WO2024245740A1

WO2024245740A1 - Circuit for driving an led string

Info

Publication number: WO2024245740A1
Application number: PCT/EP2024/063274
Authority: WO
Inventors: Carlo Fiocchi; Salvador Enrique HOSTALET LEITZKE
Original assignee: Ams-Osram Ag
Priority date: 2023-05-31
Filing date: 2024-05-14
Publication date: 2024-12-05

Abstract

Embodiments relate to a driving circuit (100) for driving a light-emitting diode, LED, string (15) comprising a first LED (105) and plurality of further LEDs (108₁,....108_n) coupled in series. The driving circuit (100) comprises a current comparator (114), the current comparator (114) being configured to compare a supply current dependent on a supply voltage supplied by a voltage source (10) with a reference current, and to generate a difference current. The driving circuit (100) further comprises a control element (112) for controlling a current steered into one of the further LEDs (108₁,....108_n). The control element (112) comprises a bypass element (120) coupled in parallel to the further LED (108₁), the bypass element (120) being controlled in dependence from the difference current.

Description

CIRCUIT FOR DRIVING AN LED STRING

The present disclosure relates to a circuit for driving an LED string .

Light-emitting diode ( LED) strings comprise a plurality of LEDs which are connected in series . For example , an LED string is driven using a battery . Usually, at low battery voltages , some of the LEDs may be switched of f . Attempts are being made to develop driving circuits which enable a smooth transition depending on the battery status .

It is an obj ect of the present invention to provide an improved circuit for driving an LED string .

According to embodiments , the above obj ect is achieved by the claimed matter according to the independent claims . Further developments are defined in the dependent claims .

Embodiments relate to a driving circuit for driving a lightemitting diode , LED, string comprising a first LED and plurality of further LEDs coupled in series . The circuit comprises a current comparator, the current comparator being configured to compare a supply current dependent on a supply voltage supplied by a voltage source with a reference current , and to generate a di f ference current . The driving circuit further comprises a control element for controlling a current steered into one of the further LEDs . The control element comprises a bypass element coupled in parallel to the further LED, the bypass element being controlled in dependence from the di f ference current . The driving circuit may further comprise a converter to generate the supply current as a function of the supply voltage .

For example , the bypass element may comprise a bypass current mirror .

According to embodiments , the control element comprises a variable transistor having a variable gain, the transistor being configured to deliver an adj ustable supply current .

For example , the control circuit is configured to control the bypass element so that i f the supply current is larger than the reference current , an LED current lied is steered across the LED and i f the supply current is smaller than the reference current , a current smaller than Ii_eu is steered across the LED .

According to embodiments , the control element comprises a first current mirror configured to replicate a di f ference current of the reference current and the supply current into a diode-connected transistor of the bypass current mirror when the supply current is smaller than the reference current .

According to further embodiments , the control element may be configured to provide an overdrive current to the diode- connected transistor of the bypass current mirror when the supply current is smaller than the reference current .

For example , the control element may comprise a third current mirror configured to replicate a di f ference current , at a second node , of an overdrive current supplied by an overdrive current source and a di f ference between the supply current and the reference current into a diode-connected transistor of the bypass current mirror, when the supply current is larger than the reference current and smaller than the overdrive current .

The driving circuit may further comprise a boosting circuit configured to add a boosting current to the di f ference current at the second node , when the supply current is smaller than the reference current .

By way of example , the driving circuit may further comprise a second current mirror configured to replicate the di f ference current to the second node .

According to embodiments , a gain of the second current mirror may be adj ustable .

For example , the driving circuit may further comprise further control elements , each of the further control elements being assigned to a corresponding one of the further LEDs .

For example , a reference current fed to the respective ones of the control elements may be individually adj ustable .

For example , each of the control elements comprises a variable transistor having a variable gain, and the gain of the variable transistors of the respective control elements is individually adj ustable .

According to embodiments , each of the control elements comprises a second current mirror having a variable gain . The gain of the second mirrors of the respective control elements may be individually adj ustable .

According to embodiments , a driving circuit for driving a light-emitting diode , LED, string comprising a first LED and plurality of further LEDs coupled in series , comprises a current comparator . The current comparator is configured to compare a supply current dependent on a supply voltage supplied by a voltage source with a reference current , and to generate a di f ference current . The driving circuit further comprises a bypass element , and circuit elements for mirroring the di f ference current to the bypass element . The bypass element is configured to subtract a bypass current depending on the di f ference current from an LED current fed to a corresponding one of the further LEDs , the LED current depending on the supply current .

