NL2018342B1

NL2018342B1 - LED driver for VLC

Info

Publication number: NL2018342B1
Application number: NL2018342A
Authority: NL
Inventors: Saes Marc; Johanna Elisabeth Otten Andy
Original assignee: Eldolab Holding Bv
Priority date: 2017-02-08
Filing date: 2017-02-08
Publication date: 2018-09-03

Abstract

An LED driver for driving a plurality of LEDs or LED groups to transmit light in a VLC mode is described, wherein the LED driver comprises a control unit that is configured to: receive, at an input terminal, a set point representing a desired illumination characteristic and data that is to be transmitted using VLC; determine a current profile for each of the plurality of LEDs or LED groups to generate the desired illumination characteristic, whereby an envelope of the combined current profile comprises a modulation in order for a corresponding intensity modulation as perceived by a sensor to represent a variable length VLC code representing the data, the VLC code comprising code words, a code word comprising a plurality of symbols; generate one or more control signals to drive the plurality of LEDs in accordance with the current profiles in order for the plurality of LEDs or LED groups to transmit the VLC code; and upon receipt of a further set point representing a different illumination characteristic during the transmission of the VLC code, determine an adjusted current profile or profiles for one or more of the plurality of LEDs or LED groups based on the different illumination characteristic, whereby the current profile is only adjusted for some, not all, symbols of a next code word to be transmitted; generate one or more control signals to drive the plurality of LEDs or LED groups in accordance with the adjusted current profile or profiles, and output, via an output terminal of the control unit, one or more control signals to drive the plurality of LEDs or LED groups in accordance with the adjusted current profile or profiles.

Description

FIELD OF THE INVENTION

The invention relates to the field of light applications, in particular to LED-based lighting applications that are applied for visual light communication (VLC).

BACKGROUND OF THE INVENTION

The present invention relates to visible light communication (VLC) as may be applied by LED based lighting applications. In general, communication by means of visual light may be realized in various manner, using different modulation schemes to code the information that is to be communicated. LED based lighting applications in general allow for a high quality illumination experience, e.g. having variable color functionality and dimming functionality. In addition, there are additional demands that have to be met, e.g. with respect to flicker. Such a combination of demands may not always be met. In particular, it may be difficult to maintain a broadcasting using VLC while an illumination set point is adjusted. Or, it may be difficult to maintain a broadcasting using VLC substantially flicker-free.

It is an object of the present invention to provide in control methods for LED based lighting applications which provide in an improved realization of the various demands.

SUMMARY OF THE INVENTION

It would be desirable to provide in an LED driver enabling to more accurately operate in a VLC mode during a transient such as an illumination characteristic transient

To address this concerns, in a first aspect of the invention, an LED driver for driving a plurality of LEDs or LED groups is provided, the LED driver being configured to drive the plurality of LEDs or LED groups to transmit light in a VLC mode, wherein the LED driver comprises a control unit that is configured to:

receive, at an input terminal, a set point representing a desired illumination characteristic;

determine a current profile for each of the plurality of LEDs or LED groups to generate the desired illumination characteristic, whereby an envelope of the combined current profile comprises a modulation in order for a corresponding intensity modulation as

-2perceived by a sensor to represent a variable length VLC code comprising code words, a code word comprising a plurality of symbols;

generate one or more control signals to drive the plurality of LEDs in accordance with the current profiles in order for the plurality of LEDs or LED groups to transmit the VLC code; and upon receipt of a further set point representing a different illumination characteristic during the transmission of the VLC code, determine an adjusted current profile or profiles for one or more of the plurality of LEDs or LED groups based on the different illumination characteristic, whereby the current profile is only adjusted for some, not all, symbols of a next code word to be transmitted;

generate one or more control signals to drive the plurality of LEDs or LED groups in accordance with the adjusted current profile or profiles, and output, via an output terminal of the control unit, one or more control signals to drive the plurality of LEDs or LED groups in accordance with the adjusted current profile or profiles.

