Circuit for driving LED string

Abstract

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

Description

[0001] This disclosure relates to a circuit for driving a string of LEDs.

[0002] A light-emitting diode (LED) string contains multiple LEDs connected in series. For example, LED strings are powered by a battery. Typically, some LEDs may be turned off when the battery voltage is low. Currently, efforts are being made to develop drive circuits that can achieve a smooth transition based on battery state.

[0003] The purpose of this invention is to provide an improved circuit for driving LED strings.

[0004] According to the embodiments, the above-mentioned objectives are achieved by the subject matter claimed in the independent claims. Further improvements are defined in the dependent claims.

[0005] The embodiment relates to a driving circuit for driving a string of light-emitting diodes (LEDs), the LED string including a first LED coupled in series and a plurality of other LEDs. The circuit includes a current comparator configured to compare a supply current, which depends on a supply voltage provided by a voltage source, with a reference current and generate a differential current. The driving circuit also includes a control element for controlling the current directed to one of the other LEDs. The control element includes a bypass element coupled in parallel with the other LEDs, the bypass element being controlled according to the differential current.

[0006] The drive circuit may further include a converter to generate the supply current based on the supply voltage.

[0007] For example, the bypass element may include a bypass current mirror.

[0008] According to an embodiment, the control element includes a variable transistor with variable gain, the transistor being configured to provide an adjustable supply current.

[0009] For example, the control circuit is configured to control the bypass element such that if the supply current is greater than the reference current, the LED current I... led The current is directed through the LED, and if the supply current is less than the reference current, then it is less than I. led The current is directed to flow through the LED.

[0010] According to an embodiment, the control element includes a first current mirror configured to copy the difference between the reference current and the supply current into a diode-connected transistor of the bypass current mirror when the supply current is less than the reference current.

[0011] According to another embodiment, when the supply current is less than the reference current, the control element can be configured to provide an overdrive current to the diode-connected transistor of the bypass current mirror.

[0012] For example, the control element may include a third current mirror configured to, when the supply current is greater than the reference current and less than the overdrive current, copy the difference current at a second node between the overdrive current provided by the overdrive current source and the difference between the supply current and the reference current to the diode-connected transistor of the bypass current mirror.

[0013] The driving circuit may further include a boost circuit, which is configured to add the boost current to the difference current of the second node when the supply current is less than the reference current.

[0014] For example, the drive circuit may further include a second current mirror configured to replicate the differential current to the second node.

[0015] According to an embodiment, the gain of the second current mirror can be adjustable.

[0016] For example, the driving circuit may also include additional control elements, each of which is assigned to a corresponding one of the additional LEDs.

[0017] For example, the reference current fed to a specific control element can be individually adjustable.

[0018] For example, each of the control elements includes a variable transistor with variable gain, and the gain of the variable transistor of each control element is individually adjustable.

[0019] According to an embodiment, each control element includes a second current mirror with variable gain. The gain of the second current mirror of each control element can be individually adjustable.

[0020] According to an embodiment, a driving circuit for driving a string of light-emitting diodes (LEDs), the LED string including a first LED coupled in series and a plurality of other LEDs, includes a current comparator. The current comparator is configured to compare a supply current, which depends on a supply voltage provided by a voltage source, with a reference current and generate a differential current. The driving circuit also includes a bypass element and circuit elements for mirroring the differential current to the bypass element. The bypass element is configured to subtract a bypass current, which depends on the differential current, from the LED current fed to a corresponding one of the other LEDs, the LED current depending on the supply current.

[0021] For example, the bypass element can be controlled such that if the supply current is greater than the reference current, the LED current I... led The current is directed through the LED, and if the supply current is less than the reference current, then it is less than I. led The current is directed to flow through the LED. Attached Figure Description

[0022] The accompanying drawings are provided to further illustrate embodiments of the invention and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles. Further embodiments of the invention and many anticipated advantages will be readily understood as they become more apparent with reference to the following detailed description. The various elements in the drawings are not necessarily drawn to scale. The same reference numerals indicate corresponding similar parts.

[0023] Figure 1 This is a schematic diagram of a circuit used to drive an LED string.

[0024] Figure 2A This is an equivalent circuit diagram of the driving circuit according to the embodiment.