For example , the bypass element may be controlled so that i f the supply current is larger than the reference current , the LED current Tied is steered across the LED and i f the supply current is smaller than the reference current , a current smaller than Tied is steered across the LED .

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of embodiments of the invention and are incorporated in and constitute a part of this speci fication . The drawings illustrate the embodiments of the present invention and together with the description serve to explain the principles . Other embodiments of the invention and many of the intended advantages will be readily appreciated, as they become better understood by reference to the following detailed description . The elements of the drawings are not necessarily to scale relative to each other . Like reference numbers designate corresponding similar parts . Fig. 1 is a schematic drawing of a circuit for driving an LED string .

Fig. 2A is an equivalent circuit diagram of a driving circuit according to embodiments.

Fig. 2B is an equivalent circuit diagram of a driving circuit according to further embodiments.

Fig. 2C is an equivalent circuit diagram of a driving circuit according to further embodiments.

Fig. 2D is an equivalent circuit diagram of a driving circuit according to further embodiments.

Fig. 3 is a schematic drawing of a circuit for driving an LED string according to embodiments.

Fig. 4 shows an example of an I/V characteristics of a driving circuit .

DETAILED DESCRIPTION

In the following detailed description reference is made to the accompanying drawings, which form a part hereof and in which are illustrated by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology such as "top", "bottom", "front", "back", "over", "on", "above", "leading", "trailing" etc. is used with reference to the orientation of the Figures being described. Since components of embodiments of the invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utili zed and structural or logical changes may be made without departing from the scope defined by the claims .

The description of the embodiments is not limiting . In particular, elements of the embodiments described hereinafter may be combined with elements of di f ferent embodiments .

Within the present disclosure , elements of an electrical circuit are described by referring to speci fic types of transistors , e . g . PMOS and NMOS transistors . As is to be clearly understood, the corresponding disclosure is not limiting . In particular, PMOS transistors may be replaced by NMOS transistor and vice versa while adapting the further circuitry where appropriate , as is generally known to the person skilled in the art . Further, where appropriate , illustrated NMOS and PMOS transistors may be replaced by further transistor types , e . g . bipolar transistors .

As employed in thi s speci fication, the terms "coupled" and/or "electrically coupled" are not meant to mean that the elements must be directly coupled together - intervening elements may be provided between the "coupled" or "electrically coupled" elements . The one or more intervening elements may be adapted for signal and/or power transmission . The one or more intervening elements may, for example , be controllable to temporarily provide a low-resistive connection in a first state and a high-resistive electric decoupling in a second state .

According to further embodiments and where appropriate , the term "electrically connected" may mean that the respective elements are "directly connected" or are "directly and permanently connected" . The term " electrically connected" may describe a permanent low-resistive connection between electrically connected elements , for example a direct contact between the concerned elements or a low-resistive connection via a metal and/or heavily doped semiconductor material . The term "electrically connected" may describe a low-ohmic electric connection between the elements electrically connected together . An ohmic contact may be a non-recti fying electrical j unction .

Fig . 1 schematically shows an LED string 15 , a driving circuit 100 for driving the LED string 15 and a voltage source 10 , e . g . a battery . For example , such an LED string 15 may be employed for automotive lighting . The LED string 15 comprises a first LED 105 and a plurality of further LEDs 108i , 108₂ , 108_n that are coupled in series . The LED string is coupled to the voltage source 10 , e . g . a battery . The term "battery" as used within the present disclosure is intended to mean a voltage source 10 that is configured to supply a voltage that may vary . For example , the supplied voltage may decrease during use and may increase after the battery has been charged . As is to be clearly understood, within the scope of the present disclosure , any other type of voltage source may be employed instead of a battery .

As is illustrated in Fig . 1 , the driving circuit 100 comprises a plurality of control elements 112 . Each of the control elements 112 is assigned to a corresponding one of the further LEDs 108i . As is indicated in Fig . 1 , the control element 112 may be configured to bypass a corresponding one of the LEDs 108i . The driving circuit 100 may comprise a converter 118 that is configured to convert the voltage supplied by the voltage source 10 into a supply current that depends on the voltage supplied by the voltage source 10 . The supply current may be fed to each of the control elements 112 . Each of the control elements 112 may be configured to compare the supply current supplied by the converter 118 with a reference current I_ref that is supplied by a corresponding one of the reference current generators

Depending on a result of comparison, a current steered into each of the LEDs 108i , 108₂ ... 108_n may be controlled . As is further illustrated in Fig . 1 , the entire driving circuit 100 may comprise one single converter 118 and a plurality of control elements 112 . For example , the number of the control elements 112 may correspond to the number of the further LEDs 108!...108_n .

