Control circuit and method for controlling light emitting diodes

Light emitting diodes, LEDs, usually have a relatively high light output with a high efficiency and small dimensions. LEDs can emit light with a light spectrum corresponding to a visible range or to an infrared range or to other non-visible frequency ranges.

LEDs can be used in backlight systems of displays of television or monitor systems. With LEDs it is possible to achieve a more evenly distributed lighting system than with a conventional neon light for example.

Controlling of the brightness of a LED can be done by varying the value of a current through the LED. This can lead to a variation of the spectral color of the LED. Another way of controlling LEDs is the use of pulse width modulated, PWM signals. In this case a mean current through the LED primarily has the same value.

FIG. 6 shows an embodiment of a conventional control circuit for controlling several strings STR 1 , STR 2 , STR 5 of LEDs. Each string comprises a LED driving circuit VS 1 , VS 2 , VS 5 , for example a voltage source, and a switch S 1 , S 2 , S 5 . The brightness of the LED is controlled by switching on or off the strings STR 1 , STR 2 , STR 5 depending on a pulse width modulated switching signal generated by a LED string switching control CCPWM. When switching the switches S 1 , S 2 , S 5 , a current flow for the LED strings STR 1 , STR 2 , STR 5 is completely switched on or off. This leads to high current peaks or spurs which cause electromagnetic interference, EMI.

When using a pulse width modulation, the switching signal in a backlight system is usually synchronized with signals derived from a video signal, for example horizontal or vertical synchronization signals as HSYNC or VSYNC. This can lead to optical interference between the switching signal and the synchronization signals. It is possible that the back lighted screen becomes dark on the same position or at the same time respectively when generating the video image from the video signal. For example, the lower half of the screen can be always darker than the upper half because of the synchronization.

A control circuit for controlling light emitting diodes comprises a switch for turning on or off a string of light emitting diodes. A sigma delta modulator comprises a signal input for receiving a control signal, a signal output coupled to a control input of the switch and a clock input for receiving a clock signal with a clock period. A combiner generates a control signal from a data signal and a noise signal. The data signal can correspond to a desired brightness of the LEDs. By using sigma delta modulation and introducing a slight variation of the data signal by combining it with a noise signal the points in time for switching the LEDs are diversified advantageously. This can lead to a more evenly distributed brightness, for example in a backlight system of a video screen.

The inventive principle can easily be adapted to a greater number of controlled strings.

Implementations are explained in more detail using exemplary embodiments with reference to the drawings, in which:

FIG. 1 shows a first embodiment of a control circuit according to one implementation,

FIG. 2 shows a second embodiment of a control circuit according to one implementation,

FIG. 3 shows a third embodiment of a control circuit according to one implementation,

FIG. 4 shows a fourth embodiment of a control circuit according to one implementation,

FIG. 5 shows an exemplary time current diagram and

FIG. 6 shows an embodiment of a conventional control circuit.

FIG. 1 shows an exemplary embodiment of a control circuit according to one implementation for controlling LEDs. A first string STR 1 of LEDs comprises several LEDs D 11 , D 12 , D 13 , D 14 . The number of LEDs is not limited to the four LEDs shown. The LEDs D 11 , D 12 , D 13 , D 14 are connected in series between a LED driving circuit VS 1 and a switch S 1 for turning on or off the first string STR 1 .

The control circuit comprises a first sigma delta modulator SD 1 which is formed by an adder A 1 and a delay element Z 11 . A first combiner CMB 1 receives a data signal DATA and a first noise signal generated by a first noise generator DNG 1 . The output of the first combiner CMB 1 is connected to an input of the first adder A 1 which forms the signal input SI 1 of the first sigma delta modulator SD 1 . A carry output of the first adder A 1 forms a signal output SO 1 of the sigma delta modulator SD 1 and is connected to a control input of the switch S 1 .

