ILLUMINATING LEDS

Abstract

A method of operating a display system consisting of a plurality of light emitting diodes (LEDs) is disclosed. The LEDs are arranged in a plurality of groups and an integrated circuit provides power to the LEDs through a plurality of output pins connected to respective groups. The integrated circuit selectively determines the states of the output pins to illuminate the groups of LEDs in a repeating sequence such that each group is illuminated for a time dependent on a number of groups and a compensation factor. The compensation factor is dependent on at least a number of LEDs in the group.

Claims

1. A method of operating a display system comprising a plurality of light emitting diodes (LEDs) arranged in a plurality of groups and an integrated circuit providing power to the LEDs through a plurality of output pins connected to respective groups, the method comprising: the integrated circuit selectively determining the states of the output pins to illuminate the groups of LEDs in a repeating sequence such that each group is illuminated for a time dependent on a number of groups and a compensation factor, wherein the compensation factor is dependent on at least a number of LEDs in the group.

2. The method of operating a display system as claimed in claim 1, wherein each group of LEDs comprises a different number of LEDs.

3. (canceled)

4. The method of operating a display system as claimed in claim 1, wherein the output pins occupy one of three different states: high; low; off, wherein ‘high’ is a high voltage, ‘low’ is a low voltage and ‘off’ is a high impedance state.

5. The method of operating a display system as claimed in claim 4, comprising determining the states of the output pins such that each group of LEDs is illuminated individually.

6. (canceled)

7. The method of operating a display system as claimed in claim 1, wherein the LEDs are electrically paired in anti-parallel such that the paired LEDs have opposite terminals connected to the same output pin.

8. The method of operating a display system as claimed in claim 1, wherein the compensation factor depends on an empirically determined performance curve of the output pins.

9. The method of operating a display system as claimed in claim 1, wherein the compensation factor accounts for a relationship between applied voltage and brightness of the LEDs.

10. The method of operating a display system as claimed in claim 1, comprising retrieving compensation factor values for each of the plurality of groups from a memory.

11. The method of operating a display system as claimed in claim 1, wherein at least one of said plurality of groups comprises a virtual group of LEDs which is a subset of a physical group of LEDs and the method comprises illuminating said virtual group of LEDs whilst a remainder of the physical group of LEDs remain unilluminated.

12. The method as claimed in claim 11, comprising selecting said compensation factor based on a number of LEDs in the virtual group.

13. A system comprising: a display system comprising a plurality of light emitting diodes (LEDs), wherein the LEDs are arranged in a plurality of groups; a plurality of output pins connected to the plurality of groups of LEDs; and an integrated circuit arranged to provide power to the LEDs through the plurality of output pins, wherein the integrated circuit is arranged to selectively determine the states of the output pins to illuminate the groups of LEDs in a repeating sequence such that each group is illuminates for a time dependent on a number of groups and a compensation factor, wherein the compensation factor is dependent on at least a number of LEDs in the group.

14. The system as claimed in claim 13, wherein each group of LEDs comprises a different number of LEDs.

15. (canceled)

16. The system as claimed in claim 13, wherein the output pins are arranged to occupy one of three different states: high; low; off, wherein ‘high’ is a high voltage, ‘low’ is a low voltage and ‘off’ is a high impedance state.

17. The system as claimed in claim 16, wherein the integrated circuit is arranged to determine the states of the output pins such that each group of LEDs is illuminated individually.

18-19. (canceled)

20. The system as claimed in claim 13, wherein the compensation factor depends on an empirically determined performance curve of the output pins.

21. The system as claimed in claim 13, wherein the compensation factor accounts for a relationship between applied voltage and brightness of the LEDs.

22. The system as claimed in claim 13, wherein the integrated circuit comprises a memory arranged to store compensation factor values for each of the plurality of groups.

23. The system as claimed in claim 13, wherein at least one of said plurality of groups comprises a virtual group of LEDs which is a subset of a physical group of LEDs and the integrated circuit is arranged to illuminate said virtual group of LEDs whilst a remainder of the physical group of LEDs remain unilluminated.

24. The system as claimed in claim 23, wherein the integrated circuit is arranged to select said compensation factor based on a number of LEDs in the virtual group.

25. (canceled)

26. An electronic device comprising: a plurality of output pins arranged to connect to a plurality of groups of light emitting diodes (LEDs); and an integrated circuit arranged to provide power to the LEDs through the plurality of output pins, wherein the integrated circuit is arranged to selectively determine the states of the output pins to illuminate the groups of LEDs in a repeating sequence such that each group is illuminated for a time dependent on a number of groups and a compensation factor, wherein the compensation factor is dependent on at least a number of LEDs in the group.