Fig . 2A shows an equivalent circuit diagram of a portion of the driving circuit 100 . As is shown in Fig . 2A, a driving circuit 100 comprises a current comparator 114 which is configured to compare a supply current which is dependent on a supply voltage supplied by a voltage source with a reference current I_ref and to generate a di f ference current . The circuit 100 further comprises a control element 112 for controlling a current steered into one of the further LEDs 108₂ . The control element comprises a bypass element . The bypass element 120 comprises a bypass current mirror 121 which is coupled in parallel to the further LED 108₂ . The bypass current mirror 121 is controlled in dependence from the di f ference current . The bypass current mirror 121 may comprise a diode-connected transistor and a further transistor having a scaling ratio or gain K with respect to the diode-connected transistor .

For example , as is illustrated in Fig . 2A, the driving circuit 100 may further comprise a converter 118 which is configured to generate a supply current which depends on the supply voltage supplied by the voltage source 10 . For example , the converter 118 may comprise a generating transistor 113 , the gate electrode of which is coupled to an output of an ampli fier 129 . A drain terminal of the generating transistor 113 is cou- pled to a f irst converter node which i s coupled to an input of the ampli fier 129 . Another input of the ampli fier 129 is coupled to a second node which is coupled via a first resistor 131 to the voltage source 10 . The drain terminal of the generating transistor 113 may be further connected via a resistor 127 to a third node . The third node is coupled via a second resistor 132 to the second node . The output of the ampli fier 129 is further connected to a gate electrode of a variable transistor 123 which may have a scaling ratio or gain N with respect to the generating transi stor 113 . As a consequence , an output o f the variable trans istor 123 may have a value N x a x VBAT/R, wherein a denotes an attenuation factor caused by the voltage divider comprising the first and the second resistor 131 , 132 , and R denotes the resistance of resistor 127 .

The current output by the variable transistor 123 is connected to a first node 124 which is also connected to a reference current generator 116i for generating a reference current I_ref . The first node 124 is also connected to a first current mirror 125 . The first node 124 implements a comparator 114 which is configured to compare the supply current with the reference current .

For example , the first current mirror 125 may comprise two PMOS transistors . A drain terminal of the diode-connected transistor of the first current mirror 125 is coupled to the first node 124 . A drain terminal of the non-diode-connected transistor of the first current mirror 125 may be coupled to a diode-connected transistor of the bypass current mirror 121 . The bypass current mirror 121 may be coupled in parallel to the corresponding LED 108i .

When the supply current is larger than the reference current at node 124 , the current Tied is fully steered into the LED 108i . When the supply current is less than I_ref , the di f ference between the reference current and the supply current is replicated by the first current mirror 125 into the diode-connected transistor of the bypass mirror 121 . Accordingly, a current that is linear in dependence from the supply voltage is supplied to the bypass current mirror 121 and subtracted from the LED current lied - When Iref is chosen to be larger than Ii_ed/K and, in addition, the battery voltage is very low, the LED current is fully steered into the bypass element 120 and the LED is totally of f .

Accordingly, this simple mirror-based implementation supports the suppression of negative di f ference contributions when the supply voltage becomes very large . This may be due to the recti fying action of the first mirror 125 . At the same time , the bypass current mirror 121 is crossed by a current that is linear in dependence from the supply voltage . Further, this arrangement makes the transition point of the LED current versus the supply voltage independent from the threshold of the bypass element 120 . As a result , a superior performance with an even simpler structure may be obtained .

As i s clearly to be understood, the elements o f Fig . 2A may be applied to the configuration of Fig . 1 . In more detail , the single elements implementing the control element 112 may be assigned to each of the single LEDs 108i . Moreover, one single converter 118 may be assigned to the entire driving circuit 100 . For example , the values of N and the reference voltage suppl ied by the reference current generator 116i may be set individually for each of the LEDs 108i . According to embodiments illustrated in Fig . 2A, the selection of I_ref determines the supply voltage where the transition is fully completed and Ii_eu is totally steered into the respective LED 108i . Further, by setting the gain N, the range of the supply voltage which is necessary to fully complete the transition may be set .

Fig . 2B is an equivalent circuit diagram of a driving circuit 100 according to further embodiments . As will be explained in the following, according to Fig . 2B, the control element 112 is further configured to provide an overdrive current to the diode-connected transistor of the bypass current mirror 121 when the supply current is smaller than the reference current .