In accordance with the first aspect of the present invention, an LED driver is provided for driving a plurality of LEDs or LED groups. In an embodiment, the LEDs or LED groups may e.g. have different illumination characteristics such as a different color.

As an example, the plurality of LEDs or LED groups may comprises one or more warm white LEDs constituting a first group and one or more cold white LEDs constituting a second group.

In an embodiment, the LED driver comprises one or more power converter for supplying power to the LEDs or LED groups. Such power converters may e.g. comprise switched mode power converters such as Buck or Boost converters which may be configured to generate a controlled output current in accordance with a control signal as received from a control unit of the LED driver. Such a control unit may e.g. comprise one or more processors or microcontrollers or the like, an input terminal for receiving input signals representing a desired illumination characteristic to be generated by the plurality of LEDs or LED groups and an output terminal for outputting control signals for a power converter of the LED driver.

In an embodiment, the control unit may further be configured to receive data or information that is to be transmitted by means of VLC by the LEDs or LED groups. Such data or information may also be received via the input terminal or may be received via a dedicated data terminal of the control unit of the LED driver.

In accordance with the present invention, the LED driver is configured to drive the LEDs so as to generate a variable length VLC code, whereby code words, representing the data or information that is to be transmitted, are transmitted by means of a modulation of the

-3overall intensity of the LEDs or LED groups. More details on such a code are provided below.

In order to generate such a VLC code, the control unit of the LED driver according to the first aspect of the present invention is configured to determine, based a set point representing a desired illumination, a required current profile for each of the plurality of LEDs or LED groups to generate the desired illumination characteristic, whereby an envelope of the combined current profile comprises a modulation in order for a corresponding intensity modulation as perceived by a sensor to represent a variable length VLC code representing the data. The control unit may thus determine current profiles for the LEDs or LED groups, such that, when these current profiles are applied, an intensity modulation is obtained which can be perceived by a sensor. Further, the current profiles are determined in such manner, that, on average, the illumination as generated by the LEDs or LED groups corresponds to the desired illumination characteristic. Note that, in general, by ensuring that the modulation occurs on a comparatively high frequency, e.g. above 250 or 300 Hz, the modulation will not be noticed by an observer, rather, the observer will perceive the mixed, i.e. the averaged intensity of the LEDs or LED groups.

In accordance with the present invention, the control unit of the LED driver according to the first aspect of the present invention is further configured to generate one or more control signals to drive the plurality of LEDs in accordance with the current profiles in order for the plurality of LEDs or LED groups to transmit the VLC code. These control signals may e.g. be provided to the one or more power converters of the LED driver.

In accordance with the present invention, the control unit of the LED driver is further configured to ensure that a transmission of a VLC code, e.g. representing data or information, is not interrupted or disturbed, when an illumination characteristic of the LEDs or LED groups is altered or to be altered. In particular, in an embodiment of the present invention, the control unit of the LED driver according to the present invention is configured to, upon receipt of a further set point representing a different, desired illumination characteristic, only gradually implement this new illumination characteristic when a VLC code is transmitted.

In an embodiment, such a gradual implementation is realized by determining an adjusted current profile for one or more of the LEDs or LED groups, based on the different desired illumination characteristic, whereby only some of the symbols in a next code word that is to be transmitted take account of the new illumination characteristic. As such, a transition from a first illumination characteristic, e.g. represented by a first desired color and intensity of the LEDs or groups of LEDs to a second illumination characteristic, e.g. represented by a second desired color and intensity of the LEDs or LED groups is introduced in multiple steps.

-4 As an example, the control unit of the LED driver may e.g. be configured to:

in a first step, determine current profiles for transmitting a code word comprising multiple symbols based in the first illumination characteristic;

in a second step, determine one or more adjusted current profiles for transmitting a second code word comprising multiple symbols based on the second illumination characteristic, whereby the some, but not all symbols of the second code word take the second illumination characteristic into account;

in third step, determine one or more further adjusted current profiles for transmitting a third code word comprising multiple symbols based on the second illumination characteristic, whereby the further adjusted current profiles result, when applied to the LEDs or LED groups, in the second illumination characteristic.