[0025] Figure 2B This is an equivalent circuit diagram of the drive circuit according to another embodiment.

[0026] Figure 2C This is an equivalent circuit diagram of the drive circuit according to another embodiment.

[0027] Figure 2D This is an equivalent circuit diagram of the drive circuit according to another embodiment.

[0028] Figure 3 This is a schematic diagram of a circuit for driving an LED string according to an embodiment.

[0029] Figure 4 An example of the I / V characteristics of a drive circuit is shown. Detailed Implementation

[0030] The following detailed description refers to the accompanying drawings, which form part of this document and illustrate specific embodiments in which the invention can be practiced. In this regard, directional terms such as "top," "bottom," "front," "rear," "across," "above," "over," "leading," and "following" are used with reference to the orientation of the described drawings. Since components of embodiments of the invention can be positioned in many different orientations, the directional terms are used for illustrative purposes and are by no means limiting. It should be understood that other embodiments can be utilized and structural or logical changes can be made without departing from the scope defined by the claims.

[0031] The description of the embodiments is not limiting. In particular, elements of the embodiments described herein can be combined with elements of different embodiments.

[0032] Within this disclosure, circuit elements are described with reference to specific types of transistors (e.g., PMOS and NMOS transistors). It should be clearly understood that the corresponding disclosure is not restrictive. In particular, a PMOS transistor can be replaced by an NMOS transistor and vice versa, and other circuits can be modified where appropriate, as is known to those skilled in the art. Furthermore, the illustrated NMOS and PMOS transistors can be replaced by other transistor types (e.g., bipolar transistors) where appropriate.

[0033] As used in this specification, the terms "coupled" and / or "electrically coupled" do not mean that the components must be directly coupled together—intermediate components may be provided between "coupled" or "electrically coupled" components. The one or more intermediate components may be suitable for signal and / or power transmission. The one or more intermediate components may, for example, be controlled to temporarily provide a low-resistance connection in a first state and to provide high-resistance electrical decoupling in a second state.

[0034] According to other embodiments and where appropriate, the term "electrical connection" may mean that the corresponding element is "directly connected" or "directly and permanently connected".

[0035] The term "electrical connection" can describe a permanent, low-resistance connection between electrically connected components, such as a direct contact between related components or a low-resistance connection via a metal and / or heavily doped semiconductor material. The term "electrical connection" can also describe a low-ohmic electrical connection between components electrically connected together. Ohmic contacts can be non-rectified electrical contacts.

[0036] Figure 1 The diagram schematically illustrates 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, this LED string 15 could be used in automotive lighting. The LED string 15 includes a first LED 105 coupled in series and a plurality of additional LEDs 1081, 1082,...108. n The LED string is coupled to a voltage source 10, such as a battery. As used in this disclosure, the term "battery" is intended to mean a voltage source 10 configured to provide a voltage that may vary. For example, the provided voltage may decrease during use and may increase after the battery is charged. It should be clearly understood that any other type of voltage source may be used instead of a battery within the scope of this disclosure.

[0037] like Figure 1As shown, the driving circuit 100 includes a plurality of control elements 112. Each control element 112 is assigned to a corresponding additional LED 108. i .like Figure 1 As shown, control element 112 can be configured to bypass the corresponding LED 108. i One of them. The drive circuit 100 may include a converter 118 configured to convert the voltage provided by the voltage source 10 into a supply current that depends on the voltage provided by the voltage source 10. This supply current may be fed to each control element 112. Each control element 112 may be configured to compare the supply current provided by the converter 118 with that provided by a corresponding reference current generator 1161...116 n One of the provided reference currents I ref A comparison is performed. Based on the comparison results, the guidance can be controlled to each LED 1081, 1082...108. n The current in it. For example, the current in... Figure 1 It is also shown that the entire driving circuit 100 may include a single converter 118 and multiple control elements 112. For example, the number of control elements 112 may correspond to additional LEDs 1081...108. n The quantity.

