EP3484248A1

EP3484248A1 - Backlight unit and method for controlling backlighting

Info

Publication number: EP3484248A1
Application number: EP17201435.9A
Authority: EP
Inventors: Matthias Koerber; Helmut Noeth
Original assignee: Delphi Technologies Inc
Current assignee: Delphi Technologies Inc
Priority date: 2017-11-13
Filing date: 2017-11-13
Publication date: 2019-05-15

Abstract

A backlight unit (10) for a vehicle (60) comprises a backlight module (12) comprising at least a first string (18) of a plurality of serials LEDs (22, 24, 26), the plurality of serials LEDs (22, 24, 26) being ranked 1 to N from a first LED (22) to a last LED (26) in series; a power supply unit (14) configured to provide an operating output voltage (34) to the backlight module (12) from a battery voltage (36); the operating output voltage (34) being electrically connected to the first LED (22, 28) of each string (18, 20); a first dimming control unit (16) electrically connected to the last LED (26) of each string (18) of the plurality of serials LEDs (22, 24, 26) and configured to control the illumination of the plurality of serials LEDs (22, 24, 26) of each string (18) representative to received first dimming levels; a backlight controller (42) electrically connected to the first dimming control unit (16) and configured to provide first dimming levels between 0% of illumination and 100% of illumination to the first diming control unit (16); and a second dimming control unit (52) electrically connected between the first LED (22) and the second LED (24) of each string (18) and configured to control the illumination of only the first LED (22) of each string (18) representative to received second dimming levels; the backlight controller (42) being electrically connected to the second dimming control unit (52) and configured to provide second dimming levels between 0% illumination and 100% illumination to the second diming control unit (52).

Description

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a backlight unit and more particularly to a method for controlling a backlight module of the backlight unit.

BACKGROUND OF THE INVENTION

Vehicles are generally equipped with displays that are backlighted by arrays of light emitting diodes (LEDs). The arrays of LEDs generally comprise several strings of a plurality of serial LEDs. Basically, the power supply of the strings of LEDs is from a boost topology that elevates the battery voltage such that the LEDs of the strings are in conduction mode and illuminate the display.
Such basic solution to backlit displays consumes lot of energy from the vehicle's battery. In some situation, the energy of the battery must be reduced to keep some of the major features of the vehicle. In some situation, energy from the battery must be reduced to keep some major vehicle features operational to avoid a situation wherein passengers are in a non-safety situation.
It is therefore important to propose a new solution to solve these problems.