In an alternative embodiment, rather than only applying an adjusted current profile to only some of the symbols of a next code word that is to be transmitted, a desired transition between a first illumination set point or characteristic and a second illumination set point or characteristic can be gradually introduced by applying one or more intermediate current profiles to a sequence of code words.

In such arrangement, a control unit as applied in an LED driver according to the present invention may be configured to, after receiving a first illumination characteristic and determining the corresponding current profiles to realize this illumination characteristic: receive a further set point representing a different second illumination characteristic; determine a further current profile for one or more of the plurality of LEDs to generate the different second illumination characteristic represented by the further set point, determine one or more intermediate current profiles for one or more of the plurality of LEDs, and determine output control signals for the LED driver to sequentially drive the plurality of LEDs or LED groups according to:

- the current profiles when transmitting a first code word;

- the one or more intermediate current profiles, when transmitting a respective one or more further code words, and

- the further current profiles when transmitting a yet further code word.

In an embodiment, the intermediate current profiles may e.g. represent illumination characteristics that are combinations of the first and second illumination characteristics. As an example, the first illumination set point may e.g. be to operate a first LED group at 50% and the second LED group at 50%, whereas the second illumination set point may e.g. be to operate a first LED group at 70% and the second LED group at 30%.

In such example, the control unit may e.g. be configured to determine and sequentially apply:

-5current profiles corresponding to operate a first LED group at 50% and the second LED group at 50% when transmitting a first code word;

current profiles corresponding to operate a first LED group at 60% and the second LED group at 40% when transmitting a second code word;

current profiles corresponding to operate a first LED group at 70% and the second LED group at 30% when transmitting a third code word.

In such embodiment, the control unit may thus implement one or more intermediate illumination set point, e.g. obtained by linear interpolation between a first set point and a second set point, thereby obtaining a gradual transition between two set points, during a VLC code transmission.

In an embodiment of the present invention, the control unit as applied in the LED driver according to the present invention is configured to substantially maintain a modulation depth during the operation in VLC mode when an illumination characteristic is adjusted.

In an embodiment, the frequency content of the current profiles as applied during the transition from a first illumination set point to a second illumination set point is kept above a predetermined value. In particular, in an embodiment, the adjusted current profiles or intermediate current profiles are constructed to have a lowest frequency that is equal to or higher than a lowest frequency of the current profiles.

These and other aspects ofthe invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description and considered in connection with the accompanying drawings in which like reference symbols designate like parts.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 depicts a current profile representing symbols to be used in a variable length VLC method.

Figure 2 depicts a detailed current profile of a symbol to be used in a variable length VLC method.

Figure 3 and 4 depict two possible current profiles for a first and second LED or LED group representing symbols to be used in a variable length VLC method.

Figures 5 and 6 depict current profiles for a sequence of two code words to be transmitted using VLC, the second code word having an amended profile.

Figures 7 and 8 depict alternative current profiles representing symbols to be used in a variable length VLC method.

Figure 9 schematically depicts an LED driver according to the present invention.

-6DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 depicts a current profile which can be applied by an LED driver for transmitting a code or code word using VLC (visible light communication) in accordance with an aspect of the present invention. In general, a VLC scheme applies a plurality of different symbols, e.g. 4-10, which can be combined into code words, each code word consisting of a combination of a predefined set of symbols. In such a VLC scheme, each symbol is represented by a particular current modulation, which may result in a modulated intensity when applied, which intensity variation may be detected by a sensor. Known examples of such modulation are on-off keying (OOK) and pulse position modulation (PPM). In the current profile as shown, a code word or code that is to be communicated consists of a combination of multiple symbols, whereby the symbols that are used have a different length. In the example as shown, 4 symbols SO - S3 are shown, each having a different duration or period TO - T3. In the example as shown, the current profile of each symbol has a first period during which the current is comparatively high, followed by the second period during which the current is comparatively low. As a result, a receiver having a light sensor may be capable, based on the intensity variations observed, to determine the instants to -13 when the symbols start and end and thus, based on the periods between these instants, identify the symbols and thus the code word that is transmitted. In particular, a receiver capable of determining the duration between two consecutive instants at which the intensity increases, would be capable to identify the symbols, based on the determined duration. In order to ensure proper detection of the instants that define the length of the symbols, a sufficient increase in intensity should be applied. In the present invention, the difference between the current at the end of a symbol and the current at the beginning of a next symbol may be referred to as the modulation depth MD. It may be a measure indicative of the ease or accuracy with which the instants tO -13 may be determined. In case the modulation depth MD is too small, it may be difficult for a receiver or sensor to notice the intensity change.

