Method, apparatus and computer program product for controlling LED backlights and for improved pulse width modulation resolution

[0001] This application claims priority from co-pending U.S. application Ser. No. 60/196,770 entitled: “Apparatus and Method of Extending Pulse Width Modulation Resolution,” filed Apr. 12, 2000, the entire text of which is incorporated by reference.

[0002] The present invention relates generally to control of light emitting diode (LED) devices and in particular to control of LED backlights using pulse width modulation.

[0003] A light emitting diode, or LED, comprises a diode that emits visible light when current passes through it. LEDs have several applications. Certain display devices, for example, but not limited to, aircraft cockpit displays, use an array of LEDs to backlight and illuminate a liquid crystal display (LCD). Controlling the amount of light emitted by the LED array is desirable to adjust the brightness of the display. The brightness level impacts the ease with which the display may be viewed under certain lighting conditions, such as bright sunlight or dark environments; and individual viewer comfort level with the display.

[0004] In some applications, the brightness level is more than a convenience factor. For example, in the aviation environment, if the display is illuminated too brightly at night, the excessive brightness may adversely impact the pilot's night vision. Impaired night vision adversely impacts the safety of flight.

[0005] The brightness level additionally impacts the amount of power required to operate the device as well as the heat given off by the display. Power consumption affects the length of time the device can operate on battery power and the electrical load placed on the vehicle power supply systems. The heat given off by the display also affects what, if any, cooling of the display and surrounding equipment is required. Cooling devices add cost and complexity to equipment and systems. In aircraft/spacecraft applications, cooling systems add unwanted additional weight to the vehicle. Furthermore, if the display generates too much heat, touching or otherwise operating the display may cause discomfort to the user.

[0006] The amount of light emitted by the diode can be controlled by controlling the amount of power supplied to the diode where power equals voltage times current (P=V*I). In certain prior art devices, a microprocessor device is coupled to drive circuitry that controls the LED display brightness. In such designs, a technique known as pulse width modulation (PWM) is used to control the power supplied to the device. Under control of the microprocessor, the drive circuitry supplies current to the LED for a predetermined amount of time, or one pulse width. In this manner, by varying the number of pulses received and the width of the pulses, the total power supplied to the LED, and hence the brightness can be controlled.

[0007] One significant limitation on this prior art design is that the pulse frequency and duration are limited by the resolution with which the pulse frequency and width can be defined by the microprocessor. For this reason, it is not always possible to control the LED display with the specificity and precision desired. This fact may result in the LED display being too bright at one setting, but too dark at the next available setting. In an aviation environment, this fact can cause the cockpit display to be illuminated too brightly at night even on the lowest available setting.

[0008] Correction of the above deficiencies cannot presently be accomplished without a complete redesign of the microprocessor/driver hardware. Redesign is frequently impractical because often, the pulse width modulation output of the microprocessor is part of a predefined set of operations purchased with the selected microprocessor chip; and its resolution is limited by the number of bits the microprocessor can output. Redesign of standard LED drive circuit hardware is also undesirable due to the cost of custom designing and fabricating such circuits.

[0009] The present invention provides a method and computer program product useful for controlling the power supplied to an LED. The present invention improves the resolution with which the brightness of LED backlit displays may be controlled. The present invention also contributes to minimizing the heat energy dissipated by the display device.

[0010] According to one aspect of the present invention, the invention may be used to improve the resolution of existing pulse width modulation systems without the need for hardware redesign.

[0011] According to another aspect of the present invention, the invention includes an additional timing source that enables the pulse duration of the pulse width modulation pulses to be varied with greater precision. A number of states are associated with the additional timing source. For each of the timer states, the resolution of the modulator is improved by log₂ K, where K=the number of timer states.

[0026] FIGS. 1A–1C contain illustrations of how pulse width modulation can be used to control power to a load such as, for example, an LED or array of LEDs. The PWM duty cycle is the ratio of the amount of time the pulse is on, to the total period of the cycle. In the example of FIG. 1A, a pulse 2 is on during the interval from t=0 seconds to t=0.25 milliseconds (ms). No pulse occurs for the interval from t=0.25 ms to t=1 ms for a total of 0.75 ms. The duty cycle in the example of FIG. 1A is therefore ¼. The duty cycle in the example of FIG. 1B is ½, and the duty cycle of FIG. 1C is ¾.