The data signal DATA can be a binary data word corresponding to a desired brightness of the LEDs. The noise generator DNG 1 can be a digital noise generator, for example with fed back shift registers and XOR outputs. The digital noise generator can generate a digital noise signal with an arbitrary word length. The word length of the noise signal determines the randomness of the switching signal. The word length of the data signal usually is greater than the word length of the noise signal.

According to the inventive principle the switch S 1 is controlled by the switching signal which is modulated according to the sigma delta principle. Thus a bit stream is generated from the desired brightness, where a time averaged mean value of the bit stream corresponds to a value of the control signal. The clock frequency of the clock signal CLK is usually relatively high compared to a change frequency of the data signal DATA.

Because the digital noise generator DNG 1 generates a pseudo-random sequence with positive and negative numbers with a mean value of zero, the mean value of the data signal DATA is not changed in average over time. Therefore the control signal provided on the signal input SI 1 of the sigma delta modulator SD 1 still corresponds to the data signal DATA, that means the desired brightness in average over time. Therefore the switching time or instance of switching is slightly varied, which reduces the effect of EMI.

FIG. 2 shows another embodiment of a control circuit according to the one implementation. In this embodiment also a second string STR 2 of LEDs is controlled by the control circuit. The string STR 2 comprises a LED driving circuit VS 2 and a second switch S 2 for turning on or off the second string STR 2 .

In addition to the control circuit shown in FIG. 1 , the control circuit comprises a delay element Z 12 which on the input side is coupled to the signal output SO 1 of the first sigma delta modulator SD 1 . An output of the delay element Z 12 is coupled to a control input of the second switch S 2 . The delay element Z 12 comprises a clock input for providing the clock signal CLK.

Thus the second switching signal at the output of the delay element Z 12 is a delayed version of the first switching signal with a delay time corresponding to or being equal to a clock period of the clock signal CLK.

A time averaged mean of the current through the second string STR 2 is not influenced by the delay, but as the first string STR 1 and the second string STR 2 are switched on or off at different times, the total current at a time is reduced. The total current of the arrangement hereby is the sum of a current through the first string STR 1 and the current through the second string STR 2 . Therefore the height of current steps of the total current is further reduced. This also leads to a reduction of EMI.

FIG. 3 shows a further embodiment of a control circuit according to one implementation. In addition to the embodiment shown in FIG. 1 a string STR 3 of LEDs comprising a LED driving circuit VS 3 , LEDs D 31 , D 32 , D 33 , D 34 and a switch S 3 , and a string STR 5 with a LED driving circuit VS 5 , diodes D 51 , D 52 , D 53 , D 54 and a switch S 5 . The control circuit comprises a second sigma delta modulator SD 2 with an adder A 2 and a delay element Z 21 , a second combiner CMB 2 and a second digital noise generator DNG 2 . A function of the second sigma delta modulator SD 2 corresponds to the function of the first sigma delta modulator SD 1 . The data signal DATA is provided to the second combiner CMB 2 along with a second noise signal generated by the second digital noise generator DNG 2 . The pseudo-random noise sequence of the second noise signal usually is different from the respective sequence of the first noise signal. Therefore the second control signal generated by the second combiner CMB 2 and provided to the sigma delta modulator SD 2 slightly differs from the first control signal. As described before for the sigma delta modulator SD 1 , the second sigma delta modulator SD 2 generates a second switching signal for controlling the second switch S 2 from the second control signal. As the first and the second control signals differ, this leads to differing first and second switching signals and differing points in time for switching the strings STR 1 and STR 3 .

As described for the second sigma delta modulator SD 2 , any number of further sigma delta modulators with respective combiners and digital noise generators can be provided, shown as an example with sigma delta modulator SD 3 , combiner CMB 3 and digital noise generator DNG 3 which control the LED string STR 5 .