In a practical implementation, periods TO - T3 of the symbols may e.g. be in a range between 200 psec and 500 psec, whereas the duration or period of a code word is e.g. smaller than or equal to 3 ms.

With respect to the implementation of a VLC scheme, it may further be important to point out that, in general, the instants at which the current level may be adjusted are fixed, e.g. fixed relative to a clock frequency. As an example, the clock frequency can be such that a new set point can only be applied upon expiring of a minimal predetermined period. This minimal period may e.g. be referred to as the incremental period, or clock period, W (see Figure 2) and may be in a range between 20 psec and 30 psec, e.g. 26 psec. As such, the length or period of the particular symbols SO - S3 as used in the coding will typically be a multiple of

-7the incremental period W, e.g. SO = 10 x W, S1 = 14 xW, S2 = 18 xW, S3 = 22 xW. In addition, the amplitude of the current I as shown in the current profile will typically also be adjustable in a discrete manner, rather than a continuous manner. In particular, as shown in Figure 2, the minimal current increment is the distance between two adjacent horizontal dotted lines, this minimal current increment being referred to, in the present invention, as the incremental current step ICS.

Figure 2 schematically shows a current profile CP representing a particular symbol, the symbol being represented or identifiable by means of the period T of the symbol. In Figure 2, the vertical dotted lines indicate instants at which the current I can be changed, the horizontal dotted lines indicate possible amplitudes of the current I.

In such an arrangement, an increase or decrease of the current I can only occur along the dotted lines.

In case a current profile as shown in Figure 1 or 2 is used to transmit information, it will be clear that this current profile will correspond to a certain illumination intensity. In order to avoid the modulation of the current to be perceived by an observer, the periods of the different symbols or the overall period of a code word should be selected to be comparatively low, such that the frequency content of the current profile is comparatively high. As an example, the periods of the symbols used can be selected in such manner that the overall length or period of a code word is less than or equal to a predetermined value, e.g. 3.3 msec.

In addition to a desired intensity, an illumination set point may further include a desired color to be generated by the light source, the light source e.g. comprising a plurality of LEDs emitting light of a different frequency or frequency spectrum. The color of the illumination as perceived by an observer can be adjusted by adjusting the relative intensity of the plurality of LEDs applied.

In case various LEDs of different color are applied in an LED fixture, VLC may still be implemented in a similar manner as described above. In particular, when differently colored LEDs, or at least some of the LEDs, are provided with a modulated current, the envelope of the overall current profile can be used to represent the desired symbols forming a code word. In such case, the overall current profile will result in an overall intensity variation which may be perceived by an optical sensor. In order to realize this, a synchronization of the modulation applied to the different LEDs may be required.

Figure 3 schematically shows current profiles applied to a first and a second LED and the combined current profile. As shown, the current profile of the first LED, 11, is a continuous

-8current with amplitude 11, the current profile of the second LED, I2 is a modulated current. Combined, the envelope of the current profile resembles the current profile of Figure 1; i.e. the combined current profile may be considered to represent the required current modulation for broadcasting four symbols SO, S1, S2, S3. Note that, in line with Figure 2, the current amplitude for both the first LED and the second LED are modulated in accordance with discrete values, as are the instants where the current amplitude can be adjusted. In case the depicted current profiles are applied in a first and a second LED, a sensor observing the light generated by the combined current profile will be able to determine the instants at which the amplitude of the current envelope changes (i.e. increase from a value 11 to a value 11 + I2, and thus the symbols that are communicated via the modulation.