[0027] If the magnitude of the pulse of FIGS. 1A–1C is 1 Volt, then the average voltage supplied to the LED in a 1 ms interval is 0.25V for FIG. 1A, 0.5V for FIG. 1B, and 0.75V in FIG. 1C. Thus, through operation of the pulse width modulation schemes of FIGS. 1A–1C, the total power supplied to the LED, and hence its brightness and thermal output can be controlled.

[0028] However, the power output mandated by the pulse width modulation scheme is limited by the resolution of the pulse width modulator. For example, if a pulse width modulator has n bits of resolution, the pulse width modulator can vary its output from 0 to 2ⁿ−1; and change its duty cycle in 1/(2ⁿ) step intervals. In the example of FIGS. 1A–1C, a pulse width modulator having a resolution of two bits was used to create the duty cycles and power outputs shown. The two bit pulse width modulator of FIGS. 1A–1C therefore has the following possible binary outputs: 00, 01, 10, and 11. Since there are four possible output values, the pulse width modulator can only change its duty cycle in intervals of 1/(2²) or ¼. Hence, the average power supplied can only be varied in ¼ V increments. Table I contains a truth table showing the output pulse as a function of modulator output for the two bit modulator used as an example throughout this document.

[0029]

TABLE I
Duty Cycle For An Example Modulator Having
Two Bits of Resolution
PWM Period = 1 ms
Modulator Binary Output	Output Pulse Duration (ms)	Duty Cycle
00	0	0
01	0.25	¼
10	0.50	½
11	0.75	¾

[0030] Increasing the bit resolution of the pulse width modulator provides greater resolution in the duty cycle that can be specified. For example, the Motorola 68HC16Z1 is a common processor used to provide pulse width modulation outputs. This Motorola processor has a resolution of n=8 bits and can thus vary its output to have values corresponding to between 0 and 255. This processor can therefore increment the PWM duty cycle in steps 1/256.

[0031] Yet, even with an 8 bit processor, the resolution provided by the pulse width modulation scheme may not be adequate for the task at hand. Suppose, for purposes of illustration, that using the two bit pulse width modulator of FIGS. 1A–1C, an increment of ⅛ V was desired. This increment is not possible using the pulse width modulator of FIGS. 1A–1C, because the smallest increment that can be specified is ¼ V. Likewise, a duty cycle smaller than 1/256 cannot be specified using the 8 bit Motorola processor described above. Absent the present invention, the only way to achieve the desired resolution is to change the pulse width modulator to one having three bit or higher resolution. Changing the hardware in such fashion may be impractical because the desired hardware is unavailable or costly due to the associated hardware and software changes.

[0032] The present invention provides a method and computer program product for virtually increasing the resolution of a pulse width modulator having n bits. In a preferred embodiment of the invention, the invention includes an additional timer with a predetermined associated number of states. During each of the timer states, the pulse width modulator output has one of 2ⁿ possible values. Thus, according to the present invention, a number of virtual bits, m, equal to the base 2 log of the number of timer states, can be added to the n existing bits of resolution. The resulting pulse width modulation has n+m bits of resolution. A better understanding of the principals of the present invention can be had with reference to the derivation below. In general, the duty cycle can be expressed as the ratio of the pulse “on” time to the total period as given in equation (1).
Duty Cycle=total pulse on time/total period  Eq.(1)
For a fixed bit modulator having n bits of resolution and a nominal period, P_n, the shortest duration pulse has a length in seconds of:

[0033] $\begin{array}{cc}\mathrm{Unit}\mathrm{Pulse}\mathrm{Length}\left(s\right)=U=\frac{P_n}{2^n}& \mathrm{Eq}.\left(2\right)\end{array}$
In the present invention, the total pulse on time in that state can be expressed as:

[0034] $\begin{array}{cc}\mathrm{ON}\mathrm{TIME}\mathrm{STATE}k=\frac{N_k{\mathrm{UP}}_T}{P_n}& \mathrm{Eq}.\left(3\right)\end{array}$