SUMMARY OF THE INVENTION

A backlight unit for a vehicle comprises a backlight module comprising at least a first string of a plurality of serials LEDs, the plurality of serials LEDs being ranked 1 to N from a first LED to a last LED in series; a power supply unit configured to provide an operating output voltage to the backlight module from a battery voltage; the operating output voltage being electrically connected to the first LED of each string; a first dimming control unit electrically connected to the last LED of each string of the plurality of serials LEDs and configured to control the illumination of the plurality of serials LEDs of each string representative to received first dimming levels; a backlight controller electrically connected to the first dimming control unit and configured to provide first dimming levels between 0% of illumination and 100% of illumination to the first diming control unit; and a second dimming control unit electrically connected between the first LED and the second LED of each string and configured to control the illumination of only the first LED of each string representative to received second dimming levels. The backlight controller is electrically connected to the second dimming control unit and configured to provide second dimming levels between 0% illumination and 100% illumination to the second diming control unit. Such configuration allows an illumination control of each string of the backlight module and of each first LED of each string.
The power supply unit may comprise a boost topology and a bypass topology and the backlight controller may be electrically connected to the power supply and may be configured to select and to control one of the boost, and the bypass topology to provide the operating output voltage to operate the first and the second dimming levels. The bypass technology allows the non-use of the boost in case of elevation of battery voltage is not needed.
The backlight unit may comprise a battery voltage supervisor electrically connected to the battery voltage and electrically connected to the backlight controller, said battery voltage supervisor being configured to provide a battery voltage value to the backlight controller such that said backlight controller may determine if boost topology is needed.
The backlight unit may comprise a voltage drop sensor or a current drop sensor electrically connected to the backlight controller and electrically connected between the first and the second LED, said voltage/current drop sensor being configured to provide a voltage drop value or a current drop value through the first LED to the backlight controller such that the backlight controller may diagnose the state of the serial LEDs, i.e. the backlight controller may determine if one LED of serial LEDs is damaged as in short circuit mode or open circuit mode.
The backlight unit may comprise a fail safe mode wherein the backlight controller is configured to provide the second dimming levels greater than 0% illumination and the first dimming levels equal to 0% illumination.
In fail safe mode, the backlight controller may be configured to determine the operating output voltage value from the current/voltage drop values and the second dimming levels; and to select the boost topology of the power supply unit if the determined operating output voltage value is greater than the battery voltage value; and to select the bypass topology of the power supply unit if the determined operating output voltage value is equal or lower than the battery voltage value. In fail safe mode, the power supply unit may comprise a buck topology, and the backlight controller may be configured to select the buck topology if the determined operating output voltage value is less than the battery voltage value instead of selecting the bypass topology.
According to one embodiment, an assembly comprises a backlight unit as described above and a warning indicator such that the first LED of a first string cooperates with the warning indicator of the vehicle such that said first LED is arranged to backlight the warning indicator of the vehicle. The backlight module may comprise a second string of a plurality of serials LEDs, said plurality of serials LEDs of the second string being ranked 1 to M from a first LED to a last LED in series; the first LED of the second string may cooperate with the warning indicator of the vehicle such that said first LED of the second string is arranged to backlight the warning indicator of the vehicle.
According to one embodiment, a method for controlling a backlight module for a vehicle comprising at least a first string of a plurality of serials LEDs, the plurality of serials LEDs being ranked 1 to N from a first LED to a last LED in series; comprises the steps of:

providing an operating output voltage to the backlight module from a battery voltage;
controlling first dimming levels of the illumination of the plurality of serials LEDs of each string;
controlling second dimming levels illumination of only the first LED of each string.

The method may comprise a fail safe mode comprising the steps of:

providing first dimming levels of 0% illumination of each LEDs ranked more than 1 of each string;
providing second dimming levels between 0% and 100% of each first LEDs of each string.

The method in that the fail safe mode may comprise the steps of:

determining the battery voltage value;
determining a current drop or voltage drop value through the first LED of each string;
determining an operating output voltage value from the determined current or voltage drop value and from the second diming levels;
selecting and operating a boost topology between the battery voltage and the operating output voltage if the determined operating output voltage value is greater than the battery voltage value;
selecting and operating a bypass topology between the battery voltage and the operating output voltage if the determined operating output voltage value is equal or lower than the battery voltage value.

The method according in fail safe mode may further comprise the steps of:

selecting and operating a buck topology between the battery voltage and the operating output voltage if the determined operating output voltage value is lower than the battery voltage value instead of the bypass topology.

According to one embodiment, a method for backlighting a warning indicator of a vehicle comprises the steps of the method for controlling a backlight module and further comprises a step of:

backlighting the warning indicator with the first LED of a first string.

The method for backlighting the warning indicator may comprise a step of:

backlighting the warning indicator with the first LED of a second string of a plurality of serials LEDs, said plurality of serials LEDs being ranked 1 to M from a first LED to a last LED in series.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features, objects and advantages of the invention will become apparent from reading the detailed description that follows, and the attached drawings, given by way of example and in which:

Figure 1 is a schematic diagram of a backlight unit according to a first embodiment;
Figure 2 is a schematic diagram of a backlight unit according to a second embodiment;
Figure 3 is a schematic diagram of an assembly of a warning indicator of a vehicle with the backlight unit of figure 2;
Figure 4 is a flowchart of a first method for controlling a backlight module of the vehicle in a fail safe mode;
Figure 5 is a flowchart of a second method for controlling the backlight module of the vehicle in the fail safe mode.