As will be understood by the skilled person, the color and intensity set point as obtained by the current profiles shown in Figure 3, may be obtained using alternative current modulations. Figure 4 schematically shows an alternative manner to arrive at the same combined current envelope using alternative modulations of the currents 11 and I2, both the alternative modulation of Figure 4 and the modulation shown in Figure 3 resulting in the same intensity and color experience for an observer. In this respect, it is assumed that there is a linear relationship between the current and the intensity.

With respect to realizing a particular illumination characteristic or set point, i.e. characterized by both a color set point and an intensity set point, it can be pointed out that, depending on the period or duration of a symbol or code word, the realization of a desired intensity and color may be realized per symbol, per code word or per multiple code word.

With respect to the sensing of the VLC, it can however be pointed out that, although the modulation depth of the current modulations shown in Figures 3 and 4 is the same, there may be a preferred manner to modulate the currents 11 and I2, e.g. to take into account that a sensor or detector that is used to detect the intensity changes may have a different sensitivity for different wavelengths.

During the transmission via VLC, there are a large number of requirements that need to be met in high end illumination systems. As mentioned above, in order to realize the VLC, a sufficient modulation depth is required. Further, in order avoid flicker, which could cause nausea for certain observers, the frequency content of the intensity modulations should be sufficiently high. In addition, during the VLC, a desired illumination and color set point should be preserved.

It would also be preferred that the mentioned requirements are maintained also during transients. Within the meaning of the present invention, transients may e.g. include a transition between a first illumination set point and a second illumination set point, an

-9illumination set point e.g. being characterized by a desired illumination intensity and an illumination color. During such a transition, it would be desirable to maintain the requirements, e.g. w.r.t. modulation depth, for using VLC, such that a transition between a first illumination set point and a second illumination set point can be performed without interruption ofthe VLC, i.e. the visible light communication. In addition, such a transient should be flicker-free, i.e. the transient should not be visible A transition for a non-VLC mode to a VLC mode or vice versa may also be considered a transient.

In an embodiment of the present invention, a transition from a first illumination set point to a second illumination set point, during VLC is performed by gradually adjusting the current profile, in particular by only adjusting, when a next code word is transmitted, the current profile of only some of the symbols that make up the code word, i.e. not all the symbols, in a first step, upon receipt of a set point representing a different illumination characteristic as represented by the current profile, only the current profile of some of the symbols. In a second step, the current profile of other symbols may be adjusted, to arrive more closely to the desired second illumination set point.

By doing so, a gradual transition from a first illumination set point to a second set point is realized, thereby maintaining an accurate VLC operation.

In an embodiment ofthe present invention, such a gradual adjustment is realized by means of transitioning via one or more intermediate current profiles.

Such embodiment may e.g. be realized by means of a control unit which may be applied to control an LED driver configured to drive a plurality of LEDs, whereby the control unit is configured to:

generate a current profile for each of the plurality of LEDs to generate the desired illumination characteristic represented by a received set point, whereby an envelope of the combined current profile is modulated in order for a corresponding intensity variation as perceived by a sensor to represent a variable length VLC code comprising code words, a code word comprising a plurality of symbols, and wherein the control unit is further configure to:

receive a further set point representing a different desired illumination characteristic as represented by the set point, determine a further current profile for one or more of the plurality of LEDs to generate the different desired illumination characteristic represented by the further set point, and determine one or more intermediate current profiles for one or more of the plurality of LEDs, and

- 10output control signals for the LED driver to sequentially drive the plurality of LEDs according to:

- the current profiles when transmitting a first code word;

- the further current profiles when transmitting a yet further code word.