[0035] Where: N_k=number of unit pulse lengths specified in that state=output of modulator for state k; and
P_T=the additional timer period in seconds
The total pulse on time can be obtained by summing equation (3) for each state k=0 to k=K−1, where K equals the total number of states; e.g. K=2^m, where m=the numbered virtual bits of resolution added.
The total time period, T, in seconds, is given as:
T=P_TK  Eq.(4)
The duty cycle of the pulse width modulation according to the present invention can therefore be expressed as:

[0036] $\begin{array}{cc}\mathrm{Duty}\mathrm{Cycle}=\frac{\sum _{k=0}^{k=K-1}\left(\frac{N_k{\mathrm{UP}}_T}{P_n}\right)}{T}=\frac{\sum _{k=0}^{k=K-1}\left(\frac{N_k{\mathrm{UP}}_T}{P_n}\right)}{P_{T}K}=\sum _{k=0}^{k=K-1}\frac{N_{k}U}{P_{n}K}.& \mathrm{Eq}.\left(5\right)\end{array}$
For the smallest possible duty cycle, only one single unit pulse will be specified and will occur in only one of the k states. By setting N_k=1 (where 1 is the smallest non-zero integer), equation 5 can thus be reduced to express the highest resolution duty cycle as:

[0037] $\begin{array}{cc}\mathrm{Minimum}\mathrm{Duty}\mathrm{Cycle}=\frac{U}{P_{n}K}& \mathrm{Eq}.\left(6\right)\end{array}$
Substituting Eq. (2) into Eq. (6) and reducing the equation yields:

[0038] $\begin{array}{cc}\mathrm{Minimum}\mathrm{Duty}\mathrm{Cycle}=\frac{1}{2^n}·\frac{1}{K}& \mathrm{Eq}.\left(7\right)\end{array}$
Thus, the present invention permits additional bits of resolution to be added by adding states to the additional timer. For the example two bit processor of FIGS. 1A–1C and Table I, additional virtual bits of resolution can be added as shown in Table II below.

[0039]

TABLE II
Pulse Width Modulator Resolution as a Function of
Number of Timer States
	No. of Bits of Virtual	Resulting Resolution For
No. of Timer States	Resolution Added	n = 2 Bit Modulator
2	1	23
4	2	24
8	3	25
16	4	26

[0040] FIG. 2 and FIGS. 3A–3B illustrates how the resolution of the two bit pulse width modulator of FIGS. 1A–1C can be improved according to the present invention. The embodiment of FIG. 2, adds a single additional timer having the same period as the pulse width modulation period. In this example, that period equals 1 ms and the total time period is therefore 2 ms. The timer has two states: 0 and 1 thereby providing 2³ bits of resolution. In timer state 0, the pulse width modulator output has a first value. In timer state 1, the modulator output has a second value for the duration of the timer state. The first value and the second value output by the pulse width modulator in each of the timer states can be equivalent if desired. The sum of the first and second values, however, equals the total number of unit pulse time intervals required to obtain the desired duty cycle.

[0041] FIG. 2 contains a truth table for creating the various duty cycles in ½³ increments. If a duty cycle of ⅜ is desired, the total number of unit pulse lengths occurring during the two timer states must equal 3. In the example truth table of FIG. 2, any one of four possible combinations of modulator output as a function of timer state may be implemented to obtain the desired three pulse units. For example, during timer state 0, the modulator output can be set to 00 and no pulse is output during the first 1 ms. During the second 1 ms period, the additional timer is in state 1 and the modulator output is binary 11, or decimal 3, and a pulse of three unit lengths are output during this time period. The total output during the two timer states is thus three pulse units yielding a duty cycle of ⅜. Optionally, a pulse of two pulse unit lengths, or 0.5 ms, may be output in timer state 0 and one pulse of 0.025 ms may be output in timer state 1 to obtain the ⅜ duty cycle. FIG. 3A shows the corresponding waveform.