[0038] Figure 2A An equivalent circuit diagram of a portion of the drive circuit 100 is shown. (e.g.) Figure 2A As shown, the drive circuit 100 includes a current comparator 114, which is configured to compare the supply current, which depends on the supply voltage provided by the voltage source, with a reference current I. ref The comparison is performed, and a differential current is generated. Circuit 100 also includes controls for directing the current to the additional LED 108. i One of the current control elements is 112. This control element includes a bypass element. The bypass element 120 includes a bypass current mirror 121, which is connected to the other LED 108. i Parallel coupling. The bypass current mirror 121 is controlled according to the differential current. The bypass current mirror 121 may include a diode-connected transistor and another transistor having a scaling ratio or gain K relative to the diode-connected transistor.

[0039] For example, such as Figure 2AAs shown, the drive circuit 100 may further include a converter 118 configured to generate a supply current that depends on the supply voltage provided by the voltage source 10. For example, the converter 118 may include a generating transistor 113, the gate of which is coupled to the output of the amplifier 129. The drain of the generating transistor 113 is coupled to a first converter node, which is coupled to the input of the amplifier 129. Another input of the amplifier 129 is coupled to a second node, which is coupled to the voltage source 10 via a first resistor 131. The drain of the generating transistor 113 may be additionally connected to a third node via a resistor 127. The third node is coupled to the second node via a second resistor 132. The output of the amplifier 129 is also connected to the gate of a variable transistor 123, which may have a scaling ratio or gain N relative to the generating transistor 113. Therefore, the output of the variable transistor 123 may have a value N×α×VBAT / R, where α represents the attenuation factor caused by the voltage divider including the first resistor 131 and the second resistor 132, and R represents the resistance of the resistor 127.

[0040] The current output by the variable transistor 123 is connected to the first node 124, which is also connected to the node used to generate the reference current I. ref Reference current generator 116 i The first node 124 is also connected to the first current mirror 125. The first node 124 implements a comparator 114, which is configured to compare the supply current with a reference current.

[0041] For example, the first current mirror 125 may include two PMOS transistors. The drain terminal of the diode-connected transistor of the first current mirror 125 is coupled to the first node 124. The drain terminal of the non-diode-connected transistor of the first current mirror 125 may be coupled to the diode-connected transistor of the bypass current mirror 121. The bypass current mirror 121 may be connected to a corresponding LED 108. i Parallel coupling.

[0042] When the supply current at node 124 is greater than the reference current, the current I... led Completely guided to LED 108 i When the supply current is less than I ref At this time, the difference between the reference current and the supply current is copied by the first current mirror 125 to the diode-connected transistor of the bypass mirror 121. Therefore, a linear current dependent on the supply voltage is supplied to the bypass current mirror 121, and a current from the LED current I is drawn from it. led Subtract the linear current from the selected Iref. When the chosen Iref is greater than I... led When the battery voltage is very low, the LED current is fully diverted to the bypass element 120, and the LED is completely turned off.

[0043] Therefore, this simple mirror-based implementation supports the suppression of negative differential contributions when the supply voltage becomes very large. This is likely due to the rectification effect of the first mirror 125. Meanwhile, the current flowing through the bypass current mirror 121 is linearly related to the supply voltage. Furthermore, this arrangement makes the transition point of the LED current relative to the supply voltage independent of the threshold of the bypass element 120. Therefore, a solution with superior performance and even a simpler structure can be obtained.

[0044] As should be clearly understood, Figure 2A The components can be applied to Figure 1 The configuration. More specifically, the individual elements implementing control element 112 can be assigned to each individual LED 108. i Furthermore, a single converter 118 can be allocated to the entire drive circuit 100. For example, the value of N is determined by the reference current generator 116. i The provided reference voltage can be applied to each LED108 i Individual settings. Based on... Figure 2A The embodiment shown, I ref The choice ensures that the conversion is fully completed and I led Fully guided to the corresponding LED 108 i The supply voltage at that time. In addition, by setting the gain N, the range of supply voltages required to complete the conversion can be set.

[0045] Figure 2B This is an equivalent circuit diagram of the drive circuit 100 according to another embodiment. As will be explained below, according to Figure 2B When the supply current is less than the reference current, the control element 112 is also configured to provide overdrive current to the diode-connected transistor of the bypass current mirror 121.