By controlling the plurality of LEDs in such manner, a gradual transition form a current profile to a further current profile is realized via one or more intermediate current profiles, whereby the current profile is adjusted per code word that is to be transmitted. By applying such a gradual transition between a first illumination set point and a second illumination set point, the transmission of VLC can more easily or more accurately sustained.

In an embodiment, care is taken that the modulation depth is not decreased during the transition from a first intensity/color set point to a second intensity/color set point.

In an embodiment, the current profile is adjusted proportional for each of the LEDs of the plurality of LEDs. By doing so, the color set point may be maintained during the transient.

In the current profiles in Figures 3 and 4, one can recognize that the current may be adjusted at only predetermined instants spaced apart by incremental period W, with predetermined amounts, spaced apart by the incremental current step ICS . The minimum increment that can be applied to a current profile is thus to increase or decrease the current with the incremental current step ICS during a period equal to the incremental period W. this minimum increment is referred to as the minimal current increment.

Figure 5 schematically shows the current profiles for a first code word (consisting of symbols S0,S1,S2,S3), followed by a current profile for transmitting the same code word, whereby a positive minimal current increment M+ is applied to the second profile, in particular at the start of the symbol S2.

In order to gradually decrease the intensity, in an embodiment of the present invention, the current profile of a next to be transmitted code word is adjusted by applying a negative minimal current increment. Such an embodiment is shown in Figure 6. Figure 6 schematically shows the current profiles for a first code word (consisting of symbols S0,S1 ,S2,S3), followed by a current profile for transmitting the same code word, whereby a negative minimal current increment M- is applied to the second profile, in particular at the end of the high current portion of symbol S2.

-11 In an embodiment ofthe present invention, the current profile of the plurality of LEDs is adjusted by only one minimal current increment per code word that is transmitted.

In an embodiment ofthe present invention, a symbol is characterized by its length in time, or period, whereby the end of a symbol, corresponding to the start of a next symbol, is triggered by a step (either positive or negative) function. Such a step function may be realized in various manners.

In the embodiments as shown in Figures 2,3,4, the step function is realized by applying a substantial increase in the total current profile at the end of a symbol. In order to realize this, the symbols as shown in Figures 2,3,4 each start with a comparatively high total current portion HCP, total current referring to the summed current of all the LED currents, and end with a comparatively low total current portion LCP.

As will be understood by the skilled person, a more gradual decrease of the total current could be implemented as well.

As such, in the embodiments shown in Figures 2,3,4, the end of a symbol can be detected as the occurrence of a positive step, i.e. an increase ofthe total current. In particular, referring to Figure 3, the total current as applied to the plurality of LEDs increases from a comparatively low value (value 11) to a comparatively high value (value 11+12) at the end of the symbols SO, S1, S2, S3. The low total current value and the high total current value should be selected in such manner that the associated intensity variation is sufficiently large that it can be detected by a sensor that is to receive the VLC.

Alternatively, one could trigger the end of a symbol by a negative step function, i.e. by applying a substantial decrease in the current symbol at the end of a symbol. Figure 7 schematically illustrates different symbols S4, S5, S6, S7 that are represented in this manner.

In the embodiment as shown, the end of the symbols S4, S5, S6 and S7 corresponds to the instants where the current I2 is reduced to zero, i.e. at instants t1, t2, t3 and t4.

At these instants, there will be an important decrease in the intensity ofthe illumination as provided by the LEDs, such decrease may be detected by a sensor, thus enabling the determination ofthe duration or periods of the different symbols.

With respect to adjusting of an intensity and/or color set point during operation in a VLC mode, i.e. during visible light communication, it can be pointed out that such an adjustment may also be realized by adjusting the relative duration ofthe high current portion HCP and the low current portion LCP.

- 12This is schematically illustrated in Figure 8. Figure 8 schematically illustrates the same symbols S4, S5, S6, S7 of Figure 7, whereby, compared to Figure 7, the HCP of both symbols S4 and S7 is decreased.

Increasing or decreasing the relative width or duration of the high current portion and the low current portion thus provides in an additional way, compared to increasing or decreasing the current amplitude, of adjusting an intensity of a light source, during the operation in VLC mode.