[0042] FIG. 3B shows a waveform for a ⅛ duty cycle constructed according to the example truth table of FIG. 2. In FIG. 3B, when the timer is in state 0, the pulse width modulator binary output is 01 and a single 0.25 ms pulse is output during the time period t=0 until t=1 ms. From the time period t=1 ms to t=2 ms the timer is in state 1 and no pulse is present during this interval. As shown in FIG. 2, the single pulse may optionally be set to occur in state 1, while no pulse is provided in state 0.

[0043] Some modulators allow for a 100% duty cycle through the use of an overflow bit. Thus, a bit modulator will have an overflow bit in the n+1 bit position, that when asserted, results in an output pulse having the length of the nominal modulator time period. Use of the overflow bit may be incorporated into the present invention. FIG. 4 illustrates how the example modulator of Table I can be used with an overflow bit to create a pulse width modulator having 3 bit resolution using an additional two state timer according to the present invention. As with the truth table of FIG. 2, various modulator output combinations are possible to obtain certain ones of the possible duty cycles.

[0044] As shown in each of the above examples, the total period of the pulse width modulator has been effectively increased from the 1 ms period of FIGS. 1A–1C to the 2 ms period of FIGS. 2 and 3A–B through the use of the additional timer. In the example of FIGS. 1A–1C, the update interval occurred every 1 ms, or 1000 Hz, whereas from the example of FIGS. 2 and 3A–B, the update interval is 2 ms, or 500 Hz. Thus, the additional resolution provided by the present invention impacts the update rate available. A lengthy update rate can cause perceptible flicker in the LCD display. However, so long as any required update rates can be maintained, additional “virtual bits” of resolution may be added according to the present invention.

[0045] For example, suppose the example two bit modulator of Table I was required to have increased resolution according to the techniques of the present invention while maintaining an update rate of at least 100 Hz. A virtual five bit pulse width modulator with an update speed of 125 Hz could be created by adding additional timer states as shown in Table II. A total of 8 states are required, which for an additional timer period of 1 ms yields an 8 ms total period. The resulting minimum duty cycle is thus ½⁵, or 1/32. This modulation scheme is shown in FIG. 5A. However, increasing the virtual modulation to six bits equates to a minimum duty cycle of ½⁶ or 1/64. For the two bit modulator of Table I, and per Table II, 16 timer states are required for a total time period of 16 ms. The resulting modulation scheme is as shown in FIG. 5B. The update rate is thus 62.5 Hz which does not meet the 100 Hz update requirements specified for the system.

[0046] In the example of FIGS. 2, 3A–3B and 5A–5B, the additional timer has a period equal to the nominal period of the pulse width modulator. Different time periods may be used with the additional timer of the present invention. Preferably, the additional timer has a period that is an integer multiple of the nominal period of the pulse width modulator period. FIG. 6 illustrates an implementation of the present invention using the example two bit pulse width modulator of Table I with a nominal period of 1 ms and an additional timer having a period of 3 ms. The example of FIG. 6 shows an effective duty cycle of ⅜ using this technique. As seen in FIG. 6, the output of the modulator is a first value, binary 10, during the initial 3 ms period when the additional timer is in state 0. During the second 3 ms time period, the additional timer is in state 1 and the modulator output is binary 01.

[0047] Constructing a pulse width modulator having an additional timer with a period not an integer multiple of the nominal period is possible, but may introduce nonlinearities in the modulator output. However, if the additional timer period is sufficiently larger than the period of the modulator output, these nonlinearities will be minimal. FIG. 7 diagrams such a modulation scheme for a pulse width modulator having a 2 ms nominal period and an additional timer period of 5 ms, to create a virtual 3 bit modulator. A three bit modulator can theoretically increment the duty cycle in increments of ⅛. In the diagram of FIG. 7, a ⅜ duty cycle is implemented, however, due to errors caused by the nonlinearities described above, the duty cycle is only approximately ⅜ and includes some error. Specifically during state 0, three 1 ms pulses occur. During state 1, three 0.5 ms pulse occur, but rest interval 600 shown in FIG. 7 is truncated in length and is less than the 1.5 ms rest interval associated with the remaining 0.5 ms pulses. The average duty cycle for the modulation scheme of FIG. 7 is thus:

[0048] $\frac{1\mathrm{ms}+1\mathrm{ms}+1\mathrm{ms}+0.5\mathrm{ms}+0.5\mathrm{ms}+0.5\mathrm{ms}}{10\mathrm{ms}}=45\%$
A 45% duty cycle is slightly larger than the ⅜, or 37.5% duty cycle desired. The resulting error in the duty cycle is therefore:

[0049] $\frac{0.45-0.375}{0.375}=20\%\mathrm{relative}\mathrm{error}$

[0050] FIG. 8 contains a flow chart of a process useful for implementing the improved pulse width modulation of the present invention. In the flow chart of FIG. 8, the desired duty cycle is specified in step 700 as a word having n=n+log²K significant bits. In steps 702 and 704, the word is truncated to the maximum number permitted if the word received is in excess of this value. In step 706, the current state of the additional timer is determined. The various steps shown grouped together by braces 708 of FIG. 8 assign a modulator output value to the given timer state. In a preferred embodiment of the invention, the modulator outputs associated with each of the various states are within one of the other. Other combinations are possible, however, in a preferred embodiment of the invention, steps 710 and 712 are used to ensure that a valid modulator output is specified at start up; and in conjunction with step 709, are used to validate that the modulator output specified is within the maximum and minimum values expected for this state. Step 714 checks if a 100% duty cycle is needed for this state and if so, step 716 asserts the modulator overflow bit. Otherwise, the desired modulator output value is set in step 718 and the overflow bit deasserted in step 720. The modulator output for the current state is now established. Step 722 increments to the next state and the modulator output for that state is set by repeating the process flow of FIG. 8.

[0051] FIG. 9 shows a table of modulator output values used to create a virtual 11 bit modulator from an n=8 bit modulator using the process of FIG. 8. In FIG. 9, a modulator output is associated with each one of eight additional timer states according to the duty cycle desired.

[0052] The present invention may be implemented as firmware, in executable code, as software stored in a memory device or as a microelectronic circuit as will be readily apparent to those of ordinary skill in the art. In addition, the present invention, may be used to control the brightness of existing LCD or other LED backlit displays with greater precision without hardware redesign of the controlling pulse modulator.

[0053] FIG. 10 contains a block diagram of an LED backlight 902 and associated drive electronics. LED backlight 902 is coupled to the positive and negative poles 904a and 904b of a power supply. In a preferred embodiment of the invention, a driver 912 and buffer 910 switch on and off in response to a control pulses 908 output by a pulse width modulator 916. When driver 912 switches on, current is drawn through array 902 powering the array. The amount of time driver 912 is “on” controls the display brightness. According to one preferred embodiment of the present invention, the LED drive electronics may additionally include a current limiter 906. Current limiter 906 prevents overheating of the LEDs comprising the display by limiting the amount of current flowing through the entire array or, optionally, through the individual array strings. Current limiter 906 may comprise a plurality of resistors arranged in series with each of the individual array strings. Optionally, current limiter 906 may be as described in copending patent application Ser. No. 09/834,277, entitled: “Apparatus and Method for Controlling LED Arrays,” filed the same day herewith and incorporated by reference; and as also described in copending patent application Ser. No. 60/237,876, entitled: “High Precision, High Efficiency Dimming Controller for LED Arrays,” also incorporated by reference.

[0054] Also according to the present invention, n bit modulator 916 is coupled to an additional timer 918 that can be used to generate K=2^m states. Modulator 916 is additionally coupled to a computing device 920 which may comprise a cpu, programmable logic device or other general purpose processor, analog or digital logic circuit. Computing device 920 may additionally include memory for storing code such as, for example, that described by FIG. 8 useful for assigning a modulator output to each of the K timer states of timer 918, wherein said code is executed by computing device 920. Computing device 920 may optionally include timer 918 or be able to assert interrupts using an internal clock to thereby function as timer 918.

[0055] The invention has now been described with reference to the preferred embodiments. Variations and modifications will be readily apparent to those of ordinary skill in the art. For these reasons, the invention is to be interpreted in view of the claims.