DETAILED DESCRIPTION OF THE PREFFERED EMBODIMENTS

Figure 1 shows a backlight unit 10 for a vehicle comprising basically a backlight module 12, a power supply unit 14 and a first dimming control unit 16.
The backlight module 12 comprises generally several strings 18, 20 of a plurality of light emitting diodes (LEDs) 22, 24, 26, 28. In order to facilitate the description, let's assume that the plurality of serials LEDs 22, 24, 26 are ranked 1 to N from a first LED 22 to a last LED 26. More particularly, each string comprises LEDs 22, 24, 26, 28 electrically connected in series. In other words, the cathode 30 of one LED 22 of a string 18 is connected to the anode 32 of the LED 24 connected with said one LED 22. In a particular embodiment, it may happen that the strings 18, 20 may have a different number N, M of LEDs 22, 24, 26, 28 in series.
The power supply unit 14 is configured to provide an operating output voltage 34 to the backlight module 12. The power supply unit 14 regulates a battery voltage 36 of a battery 38 of the vehicle such that it provides the operating output voltage 34 of the backlight module 12. More particularly, the operating output voltage 24 is electrically connected to the anode 32 of the first LED 22, 28 of each string 18, 20 such that a voltage is applied between the anode 32 of the first LED 22, 28 of each string 18, 20 and the electrical ground 40.
The first dimming control unit 16 is configured to control the illumination of the plurality of serials LEDs 22, 24, 26 of each string 18, 20 according to first dimming levels. The first dimming control unit 16 is electrically connected to the last LED 26 of each string 18, 20 of the plurality of serials LEDs 22, 24, 26. More particularly, the first dimming control unit 16 is electrically connected to the cathode 30 of the last LED 26 of each string 18, 20 such that said first dimming control unit 16 is configured to switch the cathode 30 of the last LED 26 of each string 18, 20 to the electrical ground 40.
Generally, when the cathode 30 of the last LED 26 of a string 18 is switched to the electrical ground 40, an electrical current is flowing from the first LED 22 to the last LED 26 as long as the operating output voltage 34 is high enough to get the LEDs 22, 24, 26 of said string 18 in conduction mode. Practically, the first dimming control unit 16 is applying first dimming levels between 0% and 100% such that the intensity of the plurality of serials LEDs 22, 24, 26 of each string 18, 20 may be adjusted between no illumination and maximum illumination.
Generally, the first dimming level is a duty cycle of a pulse width modulation (PWM) signal controlling a mean of switching the cathode 30 of the last LED 26 to the electrical ground 40 such that said cathode 26 of the last LED 26 may be connected to the electrical ground 40 from 0% of the time, i.e. no connection to the electrical ground 40 and so no illumination, to 100% of the time, i.e. permanent connection to the electrical ground 40 and so a maximum illumination.
As a non-limitative example, the mean of switching may be a Metal Oxide Semiconductor Field Effect Transistor (MOSFET transistor) wherein the gate is controlled by the PWM signal, the drain is electrically connected to the cathode 30 of the last LED 26, and the source is electrically connected to the electrical ground 40.
According to a first embodiment, the backlight unit 10 comprises a backlight controller 42 that provides the first dimming levels to the first dimming control unit 16.
As an option, the backlight controller 42 is also electrically connected to the power supply unit 14 such that it controls the operating output voltage 34. More particularly, the power supply unit 14 is a switch mode power supply comprising a boost topology 44 and a bypass topology 46, the backlight controller 42 being configured to select and to control one of the boost topology 44, and the bypass topology 46 to provide the operating output voltage 34 to operate the first dimming levels. In other words, the backlight controller 42 selects the boost topology 44 when an elevation of the battery voltage 36 is needed to operate the backlight module 12. An elevation of the battery voltage 36 may be needed during a start phase of the engine of the vehicle or in temperature conditions below 0 degree Celsius. An elevation of the battery voltage 36 may be also needed if a string 18 of LEDs 22, 24, 26 comprises a number N of LEDs 22, 24, 26 in series such that the operating output voltage 34 has to be greater than a nominal battery voltage value to get all the LEDs 22, 24, 26 in conduction mode. A nominal battery voltage value is considered as the most common battery voltage of the vehicle equipped with a new battery. As a non-limitative example, a battery for a thermal engine may have a nominal battery voltage value around 12.6 volts.