Description

[0032] Some embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:

[0033] FIG. 1 shows a schematic diagram of a display system in accordance with an embodiment of the invention;

[0034] FIG. 2 shows a schematic diagram of the arrangement of a pair of LEDs implemented in the arrangement shown in FIG. 1;

[0035] FIG. 3 shows in more detail electrical arrangement of LEDs and output pins in the embodiment of FIG. 1;

[0036] FIG. 4 shows the appearance of the exemplary display system;

[0037] FIGS. 5-6 are graphs of average voltage measured at the high state output pin and at the low state output pin;

[0038] FIG. 7 is a graph of voltage against current for a typical embodiment;

[0039] FIG. 8 is a graph of current against the number of LEDs in a group;

[0040] FIG. 9a is an example of an illumination of the display system of FIG. 4 when no compensation is applied;

[0041] FIG. 9b is an example of an illumination of the display system of FIG. 4 when a compensation factor is applied;

[0042] FIG. 10a is an example of an illumination of the display system when no compensation is made for only a subset of a group being illuminated; and

[0043] FIG. 10b is an example of an illumination of the display system when a compensation is made for only a subset of a group being illuminated.

[0044] FIG. 1 is a schematic diagram showing a display system including an arrangement of a plurality of LED pairs 2. The arrangement also comprises an integrated circuit in the form of a general purpose microcontroller 3 comprising a plurality of output pins L0-L7 which are connected via wires to the plurality of LED pairs. As is shown in more detail in FIG. 3, each LED pair is connected to two output pins L0-L7. For example, the LED pair 5 in the top left corner is connected to pins L7 and L0.

[0045] A voltage is applied across the pins by configuring one of the pins in a high state and the other pin in a low state. Within the plurality of pins, it is possible to have multiple pins in a high state and multiple pins in a low state at any given time. Different combinations of states pins L0-L7 will result in different LEDs being illuminated.

[0046] The integrated circuit 3 includes a voltage regulator 7 which supplies a voltage to the LEDs via the output pins L0-L7. The voltage regulator 7 supplies all the LEDs in the display system. The integrated circuit 3 also includes a memory 9 and a processor 11.

[0047] FIG. 2 shows in more details the LED pair 5 as seen in FIG. 1. The LED pair 5 includes a first LED 6 and a second LED 8. Both the first LED 6 and the second LED 8 are connected to pins L7 and L0 in anti-parallel. This arrangement results in only one of the pair being illuminated at any given time. For example, when L7 is configured in a high state and L0 is configured in a low state, the first LED 8 is illuminated and the second LED 8 is unilluminated. When L7 is configured in a low state and L0 is configured in a high level, the first LED 6 is unilluminated and the second LED 8 is illuminated.

[0048] FIG. 3 shows more details of the electrical arrangement of LEDs as seen in FIGS. 1 and 2. In FIG. 3, the paired LEDs are shown physically separated. For example, in LED pair 5 the first LED 6 is located in the upper left corner of the LED arrangement and the second LED 8 is located in the lower right corner of the LED arrangement. Paired LEDs are located in inverted positions with respect to the diagonal.

[0049] A set of groups of LEDs 10, 12, 14, 16, 18, 20, 22 can also be seen in FIG. 3. In the embodiments shown in FIG. 3 each of the groups 10, 12, 14, 16, 18, 20, 22 contains a different number of LEDs. The LEDs in each group 10, 12, 14, 18, 18, 20, 22 are arranged in a line. The groups 10, 12, 14, 16, 18, 20, 22 form a triangle composed of sets of parallel lines.

[0050] An opposing set of groups of LEDs is formed from the corresponding paired LEDs. These LEDs form a second triangle. Both sets of groups are arranged with respect to each other so as to form a square. FIG. 4 is an example of a display system incorporating the arrangement shown in FIG. 3.

[0051] Operation of the display system will now be described. To illuminate each of the groups sequentially, the output pins are drive to different states. With reference to FIG. 1, to illuminate a first group of LEDs (i.e. the top row of LEDs seen in FIG. 1), output pin L7 is driven to a low state and output pins L6-L0 are driven to a high state. With reference to FIG. 2, this causes the lowermost LED 8 of the top left/bottom right corner pair 5 (see FIG. 3) to be illuminated and similarly for the rest of the pairs in the top row. To illuminate a second group of LEDs (i.e. the second row of LEDs seen in FIG. 1), output pin L6 is driven to a low state, output pins L5-L0 are driven to a high state and output pin L7 is drive to an off (high impedance) state. The remainder of the groups of LEDs are illuminated similarly, with the final single LED at the bottom of the pattern seen in FIG. 1) being illuminated by driving output pin L1 to a low state, output pin L0 to a high state and output pins L7-L2 to an off state.