[0046] More specifically, such as Figure 2BAs shown, control element 112 may include a third current mirror 139. The third current mirror 139 is configured to replicate the difference current between the overdrive current provided by overdrive current source 135 and the difference between the supply current and the reference current into the diode-connected transistor of bypass current mirror 121 when the supply current is greater than the reference current and less than the overdrive current. More specifically, a terminal of the diode-connected transistor of the third current mirror 139 is connected to a second node 134, while a terminal of another transistor of the third current mirror 139 is connected to the diode-connected transistor of bypass current mirror 121. The second node 134 is coupled to the overdrive current source 135 and additionally coupled to a non-diode-connected transistor of the second current mirror 126. The diode-connected transistor of the second current mirror 126 is coupled to a first node 124, which serves as comparator 114. The first node 124 is connected to the output of variable transistor 123 and reference current generator 116. i As mentioned above (refer to the reference) Figure 2A As explained, the second current mirror can include an NMOS transistor.

[0047] For example, if the overdrive current equals I led / K And with unity gain for the third current mirror 139, the drive circuit 100 can operate as follows: when the supply voltage is low and the supply current is less than the reference current, no current flows through the second current mirror 126. The overdrive current provided by the overdrive current source 135 is fully supplied to the bypass current mirror 121, so that the entire current I led The current is bypassed via the bypass current mirror 121, and no current flows into LED 108. i The situation remains unchanged as the supply voltage increases, until the supply current exceeds the reference current. In this case, the current I... n That is, the difference between the supply current and the reference current is mirrored to the second node 134 by the second current mirror 126. At the second node 134, the current I is subtracted from the overdrive current. n This makes the current K×I n LED 108 begins to flow in i Until full conduction is achieved led At this point, the third mirror 139 is completely off. Therefore, the corresponding LED 108 can be activated. i A more linear I / V characteristic, as will be discussed below. Figure 4 Explanation.

[0048] When the supply current equals Iref, it can be noted that Figure 2A and 2B The functional differences between the illustrated embodiments. According to... Figure 2A In the embodiment shown, the bypass element 120 is completely turned off, and the associated LED is fully turned on.led On the contrary, according to Figure 2B In the embodiment shown, the bypass element has just begun to reduce its current. Therefore, the current supplied by the overdrive voltage source 135 is equal to I. led In the case of / K, the associated LED is turned on.

[0049] As should be clearly understood, Figure 2B The components can be applied to Figure 1 The configuration. More specifically, the individual elements implementing control element 112 can be assigned to each individual LED 108. i Furthermore, a single converter 118 can be allocated to the entire drive circuit 100. For example, the value of N is determined by the reference current generator 116. i The provided reference voltage can be applied to each LED108 i Set separately.

[0050] Depending on the implementation, due to offset and mismatch, the overdrive current should be greater than I. led / K. Otherwise, LED 108 exists. i There is always a risk of low current. For example, circuits can be designed to reduce the safety margin used.

[0051] Figure 2C This is an equivalent circuit diagram of the drive circuit 100 according to another embodiment. Besides... Figure 2B In addition to the components shown, Figure 2C The equivalent circuit diagram also includes a boost circuit 138. For example, the boost circuit 138 may include a first boost current source 136 and a second boost current source 137. For example, the first boost current source 136 may be arranged between the second node 134 and the third current mirror 139. The second boost current source 137 may be arranged between the first node 124 and the second current mirror 126. The first boost current source 136 and the second boost current source 137 may be matched or identical.

[0052] When the supply voltage is very low, especially below the voltage required for LED conversion, the second current mirror 126 is completely turned off. In this case, the current supplied by the first boost current source 136 is added to the overdrive current generated by the overdrive current source 135. Therefore, the corresponding LED 108... i The on-resistance is smaller. When the supply voltage and therefore the supply current increase, the second current mirror 126 turns on. Therefore, the current at the second node 134 decreases because 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 will not be significantly affected if the voltage drop across the bypass element 120 also increases.

[0053] At further increased supply voltages, the current flowing through the second current mirror 126 becomes greater than the current supplied by the first or second boost current source. Therefore, the effects of the two additional boost currents generated by the first boost current source 136 and the second boost current source 137 cancel each other out. Thus, the behavior of the bypass current mirror 121 relative to, for example... Figure 2B No change.