In more detail , as is shown in Fig . 2B, the control element 112 may comprise a third current mirror 139 . The third current mirror 139 is configured to replicate a di f ference current of an overdrive current which is suppl ied by an overdrive current source 135 and a di f ference between the supply current and the reference current , when the supply current is larger than the reference current and smaller than the overdrive current , into a diode-connected transistor of the bypass current mirror 121 . To be more speci fic, a terminal of a diode connected transistor of the third current mirror 139 is connected to a second node 134 , whereas a terminal of the other transistor of the third current mirror 139 is connected to the diode-connected transistor of the bypass current mirror 121 . The second node 134 is coupled to the overdrive current source 135 and further to a non-diode-connected transistor of a second current mirror 126 . A diode-connected transistor of the second current mirror 126 is coupled to the first node 124 acting as the comparator 114 . The first node 124 i s connected to an output o f the variable transi stor 123 and the reference current generator 116i as has also been explained above with reference to Fig . 2A. The second current mirror may comprise NMOS transistors .

For example , i f the overdrive current equals Ii_eci/K and there is a unity gain for the third current mirror 139 , the driving circuit 100 may be operated as follows: When the supply voltage is low and the supply current is less than the reference current, no current flows across the second current mirror 126. The overdrive current which is supplied by the overdrive current source 135 is fully supplied to the bypass current mirror 121 so that the entire current Ii_eu is bypassed via the bypass current mirror 121 and no current flows into the LED 108i. When the supply voltage increases, the situation does not change until the supply current is larger than the reference current. In this case, current I_n, i.e. the difference between the supply current and the reference current, is mirrored by the second current mirror 126 to the second node 134. At the second node 134, the current I_n is subtracted from the overdrive current so that a current K x I_n starts flowing into the LED 108i, until the full conduction Ii_eu is reached. At this point, the third mirror 139 is totally turned off. As a result, a more linear I/V characteristics of the corresponding LED 108i may be achieved, as will be explained below with reference to Fig. 4.

A difference between the functionality of embodiments illustrated in Fig. 2A and 2B may be noticed, when the supply current equals Iref. According to embodiments illustrated in Fig. 2A, the bypass element 120 is totally off and the associated LED reaches the full conduction Ii_eu- According to embodiments shown in Fig. 2B, on the contrary, the bypass element just starts reducing its current. Therefore, in case the current supplied by the overdrive voltage source 135 equals Ii_eci/K, the associated LED starts being turned on.

As is clearly to be understood, the elements of Fig. 2B may be applied to the configuration of Fig. 1. In more detail, the single elements implementing the control element 112 may be assigned to each of the single LEDs 108i. Moreover, one single converter 118 may be assigned to the entire driving circuit 100 . For example , the values of N and the reference voltage suppl ied by the reference current generator 116i may be set individually for each of the LEDs 108i .

According to implementations , due to of fset and mismatch, the overdrive current should be larger than the Ii_eci/K . Otherwise there is a risk that a small current is always present in the LED 108i . For example , the circuit may be designed to reduce the amount of safety margin taken .

Fig . 2C is an equivalent circuit diagram of the driving circuit 100 according to further embodiments . In addition to elements illustrated in Fig . 2B, the equivalent circuit diagram of Fig . 2C comprises a boosting circuit 138 . For example , the boosting circuit 138 may comprise a first boosting current source 136 and a second boosting current source 137 . For example , the first boosting current source 136 may be arranged between the second node 134 and the third current mirror 139 . The second boosting current source 137 may be arranged between the first node 124 and the second current mirror 126 . The first and the second boosting current sources 136 , 137 may be matched or identical .

When the supply voltage is very small , in particular, below the voltage which determines the LED transition, the second current mirror 126 is totally of f . In this case , the current provided by the first boosting current source 136 is added to the overdrive current generated by the overdrive current source 135 . As a consequence , the on-resistance of the corresponding LED 108i is smaller . When the supply voltage and hence the supply current increases , the second current mirror 126 is turned on . As a result , the current at the second node 134 decreases since the current replicated by the second current mirror is subtracted from the overdrive current. Since the dynamic range across the LED string 15 tracks the supply voltage, this parameter is not significantly frustrated if the drop across a bypass element 120 increases as well.

At further higher supply voltages, the current across the second current mirror 126 becomes larger than the current provided by the first or second boosting current source. As a result, the effects of the two added boosting currents generated by the first and second boosting current sources 136, 137 cancel each other. As a result, the behavior of the bypass current mirror 121 does not change with respect to e.g. Fig. 2B.

