LED lighting system with accurate current control

Abstract

A light emitting diode (LED) lighting system and method are disclosed. The LED lighting system and method include an LED controller to accurately control a current in an LED system. The LED controller includes components to calculate, based on the current and an active time period of an LED current time period, an actual charge amount delivered to the LED system wherein the LED current time period is duty cycle modulated at a rate of greater than fifty (50) Hz and to utilize the actual charge amount to modify and provide a desired target charge amount to be delivered during a future active time period of the LED current time period. The LED system and method further involve components to compare the actual charge amount to a desired charge amount for the active time period and compensate for a difference between the actual charge amount and the desired charge amount during the future active time period.

Claims

1. A light emitting diode (LED) lighting system comprising: a switching power supply for converting a phase modulated signal generated from a dimmer into an output voltage, wherein the dimmer is a phase cut dimmer and operates at a repetitive phase cut rate; an LED controller coupled to the switching power supply; and light emitting diodes (LEDs) coupled to the LED controller; and wherein the LED controller controls a current of the LEDs and includes components to: calculate, based on the current of the switching power supply, an average current amount delivered to the LEDs in a time period, wherein the time period is based on the repetitive phase cut rate; and utilize an actual charge based on the current to modify a future switching timing of the switching power supply.

2. The LED lighting system of claim 1 wherein the LED controller controls dimming of the LEDs.

3. The LED lighting system of claim 2 wherein the LED controller controls dimming of the LED system over a wide dimming frequency range.

4. The LED lighting system of claim 1 further comprising: components to utilize the actual charge based on the current to modify the future switching timing of the switching power supply.

5. The LED lighting system of claim 4 wherein the components to utilize the actual charge further comprises: components to compare the actual charge amount to a desired charge amount for an active time period and compensate for a difference between the actual charge amount and the desired charge amount during a future active time period.

6. The LED lighting system of claim 5 wherein the desired target charge is based on phase delays in a rectified input voltage.

7. The LED lighting system of claim 5 wherein the active time period and the future active time period are based on an averaging period that is based on the repetitive phase cut rate.

8. The LED lighting system of claim 7 wherein the LED controller controls dimming of the LEDs.

9. The LED lighting system of claim 8 wherein the LED controller comprises a dimming controller and a current controller wherein the dimming controller drives the current controller.

10. The LED lighting system of claim 9 wherein the dimming controller is driven by the repetitive phase cut rate and the current controller is driven by a switching frequency rate and wherein the repetitive phase cut rate is different than the switching frequency rate.

11. A method for controlling a current in light emitting diodes (LEDs) of an LED lighting system, comprising: converting, by a switching power supply, a phase modulated signal generated from a dimmer into an output voltage, wherein the dimmer is a phase cut dimmer and operates at a repetitive phase cut rate; controlling, by the LED controller coupled to the switching power supply and the light emitting diodes, a current of the LEDs; calculating, by the LED controller, based on the current of the switching power supply, an average current amount delivered to the LEDs in a time period, wherein the time period is based on the repetitive phase cut rate; and utilizing, by the LED controller, the actual charge based on the current to modify a future switching timing of the switching power supply.

12. The method of claim 11 wherein controlling, by the LED controller coupled to the switching power supply and the light emitting diodes, a current of the LEDs further comprises: controlling, by the LED controller, dimming of the LEDs.

13. The method of claim 12 wherein controlling, by the LED controller, dimming of the LEDs further comprises: controlling, by the LED controller, dimming of the LEDs over a wide dimming frequency range.

14. The method of claim 11 further comprising: utilizing, by components, the actual charge based on the current to modify the future switching timing of the switching power supply.

15. The method of claim 14 further comprises: comparing, by components, the actual charge amount to a desired charge amount for an active time period; and compensating, by the components, for a difference between the actual charge amount and the desired charge amount during a future active time period.

16. The method of claim 15 wherein the desired target charge is based on phase delays in a rectified input voltage.

17. The method of claim 15 wherein the active time period and the future active time period are based on an averaging period that is based on the repetitive phase cut rate.

18. The method of claim 11 wherein utilizing, by the LED controller, the actual charge to modify and provide a desired target charge amount to be delivered further comprises: utilizing the actual charge to modify and provide the desired target charge amount to be delivered during the future active time period of the LED current time period, wherein the future active time period is based on the repetitive phase cut rate further comprises; comparing the actual charge amount to a desired charge amount for the active time period; and compensating for a difference between the actual charge amount and the desired charge amount during the future active time period.

19. The method of claim 18 wherein controlling, by the LED controller coupled to the switching power supply and the light emitting diodes, a current, further comprises: controlling, by the LED controller, dimming of the LEDs.

20. The method of claim 19 wherein the LED controller comprises a dimming controller and a current controller and further comprising: driving, by the dimming controller, the current controller.

21. The method of claim 20 wherein driving, by the dimming controller, the current controller further comprises: driving the dimming controller at the repetitive phase cut rate; and driving the current controller at a switching frequency rate that is different than the repetitive phase cut rate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention may be better understood, and its numerous objects, features and advantages made apparent to those skilled in the art by referencing the accompanying drawings. The use of the same reference number throughout the several figures designates a like or similar element.

(2) FIG. 1 (labeled prior art) depicts an LED lighting system.

(3) FIG. 2 (labeled prior art) depicts a graphical representation of LED current in the LED lighting system of FIG. 1 for various dimming levels.

(4) FIG. 3 (labeled prior art) depicts a graphical relationship between an enable signal, output voltage, and LED current plotted against dimming voltage values for a prior art LED switch mode controller.

(5) FIG. 4 (labeled prior art) depicts another graphical relationship between an enable signal, output voltage, and LED current plotted against dimming voltage values for a prior art LED switch mode controller.