[0054] Because of this configuration, the performance can become more similar to a drive circuit that includes a voltage comparator, especially when the switching threshold is large. Furthermore, any potentially necessary clamping circuitry to limit the gate-source voltage of the bypass element 120 can be eliminated. Therefore, the bypass element 120 can be implemented in a more compact manner, with a reduced size.

[0055] As should be clearly understood, Figure 2C The components can be applied to Figure 1 The configuration. More specifically, the individual elements implementing control element 112 can be assigned to each individual LED 108. i Furthermore, a single converter 118 can be allocated to the entire drive circuit 100. For example, the value of N is determined by the reference current generator 116. i The provided reference voltage can be applied to each LED108 i Set separately.

[0056] According to all embodiments described herein, the conversion region, such as the range of supply voltages required to fully complete the conversion, can be set, for example, by setting the size of the mirror, and more specifically by setting the gain N of the variable transistor 123.

[0057] Figure 2D An equivalent circuit diagram of the components of a drive circuit 100 according to another embodiment is shown. Figure 2D The equivalent circuit diagram is similar to Figure 2C The equivalent circuit diagram. Furthermore, such as... Figure 2D As further shown, the gain S of the second current mirror 126 can be set. By setting the value of S, the slope of the I / V characteristic, i.e., the sensitivity to the supply voltage, can be set.

[0058] According to further explanation, a driving circuit 100 for driving a string of light-emitting diodes (LEDs) 15 includes a current comparator 114 and a bypass element 120. The current comparator 114 is configured to compare a supply current, which depends on the supply voltage provided by the voltage source 10, with a reference current and generate a differential current. The driving circuit 100 also includes circuit elements for mirroring the differential current to the bypass element 120. The bypass element 120 is configured to feed the differential current from the feed to the additional LED 108 according to the differential current. iThe bypass current is subtracted from the corresponding LED current, which depends on the supply current. For example, the drive circuit may also include a converter 118 to generate the supply current based on the supply voltage. For example, the bypass element 120 may be controlled such that if the supply current is greater than the reference current, the LED current I... led The flow is directed through the LED 108 i And if the supply current is less than the reference current, then it is less than I. led The current is directed to flow through the LED.

[0059] Figure 3 An implementation of a driving circuit 100 for driving the LED string 15 is shown. As shown, it is similar to the above reference. Figure 1 The discussion focuses on LED string 15, which includes additional LEDs 1081, 1082...108. n The control element 112 is connected to each of the other LEDs 108. i The value of S and the corresponding reference current I can be set for each control element. ref For example, using digital commands. The group of control elements 112 is connected to a converter 118, which is configured to generate a supply current based on the supply voltage provided by the voltage source 10. Therefore, the drive circuit includes a power supply for each additional LED 108. i Individual control elements 112 are provided, which can be configured by setting the values ​​of N, S, and the reference current. Furthermore, control elements 112 share a common converter 118.

[0060] Figure 4 A single light-emitting diode 108 is shown. i An example of I / V characteristics. As shown in the figure, by employing any of the drive circuits shown above, the threshold V can be... t and saturation value V s This makes the I / V characteristics more linear. Furthermore, as mentioned above, by setting the value of S, the I / V characteristics can be adjusted for each LED 108. i The slope of the I / V characteristic can be set individually. Furthermore, by setting the value of N, the slope can be customized for each LED (108). i Set V s and V t The range of conversions between them.

[0061] As described above, a bypass current based on the supply current is injected into the path connected in parallel with the other LED. Therefore, the I / V characteristics can be made linear at each stage of the transition. Furthermore, smooth transitions in the LEDs can be achieved. Moreover, the parameters of the I / V characteristics can be easily set by adjusting the gain of the current mirror. Specifically, for each of the other LEDs 108... i The activation point can be set independently. Due to the specific implementation of the drive circuit, the temperature dependence of the I / V characteristics can be reduced.

[0062] While embodiments of the invention have been described above, it will be apparent that other embodiments can be implemented. For example, other embodiments may include any sub-combination of the features recited in the claims or any sub-combination of the elements described in the examples above. Therefore, the spirit and scope of the appended claims should not be limited to the embodiments described herein.

