENSE representing the string current I 3 in order to adjust the voltage threshold signal S 3 TH dependent on the string current. This, however, is only an example. According to another example (not shown) the controller 47 receives the temperature signal S TEMP and drives the switches 47 2 - 47 5 such that the number of resistors that are bypassed by switching on a respective one of the switches 47 2 - 47 5 increases as the temperature signal S TEMP increases. In this way, the voltage threshold signal S 3 TH decreases in discrete steps as the temperature signal S TEMP increases.

In the example illustrated in FIG. 13 , the string current I 3 is measured and the voltage threshold signal S 3 TH is adapted based on the measured string current I 3 . FIG. 14 shows a modification of the circuit illustrated in FIG. 13 . In the example illustrated in FIG. 14 , the current source 2 is a controlled current source that provides the string current I 3 dependent on a control signal. More specifically, the current source 2 illustrated in FIG. 14 includes three sub-sources 2 A, 2 B, 2 C connected in parallel. The control signal includes three sub-signals S 2 A, S 2 B, S 2 C, wherein each of these sub-signals controls one of the sub-sources 2 A, 2 B, and 2 C. Each of these current sources 2 A, 2 B, 2 C is configured to provide a sub-current I 3 A, I 3 B, I 3 C, wherein the string current I 3 is given by the sum of the sub-currents I 3 A, I 3 B, I 3 C provided by the current source 2 A, 2 B, 2 C. According to one example, the current provided by each of the sub sources 2 A, 2 B, 2 C is either zero or a predefined current level, wherein this predefined current level can be the same for each of the sub-sources 2 A, 2 B, and 2 C or can be different. Whether the current provided by one sub-source 2 A, 2 B, and 2 C is zero or has the predefined current level different from zero is dependent on the respective control signal S 2 A, S 2 B, S 2 C. Using a current source 2 of the type illustrated in FIG. 14 , 2 N −1 different current levels, each being different from zero, can be generated. N is the number of sub-sources ″−C, wherein N=3 in the example shown in FIG. 14 .

Referring to FIG. 14 , the defect detection circuit 4 receives the control signals S 2 A, S 2 B, S 2 C, and adjusts the voltage threshold signal S 3 TH dependent on these control signals S 2 A-S 2 C. More specifically, a logic circuit 61 receives the control signals S 2 A, S 2 B, S 2 C and adjusts the voltage threshold signal S 3 TH by bypassing two or more resistors of a resistive voltage divider 44 1 - 44 4 dependent on the control signal S 2 A-S 2 C. The arrangement shown in FIG. 14 includes 2 N −1 resistors connected in series and 2 N −2 switches 47 2 - 47 7 so that 2 N −1 different resistances and, therefore, 2 N −1 different levels of the voltage threshold signal S 3 TH can be adjusted. Each of these levels of the voltage threshold signal S 3 TH is associated with one of the different current levels of the string current I 3 .

FIG. 15 illustrates a defect detection circuit 4 according to another example. In this example, the defect detection circuit 4 is configured to detect a forward voltage V F3m of one of the plurality of LEDs 3 1 - 3 m of the LED string 3 and adjusts the voltage threshold signal S 3 TH based on the detected forward voltage V F3m . According to one example, the voltage threshold signal S 3 TH is generated such that the voltage threshold signal S 3 TH increases as the detected forward voltage V F3m increases. In the example, shown in FIG. 15 , an amplifier 71 receives a forward voltage signal S F3m that represents the forward voltage V F3m and generates the voltage threshold signal S 3 TH at an output. According to one example (not shown), the forward voltage signal S F3m is the forward voltage V F3m , that is, the amplifier may be connected to the LED 3 m . According to another example, illustrated in FIG. 15 , a buffer 72 receives the forward voltage V F3m and provides forward voltage signal SF 3 m either direct or, as illustrated, via an optional voltage divider 73 1 , 73 2 .

Like in the examples explained herein before, the string voltage signal S 3 , which equals the string voltage V 3 in this example, and the voltage threshold signal S 3 TH are received by a comparator 41 that generates the defect signal S DEF . Optionally, a further comparator 74 compares the forward voltage V F3m with a minimum voltage V MIN , wherein an output signal of the comparator 41 and the further comparator 74 are received by a logic gate such as an OR-gate 75 and the defect signal S DEF is provided by the logic gate 75 . In this example, a defect level of the defect signal S DEF is generated each time the forward voltage V F3m falls below a voltage represented by the voltage threshold signal S 3 TH or the forward voltage V 3m falls below the minimum voltage V MIN . The latter may occurs an LED short in the LED 3 m occurs.

According to another example illustrated in FIG. 16 , the electronic circuit includes several LED strings, the monitored LED string and at least one further LED string 3 1 , 3 2 , wherein in the example shown in FIG. 16 there are two further LED strings 3 1 , 3 2 . Each of the further LED strings 3 1 , 3 2 is connected in series with a respective further current source 2 1 , 2 2 . These further LED strings 3 1 , 3 2 are driven such that—when there is no defect in the further LED strings 3 1 , 3 2 —are essentially the same. Further, the string voltage V 3 of the monitored LED string 3 —when there is no defect in the monitored LED string 3 —is essentially the same as the string voltages V 3 1 , V 3 2 of these further LED strings 3 1 , 3 2 or there is a predefined ratio between the string voltage V 3 of the monitored LED string 3 and the string voltages V 3 1 , V 3 2 of the further LED strings 3 1 , 3 2 .

It can be assumed that dependent on the temperature the string voltage V 3 of the monitored LED string 3 and the string voltages V 3 1 , V 3 2 of the further LED strings 3 1 , 3 2 vary in the same way. Thus, in the example shown in FIG. 16 , the voltage threshold signal S 3 TH is generated based on the string voltages V 3 1 , V 3 2 of the further LED strings 3 1 , 3 2 . According to one example, a maximum selector 81 receives the string voltage V 3 1 , V 3 2 of each of the further LED strings 3 1 , 3 2 and outputs a signal representing the maximum of the further strings voltages V 3 1 , V 3 2 . In this way, generating a proper voltage threshold signal S 3 TH is ensured even if an LED short in one of the further LED strings 3 1 , 3 2 occurs. A resistive voltage divider 82 1 , 82 2 generates the voltage threshold signal based S 3 TH based on the output signal generated by the maximum selector 81 . In this example, generating the voltage threshold signal S 3 TH considers the temperature although the temperature is not directly measured.

According to yet another example illustrated in FIG. 17 , each of the LED strings 3 , 3 1 , 3 2 illustrated in FIG. 16 is monitored for the occurrence of a defect. In this example, the maximum selector 81 receives each of the string voltages V 3 , V 3 1 , V 3 2 and a minimum selector 83 receives each of the string voltages V 3 , V 3 1 , V 3 2 . An output signal of the minimum selector 83 is received by the second input of the comparator 41 , which is the inverting input in this example.

While the invention has been described with reference to illustrative examples, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative examples, as well as other examples of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or examples.