As an option, the backlight unit 10 further comprises a battery voltage supervisor 48 electrically connected to the battery voltage 36 and electrically connected to the backlight controller 42, said battery voltage supervisor 48 being configured to measure the battery voltage 36 and to provide a battery voltage value to the backlight controller 42. According to the battery voltage value, the backlight controller 42 may select the bypass topology 46 or the boost topology 44 of the power supply unit 14 such that the operating output voltage 34 is high enough to get the LEDs 22, 24, 26 of each string 18, 20 in conduction mode. The backlight controller 42 controls the boost topology 44 by controlling the switching frequency of the switching mode power supply.
As an option, the backlight unit 10 further comprises a first voltage drop sensor 50 or a first current drop sensor electrically connected to the backlight controller 42 and electrically connected to the cathode 30 of the last LED 26 of each string 18, 20, said first voltage drop sensor 50 or first current drop sensor being configured to measure a first voltage drop value or a first current drop value through each string 18, 20 and to provide the first voltage drop values or the first current drop values to the backlight controller 42. According to the first voltage drop value or the first current drop value of each string 18, 20, the backlight controller 42 may diagnose each string 18, 20, i.e. said backlight controller 42 may identify if one or several LEDs 22, 24, 26 of each string 18, 20 are damaged such that one or several LEDs 22, 24 of the plurality of serial LEDs 22, 24, 26 are in short circuit mode or in open circuit mode. According to said diagnostic, the backlight controller 42 may adjust the first dimming levels, and/or selecting the bypass topology 46 or the boost topology 44 of the power supply unit 14 such that the operating output voltage 34 is high enough to get the LEDs 22, 24, 26 of each string 18, 20 in conduction mode.
As a non-limiting example, the first voltage drop sensor 50 or the first current drop sensor may comprise a resistor switched to the electrical ground 40 during measurement such that a voltage drop value or a current drop value may be measured and converted digitally by an analog to digital converter (ADC).
Thus, according to the first dimming levels, and according to the battery voltage value, and according to the first voltage drop values or the first current drop values, the backlight controller 42 may select the bypass topology 46 or the boost topology 44 of the power supply unit 14 such that the operating output voltage 34 is high enough to get the LEDs 22, 24, 26 of each string 18, 20 in conduction mode.
Figure 2 illustrates a further refined and preferred embodiment of the invention. The drawing shows similar components and reference numerals as before. The embodiment further however includes a second dimming control unit 52 and optionally, a second voltage drop sensor 54 or a second current drop sensor.
The second dimming control unit 52 is electrically connected between the first LED 22 and the second LED 24 of each string 18, 20 and is configured to control the illumination of only the first LED 22 of each string 18, 20 representative to received second dimming levels. The backlight controller 42 is electrically connected to the second dimming control unit 52 and configured to provide second dimming levels between 0% illumination and 100% illumination to the second diming control unit 52.
The second voltage drop sensor 54 or second current drop sensor is electrically connected to the backlight controller 42 and electrically connected between the first LED 22 and the second LED 24, said second voltage/current drop sensor 54 being configured to measure a second voltage drop value or a second current drop value through the first LED 22, 28 of each string 18, 20 and to provide the second voltage drop value or the second current drop value to the backlight controller 42.