Due to this configuration, the performance may be made more similar to a driving circuit comprising voltage comparators, especially if the transition threshold is large. Further, any clamping circuitry may be eliminated which may be necessary to limit the gate-to-source voltage of a bypass element 120. Accordingly, the bypass element 120 may be implemented in a more compact manner having a reduced size.

As is clearly to be understood, the elements of Fig. 2C may be applied to the configuration of Fig. 1. In more detail, the single elements implementing the control element 112 may be assigned to each of the single LEDs 108i. Moreover, one single converter 118 may be assigned to the entire driving circuit 100. For example, the values of N and the reference voltage supplied by the reference current generator 116i may be set individually for each of the LEDs 108i.

According to all embodiments described herein, the transition zone, e.g. the determination of the range of the supply voltage which is necessary to fully complete the transition may be set, e.g. by setting the size of a mirror, in more detail the gain N of the variable transistor 123.

Fig. 2D shows an equivalent circuit diagram of components of a driving circuit 100 according to still further embodiments. The equivalent circuit diagram of Fig. 2D is similar to the equivalent circuit diagram of Fig. 2C. As is further shown in addition, the gain S of the second current mirror 126 may be set. By setting a value of S, the slope of the I/V characteristics may be set, i.e. the sensitivity to a supply voltage.

According to a further interpretation, a driving circuit 100 for driving a light-emitting diode, LED, string 15 comprises a current comparator 114 and a bypass element 120. The current comparator 114 is configured to compare a supply current dependent on a supply voltage supplied by a voltage source 10 with a reference current, and to generate a difference current. The driving circuit 100 further comprises circuit elements for mirroring the difference current to the bypass element 120. The bypass element 120 is configured to subtract a bypass current depending on the difference current from an LED current fed to a corresponding one of the further LEDs 108i, the LED current depending on the supply current. For example, the driving circuit may further comprise a converter 118 to generate the supply current as a function of the supply voltage. For example, the bypass element 120 may be controlled so that, if the supply current is larger than the reference current, an LED current lied is steered across the LED 108i and if the supply current is smaller than the reference current, a current smaller than Ii_eu is steered across the LED.

Fig. 3 shows an implementation of a driving circuit 100 for driving the LED string 15. As is shown in a similar manner, as has been discussed above with reference to Fig. 1, the LED string 15 comprises further LEDs 108i, 108₂ ... 108_n. Control elements 112 are connected to each of the further LEDs 108₂. A value of S and a corresponding reference current I_ref may be set for each of the control elements, e.g. using a digital command. The set of control elements 112 is connected to the converter 118 which is configured to generate a supply current in dependence from a supply voltage supplied by the voltage source 10. Accordingly, the driving circuit comprises individual control elements 112 for each of the further LEDs 108₂ which may be set by setting a value of N, a value of S and the reference current. Moreover, the control elements 112 share a common converter 118.

Fig. 4 shows an example of an I/V characteristics of a single light emitting diode 108i. As is shown by employing any of the driving circuits as illustrated above, the I/V characteristics may be made more linear between the threshold V_t and a saturation value V_s . Moreover, as has been explained above, by setting a value of S, the slope of the I/V characteristics may be individually set for each of the LEDs 108i. Further, by setting a value of N, the transition range between V_s and V_t may be set for each of the LEDs 108i.

As has been discussed above, a bypass current which is a function of the supply current is injected in parallel to the further LEDs. As a result, the I/V characteristics can be made linear in each stage of the transition. Further a smooth transition in the LED may be achieved. Moreover, parameters of the I/V characteristics may be easily set by setting a gain of current mirrors. In particular, for each of the further LEDs 108i, a turn-on point may be individually set. Due to the specific implementation of the driving circuit, a temperature dependence of the I/V characteristics may be reduced. While embodiments of the invention have been described above , it is obvious that further embodiments may be implemented . For example , further embodiments may comprise any subcombination of features recited in the claims or any subcombination of el- ements described in the examples given above . Accordingly, this spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein .

LIST OF REFERENCES voltage source LED string 0 driving circuit 5 first LED 8_X , . 108_n LED 2 control element 3 generator transistor 4 current comparator 6 , 116i , ...116_n reference current generator8 converter 0 bypass element 1 bypass current mirror 3 variable transistor 4 first node 5 first current mirror 6 second current mirror 7 resistor 9 amp 1 i f i e r 1 first resistor 2 second resistor 4 second node 5 overdrive current source 6 first boosting current source 7 second boosting current source 8 boosting circuit 9 third current mirror

Claims

1. A driving circuit (100) for driving a light-emitting diode, LED, string (15) comprising a first LED (105) and plurality of further LEDs ( 108i, ....108_n) coupled in series, the driving circuit (100) comprising: a current comparator (114) , the current comparator (114) being configured to compare a supply current dependent on a supply voltage supplied by a voltage source (10) with a reference current, and to generate a difference current; and a control element (112) for controlling a current steered into one of the further LEDs ( 108i, ....108_n) , the control element (112) comprising a bypass element (120) coupled in parallel to the further LED (108i) , the bypass element (120) being controlled in dependence from the difference current, wherein the bypass element (120) comprises a bypass current mirror (121) , and the control element (112) comprises a current mirror (125, 126) configured to replicate the difference current of the reference current and the supply current to a node connected to the bypass current mirror (121) when the supply current is smaller than the reference current.

2. The driving circuit (100) according to claim 1, further comprising a converter (118) to generate the supply current as a function of the supply voltage.

3. The driving circuit (100) according to claim 1 or 2, wherein the control element (112) comprises a variable transistor (123) having a variable gain, the variable transistor (123) being configured to deliver an adjustable supply current .

4. The driving circuit (100) according to any of the preceding claims, wherein the control circuit (112) is configured to control the bypass element (120) so that if the supply current is larger than the reference current, an LED current lied is steered across the LED (108i) and if the supply current is smaller than the reference current, a current smaller than Ii_eu is steered across the LED , the LED current Ii_eu depending on the supply current.

5. The driving circuit (100) according to any of the preceding claims, wherein the current mirror (125) is configured to replicate the difference current of the reference current and the supply current into a diode-connected transistor of the bypass current mirror (121) when the supply current is smaller than the reference current.

6. The driving circuit (100) according to any of the preceding claims 5, wherein the control element (112) comprises an overdrive current source (135) configured to provide an overdrive current to the diode-connected transistor of the bypass current mirror (121) when the supply current is smaller than the reference current.

7. The driving circuit (100) according to claim 6, wherein the current mirror (126) is configured to replicate the difference current to a third node (134) connected to the overdrive current source (135) and the control element (112) comprises a further current mirror (139) configured to replicate a further difference current, at the second node (134) , of the overdrive current supplied and the difference current into the diode-connected transistor of the bypass current mirror (121) , when the supply current is larger than the reference current and smaller than the overdrive current.

8. The driving circuit (100) according to claim 6 or 7, further comprising a boosting circuit (138) configured to add a boosting current to the further difference current at the second node (134) , when the supply current is smaller than the reference current.

The driving circuit (100) according to claim 7 or 8, wherein a gain of the current mirror (126) is adjustable.

10. The driving circuit (100) according to any of the preceding claim, further comprising further control elements (112) , each of the further control elements (112) being assigned to a corresponding one of the further LEDs ( 108i, ...108_n) .

11. The driving circuit (100) according to claim 10 the control elements (112) is individually adjustable.

12. The driving circuit (100) according to claim 10 or 11, wherein each of the control elements (112) comprises a variable transistor (123) having a variable gain, the gain of the variable transistors (123) of the respective control elements (112) being individually adjustable.

13. The driving circuit (100) according to any of claims 10 to 12, wherein each of the control elements (112) comprises a current mirror (126) having a variable gain, the gain of the current mirrors (126) of the control elements (112) being individually adjustable.

14. A driving circuit (100) for driving a light-emitting diode, LED, string (15) comprising a first LED (105) and plurality of further LEDs ( 108i, ...108_n) coupled in series, the cir- cult (100) comprising: a current comparator (114) , the current comparator (114) being configured to compare a supply current dependent on a supply voltage supplied by a voltage source (10) with a reference current, and to generate a difference current; a bypass element (120) , and circuit elements for mirroring the difference current to the bypass element (120) , the bypass element (120) being configured to subtract a bypass current depending on the difference current from an LED current Ii_eu fed to a corresponding one of the further LEDs (108i) , the LED current Ii_eu depending on the supply current.

15. The driving circuit (100) according to claim 14, wherein the bypass element (120) is controlled so that, if the supply current is larger than the reference current, the LED current lied is steered across the corresponding one of the further LEDs (108i) and if the supply current is smaller than the reference current, a current smaller than Ii_ed is steered across the further LED (108i) ,the LED current lied depending on the supply current.