[0063] List of reference numerals

[0064] 10 voltage sources

[0065] 15 LED strings

[0066] 100 drive circuit

[0067] 105 First LED

[0068] 1081,....108 n led

[0069] 112 control elements

[0070] 113 generates transistors

[0071] 114 Current Comparator

[0072] 116,1161,...116 n Reference current generator

[0073] 118 converter

[0074] 120 bypass components

[0075] 121 Bypass Current Mirror

[0076] 123 Variable Transistor

[0077] 124 First Node

[0078] 125 First Current Mirror

[0079] 126 Second Current Mirror

[0080] 127 resistor

[0081] 129 amplifier

[0082] 131 First Resistor

[0083] 132 Second Resistor

[0084] 134 Second Node

[0085] 135 Overdrive Current Source

[0086] 136 First Boost Current Source

[0087] 137 Second Boost Current Source

[0088] 138 boost circuit

[0089] 139 Third Current Mirror

Claims

1. A driving circuit (100) for driving a string of light-emitting diodes (LEDs) (15), the LED string comprising a first LED (105) coupled in series and a plurality of other LEDs (1081, ... 108...). n The driving circuit (100) includes: A current comparator (114) is configured to compare a supply current with a reference current and generate a difference current, the supply current depending on the supply voltage provided by the voltage source (10); as well as Control element (112) for controlling the LEDs (1081, ... 108) that are directed to the other LEDs. n The current in another LED, The control element (112) includes the additional LED (108) i A parallel-coupled bypass element (120), said bypass element (120) being controlled according to said differential current, wherein said bypass element (120) includes a bypass current mirror (121), and The control element (112) includes current mirrors (125, 126) configured to copy the difference between the reference current and the supply current to a node connected to the bypass current mirror (121) when the supply current is less than the reference current.

2. The driving circuit (100) according to claim 1 further includes a converter (118) for generating the supply current according to the supply voltage.

3. The drive circuit (100) according to claim 1 or 2, wherein the control element (112) includes 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 one of the preceding claims, wherein the control circuit (112) is configured to control the bypass element (120) such that if the supply current is greater than the reference current, the LED current I led The flow is directed through the LED (108) i And if the supply current is less than the reference current, then it is less than I. led The current is directed to flow through the LED, and the LED current I led It depends on the supply current.

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

6. The drive circuit (100) according to claim 5, wherein the control element (112) includes 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 less than the reference current.

7. The drive circuit (100) of claim 6, wherein the current mirror (126) is configured to copy the differential current to a third node (134) connected to the overdrive current source (135), and the control element (112) includes an additional current mirror (139) configured to copy an additional differential current between the supplied overdrive current and the differential current at the second node (134) to a diode-connected transistor of the bypass current mirror (121) when the supply current is greater than the reference current and less than the overdrive current.

8. The drive circuit (100) according to claim 6 or 7 further includes a boost circuit (138) configured to add the boost current to the additional differential current at the second node (134) when the supply current is less than the reference current.

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

10. The driving circuit (100) according to any one of the preceding claims further includes additional control elements (112), each of the additional control elements (112) being assigned to the additional LEDs (1081,...108). n ( ) corresponds to another LED.

11. The drive circuit (100) according to claim 10, wherein the control element (112) is individually adjustable.

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

13. The drive circuit (100) according to any one of claims 10 to 12, wherein each control element of the control element (112) includes a current mirror (126) with variable gain, the gain of the current mirror (126) of the control element (112) being individually adjustable.

14. A driving circuit (100) for driving a string of light-emitting diodes (LEDs) (15), the LED string comprising a first LED (105) coupled in series and a plurality of other LEDs (1081,...1082). n The circuit (100) includes: A current comparator (114) is configured to compare a supply current with a reference current and generate a difference current, the supply current depending on the supply voltage provided by the voltage source (10); Bypass element (120), and Circuit elements used to mirror the differential current to the bypass element (120), The bypass element (120) is configured to receive power from the additional LED (108) via a circuit. i The corresponding LED current I of another LED in ) led Subtract the bypass current, which depends on the differential current, from the LED current I. led It depends on the supply current.

15. The driving circuit (100) according to claim 14, wherein the bypass element (120) is controlled such that if the supply current is greater than the reference current, the LED current I... led The flow is directed through the additional LED (108) i The corresponding additional LED in ) and if the supply current is less than the reference current, then less than I. led The current is directed to flow through the other LED (108i), the LED current I led It depends on the supply current.

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