According to the preferred embodiment shows figure 2, the backlight unit 10 is able to behave in a fail safe mode such that minimum power consumption of the backlight unit 10 is reached. A fail safe mode, in the context of vehicle behavior, may be defined as a mode to prevent major failure of the vehicle when a device of the vehicle has a major failure. In the particular context of the present invention, and as a non-limiting example, the fail safe mode is a mode to save energy of the battery 38 when the battery voltage 36 of the vehicle becomes low such that some major functionalities of the vehicle may not operate in a nominal way. As non-limiting examples, a low battery voltage 36 of an electrical vehicle may reduce the remaining distance that may be traveled by the vehicle; a low battery voltage 36 of a vehicle may reduce the lighting power of the front and rear high/low beam.
Thus, in a fail safe mode, the backlight unit 10 is limited on providing the operating output voltage 34 to only the first LED 22, 28 of each string 18, 20. In fail safe mode, the first dimming levels are equal to 0%. The second dimming levels are between 0% and 100%.
By including the second voltage/current drop sensor 54 and the battery voltage supervisor 48, and according to the second dimming levels, the battery voltage value, and the second voltage drop values or the second current drop values, the backlight controller 42 may select the bypass topology 46 or the boost topology 44 of the power supply unit 14 such that the operating output voltage 34 is high enough to get the LEDs 22, 24, 26 of each string 18, 20 in conduction mode.
More particularly, the backlight controller 42 is configured to determine the operating output voltage 34 from the current/voltage drop values and the second dimming levels such that said backlight controller 42 may select the boost topology 44 of the power supply unit 14 if the determined operating output voltage 34 is greater than the battery voltage value and may select the bypass topology 46 of the power supply unit 14 if the determined operating output voltage 34 is equal or lower than the battery voltage value.
In order to optimize the power dissipation of the backlight unit 10, in addition to the boost topology 44 and to the bypass topology 46, the power supply unit 14 may comprise a buck topology 56 that provides an operating output voltage value lower that the battery voltage value. In fail safe mode, the backlight controller 42 may be configured to select the buck topology 56 if the determined operating output voltage 34 is less than the battery voltage value instead of selecting the bypass topology 46.
In fail safe mode, and according to the second voltage drop value or the second current drop value of each string 18, 20, the backlight controller 42 may diagnose the first LED 22, 28 of each string 18, 20, i.e. said backlight controller 42 may identify if the first LED 22, 28 of each string 18, 20 is damaged such that the first LED 22, 28 of each string 18, 20 is in short circuit mode or in open circuit mode. According to said diagnostic, the backlight controller 42 may adjust the second dimming levels, and/or selecting one of the bypass topology 46, the boost topology 44 and the buck topology 56 of the power supply unit 14 such that the operating output voltage 34 is just high enough to get the first LED 22, 28 of each string 18, 20 in conduction mode.
Figure 3 illustrates a third embodiment. The drawing shows similar components and reference numerals as before. Said third embodiment is an assembly of the embodiment of figure 2 with a warning indicator 58 of the vehicle 60. The warning indicator 58 is configured to inform passengers of the vehicle 60 on the fail safe mode behavior. The warning indicator 58 cooperates with at least one first LED 22 of a string 18 of the backlight module 12 such that the first LED 22 is switched ON during fail safe mode. In other words, the first LED 22 illuminates the warning indicator 58 such that the second dimming control unit 52 controlling the illumination of only the first LED 22 of said string 18 is applying a second dimming level for said first LED 22 more than 0%.