With respect to increasing or decreasing an amplitude of the current, e.g. either current 11 or I2 during a part of the HCP, as illustrated in Figures 5 and 6, it can be pointed out that it may be advantageous, for the symbols that start with a HCP, to implement a positive current increment, such as the positive minimal current increment M+ shown in Figure 5, at the beginning of the HCP of a symbol and to implement a negative current increment, such as the negative minimal current increment M- shown in Figure 6, at the end of the HCP of a symbol.

In case of the application of the positive minimal current increment M+ shown in Figure 5 at the beginning of the symbol S1, this increment result in an increase in the modulation depth MD for this symbol. As can be seen, the modulation depth MD for symbol S1 is larger than then the modulation depth MD for symbol SO. Applying a negative minimal current increment M-, as shown in Figure 6, at the end of the HCP of a symbol enables to avoid that the modulation depth would become smaller.

As indicated above, in order to adjust an intensity of the light emitted by the LEDs or LED groups, the current profiles can be adjusted, either by adjusting an amplitude of the current, e.g. an amplitude of a current profile, or by adjusting a width of a portion of the current profile. In an embodiment, the current profiles are adjusted such that the frequency content of the profile is not lowered, i.e. care is taken that no lower frequencies are introduced by determining the adjusted or intermediate current profiles. When considering this, it may e.g. be preferred to introduce one or more current increments to a current profile, rather than widening or narrowing an HCP of a current profile.

In an embodiment, an LED driver according to the present invention comprises one or more power converters, e.g. switched mode power converters such as Buck or Boost converters. Such an LED driver is schematically shown in Figure 9. Figure 9 schematically shows an LED driver 710 according to the present invention. The LED driver may e.g. be connected to a light engine 720 comprising two LED groups 722 and 724. In the embodiment as shown, the LED driver 710 may be configured to output three currents la, lb, Ic, at respective output

- 13terminals 710.1, 710.2 and 710.3, the output currents being provided by three power converters, e.g. switched mode power converters, 750.1,750.2 and 750.3 of the LED driver. The LED driver 710 further comprises a control unit 730 which may be configured to provide, at an output terminal 730.2, control signals 730.3 to control the power converters of the LED driver, the control signal e.g. representing the desired current profiles. The control unit 730 further comprises an input terminal 730.1 configured to receive an input signal representative of a desired illumination set point and data that is to be transmitted via VLC. In the embodiment as shown, the output terminals 710.1, 710.2 of the LED driver 710 may be connected input terminals 720.1, 720.2 of the light engine 720 to power the LED groups 724 and 722. As such, in the embodiment as shown, currents la and lb of the LED driver may be provided, to LED groups 722 and 724 of the light engine.

As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting, but rather, to provide an understandable description of the invention.

The terms a or an, as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language, not excluding other elements or steps). Any reference signs in the claims should not be construed as limiting the scope of the claims or the invention.

The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically.

A single processor or other unit may fulfil the functions of several items recited in the claims.

The terms program, software application, and the like as used herein, are defined as a sequence of instructions designed for execution on a computer system. A program, computer program, or software application may include a subroutine, a function, a procedure, an object method, an object implementation, an executable application, an applet, a servlet, a source code, an object code, a shared library/dynamic load library and/or other sequence of instructions designed for execution on a computer system.

- 14A computer program may be stored and/or distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

Claims

CONCLUSIONS

A LED driver for controlling a plurality of LEDs or LED groups to emit light in a VLC mode, the LED driver comprising a control unit adapted to:

- receiving, at an input terminal, a set point representing a desired exposure characteristic, and data to be transmitted using VLC;

- determining a current profile for each of the plurality of LEDs or LED groups to generate the desired exposure characteristic, wherein an envelope of the combined current profile comprises a modulation such that a corresponding intensity modulation, as observed by a sensor, represents a variable length VLC code of the data, wherein the VLC code comprises code words, a code word comprises a plurality of symbols;