[0052] The sequence then continues by illuminating the other member of each anti-parallel pair. With reference to FIG. 2, it will be seen that in the case of the corner pair 5, by driving L7 high and L0 low, the uppermost LED 6 is illuminated and similarly for the other pairs in the top row. As before the sequence continues to the second row but with L6 high and L5-L0 low and so on until L1 is high and L0 is low to illuminate the single LED in the top right (see FIG. 3)

[0053] The sequence of illuminating all the groups of LEDs described above is repeated in a cycle. The processor 11 controls the sequence of illuminations in the cycle by changing the states of the output pins L7-L0 as described above. The cycle may be suitably short, with each group of LEDs only illuminated for a short time, such that all the groups appear to be illuminated simultaneously. The cycle may, for example, be repeated more than thirty times a second.

[0054] What the Applicant has realised is that if each group of LEDs is illuminated for the same period of time within the cycle, e.g. all groups of LEDs have the same duty cycle, the display system appears non-uniform in brightness, as groups comprising a smaller number of LEDs appear brighter because the voltage regulator 7 is able to provide a voltage close to the nominal value as less overall current is draw. Conversely when more LEDs are illuminated, the extra current drain causes the voltage supplied to dip. The effect of this is seen in FIG. 9a and explained in detail below. To compensate for this, in accordance with the invention the duty cycle of each group is adjusted such that groups comprising a larger number of LEDs have a greater duty cycle than groups containing smaller numbers of LEDs. The calculation of the compensation factor is described in more detail below with reference to FIGS. 5-8 and Equations 1-4.

[0055] FIGS. 5 and 6 are graphs of the measured voltage against the number of LEDs in a group for an arrangement as shown in FIG. 3. Both Figures contain a plots corresponding to the voltage at a high state pin (also known as the sourcing pin) 24, 28 and a plot corresponding to the voltage at a low state pin 26, 30 (also known as the sinking pin). In FIG. 5, multiple pins are driven high (multiple source) and only a single pin is driven low (single sink). In FIG. 8, multiple pins are driven low (multiple sink) and only a single pin is driven high (single source). When multiple pins are in the same state, the voltage of each of these pins is averaged.

[0056] From FIG. 5 it can be seen that when more LEDs are sinked to a single pin, a higher voltage is measured at the high state pin and at the low state pin as the number of source pins (i.e. high state pins) is increased. From FIG. 6 it can be seen that when more LEDs are sourced from a single pin, a lower voltage is measured at the high state pin and the low state pin. However, the voltage across each LED is constant in both FIGS. 5 and 6.

[0057] For a typical LED the relationship between current and voltage is not linear, as shown in FIG. 7. The data shown in FIG. 7 is determined empirically. FIG. 7 may be used to determine the current through a group of LEDs based on the voltage at the high state pin(s) and the low state pin(s). The current through any individual LED within a group can then be determined by dividing the current through the whole group by the number of LEDs in the group.

[0058] This relationship is shown in Equation 1:

[00001]$\begin{array}{cc}{I}_N=\frac{f}{N}=\frac{f\left(V_{\mathrm{OL}}\left(N\right)\right)}{N}& \left(1\right)\end{array}$

[0059] Where I is the current through a group, N is the number of LEDs in a group, I.sub.N is the current through a single LED in a group and f(V.sub.OL(N)) is the function shown in FIG. 7.

[0060] FIG. 8 shows graphically the relationship between current through a group of LEDs 32 and current through a single LED within the group for various numbers of LEDs 34. The current through a single LED decreases as the number of LEDs in a group increases. This means that LEDs in a group with a large number of LEDs appear dimmer than the LEDs in a group with a smaller number of LEDs. This effect can be seen in FIG. 9a, which shows the variation in brightness of LEDs across a display system. For example, the LED in the lower right corner of the array and the LED is the upper left corner of the array appear brightest as there is only one LED in the group these LEDs belong to.