The warning indicator 58 of the vehicle 60 is generally a dashboard warning light. In our context, as a non-limitative example, the warning indicator 58 is generally a red color dashboard warning light that informs the passenger of an important reminder that needs immediate attention. The warning indicator 58 may also be a yellow (sometimes orange) dashboard warning light that usually means something needs to be repaired or serviced soon and to operate with caution. In our particular context, it may indicate the need of changing the battery 38 of the vehicle 60, or in the case of hybrid or electrical vehicle, the need to charge the battery of the electrical engine.
The warning indicator 58 may cooperate with other first LEDs 28 of other strings 20 of the backlight module 12. Preferably, by balancing technical issues, economical issues and functional safety issues, the warning indicator 58 may cooperate with two first LEDs 22, 28 of two strings 18, 20 of the backlight module 12. That redundancy may prevent failures of one of the two LEDs 22, 28 such that the non-damaged first LED 22 may still illuminate the warning indicator.
As a particular non-limitative example not shown, the warning indicator 58 may be a combination of a left and a right turn indicator such that each turn indicator may cooperate with two first LEDs 22, 28.
According to the three embodiments illustrated by figures 1, 2 and 3, it may be possible to integrate inside the backlight controller 42 the first and/or the second dimming control unit 16, 52, and/or the first and/or the second voltage/ current drop sensor 50, 54, and/or the battery voltage supervisor 48.
Figure 4 shows a method for controlling the backlight module 12 in the fail safe mode. The first three steps are steps that may occur out of fail safe mode. Said three first steps comprise providing 110 an operating output voltage 34 to the backlight module 12 from a battery voltage 36, controlling 112 first dimming levels of the illumination of the plurality of serials LEDs 22, 24, 26 of each string 18, 20 and controlling 114 second dimming levels illumination of only the first LED 22, 28 of each string 18, 20. The last two steps may occur during a fails safe mode. Said last two steps comprises providing 116 first dimming levels of 0% illumination of each LEDs 24, 26 ranked more than 1 of each string 18, 20 and providing 118 second dimming levels between 0% and 100% of each first LED 22, 28 of each string 18, 20.
Figure 5 shows a further refined and preferred method for controlling the backlight module 12 in the fail safe mode. The five first steps 110, 112, 114, 116, 118 are similar steps and reference numerals as before. In addition, the method comprises three steps wherein the backlight controller 42 is determining 120 the battery voltage value, is determining 122 a current drop or voltage drop value through the first LED 22, 28 of each string 18, 20, and is determining 124 an operating output voltage value from the determined current or voltage drop value and from the second diming levels. According to the three step of determining values 120, 122, 124, the method comprises a step of selecting and operating 126 the boost topology 44 between the battery voltage 36 and the operating output voltage 34 if the determined operating output voltage value is greater than the battery voltage value, and a step of selecting and operating 128 a bypass topology between the battery voltage 36 and the operating output voltage 34 if the determined operating output voltage value is equal or lower than the battery voltage value.
Optionally, the method may comprise a step of selecting and operating the buck topology 56 between the battery voltage 36 and the operating output voltage 34 if the determined operating output voltage value is lower than the battery voltage value instead of the bypass topology 46.
The method illustrated by figure 4 may be included in a method for backlighting the warning indicator 58 of the vehicle 60. In addition, the method for backlighting the warning indicator 58 of the vehicle 60 comprises a step of backlighting the warning indicator with the first LED 22 of a first string 18. Optionally, for functional safety issue, a redundancy of illumination of the warning indicator 58 is needed such that the method for backlighting the warning indicator 58 of the vehicle 60 further comprises a step of backlighting the warning indicator 58 with the first LED 28 of the second string 20 of the plurality of serials LEDs 28, 29, 31 said plurality of serials LEDs 28, 29, 31 being ranked 1 to M from a first LED 28 to a last LED 31 in series.