- generating one or more control signals to control the multiple LEDs according to the current profiles, so that the multiple LEDs or LED groups emit the VLC code; and

- after receiving a further set point representing a different exposure characteristic during the transmission of the VLC code, determining an adjusted current profile or profiles for one or more of the multiple LEDs or LED groups based on the different exposure characteristic, the current profile corrected only for some, not all, symbols of a subsequent code word to be transmitted;

- generating one or more control signals to control the multiple LEDs or LED groups according to the adjusted current profile or profiles, and

- outputting, via an output terminal of the control unit, one or more control signals to drive the plurality of LEDs or LED groups in accordance with the adapted current profile or profiles.

The LED driver according to claim 1, wherein the LED driver is adapted to adjust a current for any LED from the plurality of LEDs or LED groups to multiple discrete values at multiple discrete times.

The LED driver according to claim 2, wherein the LED driver is adapted to adjust the current profile of only one symbol of a following code word with a minimum current increment per time point.

The LED driver according to claim 1 or 2, wherein each symbol is characterized by a predetermined period and at least two different intensities applied during the predetermined period.

The LED driver according to claim 4, wherein each symbol is characterized by applying a relatively high intensity during a first portion of the predetermined period, followed by applying a relatively low intensity during a second portion of the predetermined period.

The LED driver according to any one of the preceding claims, wherein the LED driver comprises one or more switched power converters, arranged for:

- receiving the one or more control signals from the control unit and

- generating output currents for the multiple LEDs or LED groups according to the determined current profiles or adapted current profile or profiles.

7. A control unit for controlling an LED driver to control a plurality of LEDs to emit light in a VLC mode, the control unit comprising:

- an input terminal adapted to receive a set point representing a desired exposure characteristic and data to be transmitted using VLC; and

- an output terminal arranged for outputting one or more control signals for controlling the LED control, the control unit being arranged for:

- determining a current profile for each of the plurality of LEDs around the desired exposure characteristic represented by the set point, wherein an envelope of the combined current profile is modulated such that a corresponding intensity variation as perceived by a sensor represents a variable length VLC code, comprising code words, code word comprising several symbols,

- receiving a further set point representing a different desired illumination characteristic, determining a further current profile for one or more of the multiple LEDs on the basis of the different desired illumination characteristic, and

- determining one or more intermediate current profiles for one or more of the multiple LEDs and

- determining output control signals for the LED control to successively control the multiple LEDs or LED groups according to:

- the flow profiles when sending a first code word;

the one or more intermediate stream profiles, when transmitting a respective one or more further code words, and

- the further flow profile when sending a further code word.

The LED driver according to claim 7, wherein each symbol is characterized by a predetermined period and at least two different intensities applied during the predetermined period.

The LED driver of claim 8, wherein each symbol is characterized by applying a relatively high intensity during a first portion of the predetermined period, followed by applying a relatively low intensity during a second portion of the predetermined period.

A visible-light communication method for a lighting system comprising an LED driver and a plurality of LEDs or LED groups, the method comprising the steps of:

- receiving a set point representing a desired exposure characteristic and data to be sent with VLC; and

- determining a current profile for each of the plurality of LEDs to generate the desired exposure characteristic as proposed by the set point, wherein an envelope of the combined current profile is modulated such that a corresponding intensity variation as perceived by a sensor represents a variable length VLC code representing the represents data, the VLC code comprising code words, a code word comprising a plurality of symbols.

The method of claim 10, wherein each of the plurality of symbols is characterized by a predetermined period.

The method of claim 10 or 11, further comprising the steps of:

- receiving a further set point representing a different desired exposure characteristic,

- determining a further current profile for one or more of the plurality of LEDs to generate the different desired illumination characteristic represented by the further set point, and

- determining one or more intermediate current profiles for one or more of the multiple LEDs and determining output control signals for the LED driver to successively control the multiple LEDs or LED groups according to:

- the flow profiles when sending a first code word;

- the further flow profile when sending a further code word.

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Figure 8

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Figure 9