[0061] In order for all the LEDs in each group to appear to have a uniform brightness, a compensation factor is applied in accordance with the invention. The duty cycle for a particular group is defined as the fraction of the total time taken for a cycle of illuminating for which the group is illuminated. The duty cycle D.sub.N for a groups of LED is defined by Equation 2:

[00002]$\begin{array}{cc}{D}_N=\frac{t_N}{T}& \left(2\right)\end{array}$

[0062] Where N is the number of LEDs in a group, t.sub.N is the time period over which the LEDs in a group are illuminated and T is the total time taken for a cycle of illuminating all the groups individually. For all the groups to be have an equal brightness regardless of the number of LEDs in a group, the product of the duty cycle for any group and the current through the group must be equal for all groups. This required relationship is expressed in Equation 3.

D.sub.NI.sub.N=D.sub.1I.sub.1  (3)

[0063] Therefore, as the current through an LED in a group varies depending on the number of LEDs in the group, varying the duty cycle depending on the number of LEDs in a group enables the all LEDs appear equally bright. Combining Equations 1, 2 and 3, and re-arranging provides an equation for t.sub.N as seen in Equation 4:

[00003]$\begin{array}{cc}{t}_N=t_{1}N\frac{f\left(V_{\mathrm{OL}}\left(1\right)\right)}{f\left(V_{\mathrm{OL}}\left(N\right)\right)}& \left(4\right)\end{array}$

[0064] Where f(V.sub.OL(N)) and f(V.sub.OL(1)) are determined from FIG. 7. The compensation factor is therefore defined as

[00004]$\frac{f\left(V_{\mathrm{OL}}\left(1\right)\right)}{f\left(V_{\mathrm{OL}}\left(N\right)\right)}.$

[0065] Whilst the selected value of t.sub.1 can be chosen to suit the particular application, for embodiments in which it is desirable for all groups of LEDs to be able to appear to be illuminated simultaneously, t.sub.1 should be selected to be suitably short such that the total duration of the cycle T is short enough to benefit from human persistence of vision (e.g. a refresh rate of at least thirty times per second).

[0066] Each group of LEDs is then individually illuminated for a period of time equal to t.sub.N as calculated using the above equations until all the groups have been illuminated.

[0067] This pattern of illumination is then repeated such that all the LEDs appear to be illuminated simultaneously.

[0068] FIG. 8 also shows the plot of the duty cycle of a group as a function of the number of LEDs 36 in a group. As can be seen from FIG. 8, the required duty cycle increases as the number of LEDs in a group increases. FIG. 8 further includes a plot of the product of the duty cycle and the current through each LED in a group (D.sub.NI.sub.N) 38. As can be seen from FIG. 8, this product is constant regardless of the number of LEDs in a group. Therefore, when the duty cycle calculated using the method above is implemented, all the LEDs appear equally luminous.

[0069] The resulting appearance of the display system implementing the aforementioned duty cycles is seen FIG. 9b. In contract to the display system seen in FIG. 9a, all the LEDs within the display system appear to be equally bright.

[0070] Typically it will be desirable to illuminate only a subset of LEDs in a group in order to form a pattern on the display system. This is shown in FIG. 10a, where the aforementioned duty cycle (calculated assuming all LEDs would be illuminated) is implemented but only one LED is illuminated in each of the groups. The LEDs in groups 40-52 become progressively dimmer ascending through the groups as the duty cycle applied is disproportionate as only one LED is illuminated in each group.

[0071] In order to implement the correct duty cycle, each group is considered to comprise only the subset of LEDs illuminated. This subset of the physical group is a virtual group. The virtual groups forming a subset of groups 40-50 comprise one LED. The duty cycles required for all virtual groups to appear equally luminous is then calculated by selecting a compensation factor according to the number of LEDs in the virtual groups. In the particular arrangement shown in FIG. 10a this allows for the duty cycle of the groups where more than one LED is illuminated to be compensated for appropriately such that all the LEDs appear to be equally bright.

[0072] The resulting appearance of display system where the duty cycles are calculated for the virtual groups can be seen in FIG. 10b. In contrast to FIG. 10a, all of the LEDs appear to be equally bright. This allows for different patterns of LEDs to be displayed by illuminating different numbers of LEDs. Every time the pattern is changed, a new compensation factor is used by changing the duty cycles according to the number of LEDs in the virtual groups. These compensation factors may be calculated design and testing of the display system and stored in the memory 9 in the form of a lookup table. The processor then simply needs to select the appropriate compensation factor for the number of LEDs being illuminated at any moment.

[0073] In embodiments in which RGB LEDs are driven the Applicant has observed that the hue of the RGB LEDs is not affected significantly by their intensity and thus the invention is equally applicable to RGB LEDs.