Claims

A backlight unit (10) for a vehicle (60) comprising
a backlight module (12) comprising at least a first string (18) of a plurality of serials LEDs (22, 24, 26), the plurality of serials LEDs (22, 24, 26) being ranked 1 to N from a first LED (22) to a last LED (26) in series;
a power supply unit (14) configured to provide an operating output voltage (34) to the backlight module (12) from a battery voltage (36); the operating output voltage (34) being electrically connected to the first LED (22, 28) of each string (18,20);
a first dimming control unit (16) electrically connected to the last LED (26) of each string (18) of the plurality of serials LEDs (22, 24, 26) and configured to control the illumination of the plurality of serials LEDs (22, 24, 26) of each string (18) representative to received first dimming levels;
a backlight controller (42) electrically connected to the first dimming control unit (16) and configured to provide first dimming levels between 0% of illumination and 100% of illumination to the first diming control unit (16); and
characterized in that
a second dimming control unit (52) is electrically connected between the first LED (22) and the second LED (24) of each string (18) and is configured to control the illumination of only the first LED (22) of each string (18) representative to received second dimming levels;
the backlight controller (42) is electrically connected to the second dimming control unit (52) and configured to provide second dimming levels between 0% illumination and 100% illumination to the second diming control unit (52).
Backlight unit (10) according to the preceding claim characterized in that
the power supply unit (14) comprises a boost topology (44) and a bypass topology (46);
the backlight controller (42) is electrically connected to the power supply unit (14) and is configured to select and to control one of the boost topology (44), and the bypass topology (46) to provide the operating output voltage (34) to operate the first and the second dimming levels.
Backlight unit (10) according to any one of the preceding claims characterized in that the backlight unit (10) comprises a battery voltage supervisor (48) electrically connected to the battery voltage (36) and electrically connected to the backlight controller (42), said battery voltage supervisor (48) being configured to provide a battery voltage value to the backlight controller (42).
Backlight unit (10) according to any one of the preceding claims characterized in that the backlight unit (10) comprises a voltage drop sensor (52) or a current drop sensor electrically connected to the backlight controller (42) and electrically connected between the first LED (22) and the second LED (24) of each string (18), said voltage/current drop sensor (52) being configured to provide a voltage drop value or a current drop value through the first LED (22) to the backlight controller (42).
Backlight unit (10) according to any one of the preceding claims characterized in that the backlight unit (10) comprises a fail safe mode wherein the backlight controller (42) is configured to provide the second dimming levels greater than 0% illumination and the first dimming levels equal to 0% illumination.
Backlight unit (10) according to each of the previous claims, wherein in fail safe mode, the backlight controller (42) is configured to determine an operating output voltage value from the current/voltage drop values and the second dimming levels; and to select the boost topology (44) of the power supply unit (14) if the determined operating output voltage value is greater than the battery voltage value; and to select the bypass topology (46) of the power supply unit (14) if the determined operating output voltage value is equal or lower than the battery voltage value.
Backlight unit (10) according to claim 6 characterized in that the power supply unit (14) comprises a buck topology (56), and wherein in fail safe mode, the backlight controller (42) is configured to select the buck topology (56) if the determined operating output voltage value is less than the battery voltage value instead of selecting the bypass topology (46).
Assembly comprising a backlight unit (10) according to any one of the preceding claims and a warning indicator (58) characterized in that the first LED (22) of a first string (18) cooperates with the warning indicator (58) of the vehicle (60) such that said first LED (22) is arranged to backlight the warning indicator (58) of the vehicle (60).
Assembly according to claim 8 characterized in that
the backlight module (12) comprises a second string (20) of a plurality of serials LEDs (28, 29, 31), said plurality of serials LEDs (28, 29, 31) of the second string (20) being ranked 1 to M from a first LED (28) to a last LED (31) in series;
the first LED (28) of the second string (20) cooperates with the warning indicator (58) of the vehicle (60) such that said first LED (28) of the second string (20) is arranged to backlight the warning indicator (58) of the vehicle (60).
Method for controlling a backlight module (12) for a vehicle (60) comprising at least a first string (18) of a plurality of serials LEDs (22, 24, 26), the plurality of serials LEDs (22, 24, 26) being ranked 1 to N from a first LED (22) to a last LED (26) in series; said method comprising the steps of:
providing (110) an operating output voltage (34) to the backlight module (12) from a battery voltage (36);

controlling (112) first dimming levels of the illumination of the plurality of serials LEDs (22, 24, 26) of each string (18);

controlling (114) second dimming levels illumination of only the first LED (22) of each string (18).
Method according to claim 10 comprising a fail safe mode comprising the steps of:
providing (116) first dimming levels of 0% illumination of each LEDs (24, 26) ranked more than 1 of each string (18);

providing (118) second dimming levels between 0% and 100% of each first LED (22) of each string (18).
Method according to claim 11 characterized in that the fail safe mode comprises the steps of:
determining (120) the battery voltage value;

determining (122) a current drop or voltage drop value through the first LED (22) of each string (18);

determining (124) an operating output voltage value from the determined current or voltage drop value and from the second diming levels;

selecting and operating (126) a boost topology (44) between the battery voltage (36) and the operating output voltage (34) if the determined operating output voltage value is greater than the battery voltage value;

selecting and operating (128) a bypass topology (46) between the battery voltage (36) and the operating output voltage (34) if the determined operating output voltage value is equal or lower than the battery voltage value.
Method according to claim 12 characterized in that the fail safe mode comprises the steps of:
selecting and operating a buck topology (56) between the battery voltage (36) and the operating output voltage (34) if the determined operating output voltage value is lower than the battery voltage value instead of the bypass topology (46).
Method for backlighting a warning indicator (58) of a vehicle (60) comprising the steps of the method for controlling a backlight module (12) according to any one of the claims 11 to 13 and further comprising a step of:
backlighting the warning indicator (58) with the first LED (22) of a first string (18);
Method for backlighting the warning indicator (58) according to claim 14 characterized in that it comprises a step of:
backlighting the warning indicator (58) with the first LED (28) of a second string (20) of a plurality of serials LEDs (28, 29, 31), said plurality of serials LEDs (28, 29, 31) being ranked 1 to M from a first LED (28) to a last LED (31) in series.