All combiners are provided with the same data signal DATA. Because every combiner CMB 1 , CMB 2 , CMB 3 is coupled to an independent digital noise generator DNG 1 , DNG 2 , DNG 3 , the control signals generated by the combiners differ from each other. This results in differing switching signals accordingly. The average value of the LED current in each of the strings STR 1 , STR 3 , STR 5 in general is determined by the data signal DATA only. A total current of all strings at a time is reduced because the strings are not switched synchronously. Accordingly, using the inventive principle the synchronous switching of LED strings can be circumvented although the respective average current in each LED string is unchanged.

FIG. 4 shows another embodiment of a control circuit according to one implementation controlling several LED strings STR 1 , STR 2 , STR 3 , STR 4 , STR 5 . Also the LED string STR 4 comprises a LED driving circuit VS 4 , LEDs D 41 , D 42 , D 43 , D 44 and a switch S 4 for turning on or off the string STR 4 . The number of LEDs in each of the strings is not limited to the shown number of four LEDs. It can be equal for all of the strings, for example when used in a backlight system for a rectangular screen. The number of LEDs could also be different and depends on the respective lighting application. The LEDs or strings of LEDs respectively can be distributed equally over a screen area.

Controlling of the LED strings STR 1 and STR 2 corresponds to the embodiment shown in FIG. 2 . In this the first switching signal for controlling the switch S 1 is generated by the first sigma delta modulator SD 1 and the second switch is controlled by the second switching signal which is a delayed version of the first switching signal.

LED strings STR 3 and STR 4 are controlled in a similar manner as LED strings STR 1 and STR 2 . A third switching signal is generated by the sigma delta modulator SD 2 depending on the data signal DATA and the second noise signal generated by the digital noise generator DNG 2 . A fourth switching signal for controlling the fourth LED string STR 4 is generated by delaying the third switching signal for one clock period of the clock signal CLK by means of the delay element Z 22 .

Also further strings of LEDs could be controlled in a similar manner. As an example, a further string STR 5 is controlled by a further switching signal generated by the sigma delta modulator SD 3 , as described for FIG. 3 .

The combiners CMB 1 , CMB 2 , CMB 3 and the adders A 1 , A 2 , A 3 of the respective sigma delta modulators can be implemented as accumulators with a respective word length corresponding to a word length of the data signal DATA and a noise signal. The accumulators can be formed in hardware using for example simple logical circuits or can be realized in a digital signal processor, DSP. Also the sigma delta modulation can be performed using digital signal processing.

FIG. 5 shows a current time diagram comparing a total current of a control circuit according to one implementation and a conventional control circuit. In the lower half of FIG. 5 a total current IPWM of a control circuit using a pulse width modulated switching signal is shown. As an example, the pulse ratio is about 60% according to a desired value for a brightness of 0.6. All LEDs or strings of LEDs respectively are switched at the same time. This results in a large current peak when switching the LEDs on or off respectively. The current peaks cause high electromagnetic interference with high spurs in the electromagnetic frequency spectrum.

The upper half of FIG. 5 shows a total current ISD for LED strings controlled with a control circuit according to one implementation. The desired brightness value corresponds to the respective brightness value of the current IPWM, i.e. about 60%. It can be seen that switching off LED strings is performed more often using the inventive principle which leads to a distribution of the signal energy of the total current ISD over a broader frequency range. Because the current peak ΔISD is smaller than the current peak ΔIDWM, peak values of the frequency spectrum of the total current ISD are smaller. Accordingly, the electromagnetic interference is reduced.

Providing a greater number of strings of LEDs, a total current ISD can be regarded almost constant with current peaks ΔISD being relatively small. Therefore unwanted pulsed currents are avoided.

The inventive principle can be used in television or monitor backlight systems. The strings of LEDs can comprise white LEDs which emit light in the full visible frequency range which is seen as white light by a human eye. Color information could be added by local filtering of the respective spectral components.

As an alternative, the strings of LEDs could comprise colored LEDs, for example red, green, blue LEDs, also known as RGB-LEDs.

Optical interferences between a switching signal and a synchronization signal used in a conventional control circuit which would result in unwanted patterns on the screen can be avoided using the inventive principle.