(6) FIG. 5 (labeled prior art) depicts a graphical relationship between an enable signal, output voltage, and LED current plotted against time for another prior art LED switch mode controller showing the ramp-up of the LED current.

(7) FIG. 6 (labeled prior art) depicts another graphical relationship between an enable signal, output voltage, and LED current plotted against time for the prior art LED switch mode controller which is the ramp-down of the LED current shown in FIG. 5.

(8) FIG. 7 depicts an LED lighting system having accurate current control in accordance with the present invention.

(9) FIG. 8 depicts a timing diagram for the current controller of the controlled LED lighting system which implements the principles of the present invention.

(10) FIG. 9 depicts time plots showing a current active period of the LED current in which a Pulse Width Modulation (PWM) control signal controls the current levels of the LED current over the active time period of the LED current time period.

(11) FIG. 10 depicts a time plot of LED current showing an exemplary train of active and inactive periods for the LED current controlled by a pulse width modulated dimming controller.

(12) FIG. 11 depicts a time plot of LED current showing an exemplary train of active and inactive periods for the LED current controlled by a delta sigma modulated dimming controller.

DETAILED DESCRIPTION

(13) A light emitting diode (LED) lighting system includes an LED controller to accurately control a current in an LED system. The LED controller includes components to calculate, based on the current and an active time period of an LED current time period, an actual charge amount delivered to the LED system wherein the LED current time period is duty cycle modulated at a rate of greater than fifty (50) Hz and to utilize the actual charge amount to modify and provide a desired target charge amount to be delivered during a future active time period of the LED current time period. The LED system further has components to calculate for an active time period of the LED current time period an actual charge amount delivered to the LED system and also has components to compare the actual charge amount to a desired charge amount for the active time period of the LED current time period and compensate for a difference between the actual charge amount and the desired charge amount during the future active time period. By being able to accurately control the desired charge amount, the average LED current is better controlled, and thus, the light intensity of the LED(s) is more effectively controlled.

(14) The accurate control and charge compensation of the LED current in this manner and in accordance with the principles of the present invention allows the LED lighting control system to operate linearly across wide dimming ranges while maintaining high efficiency. By accurately controlling and charge compensating the LED current, flicker caused by a slow PWM operation frequency (e.g., below 200 Hz) for pulsing the enable input signal can be avoided. Additionally, start-up and shut down problems caused by an overly fast PWM operation frequency for pulsing the enable input signal are also avoided by accurately calculating charge compensation for the LED current. Other negative effects caused by an overly fast PWM operation frequency, such as non-uniform dimming control, unpredictable control, and non-linear behavior, are also eliminated because of the accurate control and charge compensation of the LED current. By being able to accurately balance a slow and fast PWM operation frequency, the ability to provide a desired resulting LED color is no longer limited.

(15) FIG. 7 depicts an LED lighting system 700 that includes a current control system 702 to control the LED current i.sub.LED. The LED lighting system 700 also includes a dimming strategy module 704 to vary and modulate an active value of LED current i.sub.LED in response to varying dimming levels and in accordance with a dimming strategy described subsequently in more detail. In at least one embodiment, the LED lighting system 700 also includes the voltage source 104, dimmer 106, rectifier 108, and switching power supply 110, which operate as previously described.

(16) The current control system 702 (shown in a dotted-line border) includes an LED controller 706 to generate a duty cycle modulated gate control signal C.sub.G1 to control conductivity of FET Q1 and, thus, control LED current i.sub.LED. LED controller 706 includes a dimming controller 707 and current controller 709. Dimming controller 707 drives current controller 709. Dimming controller 707 can be a pulse width modulation (PWM) dimming controller or a delta-sigma modulated dimming controller. Control signal C.sub.G1 charges and discharges a gate of FET Q.sub.1. A logical 1 value (e.g., a first state) of control signal C.sub.G1 causes FET Q.sub.1 to conduct and draw LED current i.sub.LED through an LED system that comprises a number of LEDs 102 and also through an inductor L1. A logical 0 value of control signal C.sub.G1 causes FET Q1 to be non-conductive (e.g., a second state). FET Q.sub.1 represents one embodiment of a switch and can be replaced by any type of switch.

(17) In at least one embodiment, the LED lighting system 700 dims the LED system (e.g., the LEDs 102) in conformity with a dimming level input generated by a dimmer such as phase cut dimmer 106. The number of LEDs 102 is a matter of choice. LEDs 102 can be replaced by a single LED. The LED lighting system 700 can receive dimmer signals indicating dimming levels from LEDs 102 from any type of dimmer. For example, dimmer 106 can be omitted, and LED lighting system 700 can include a dimmer, such as digital dimmer 708 or a dimmer 106 having a direct current (DC) dimming control voltage (not shown). In at least one embodiment, the digital dimmer 708 is a digital addressable lighting interface (DALI) compatible dimmer. Digital dimmer 708 is depicted with dashed lines because generally LED lighting system 700 includes one dimmer or another dimmer but not two dimmers. Thus, in at least one embodiment, digital dimmer 708 is a substitute for dimmer 106 and phase delay detector 710. The dimmers, such as dimmer 106 and digital dimmer 708, receive inputs, either manually or automatically, that set the dimming level values to be output by the dimmers.

(18) In at least one embodiment, the LED controller 706 responds to a dimming level input and generates the control signal C.sub.G1 in accordance with a dimming strategy that, in at least one embodiment, includes two modes of operation. In an active value varying mode of operation, the LED controller 706 varies an active value of the LED current i.sub.LED in conformity with the dimming level for a first set of dimming levels. In an active value, duty cycle modulation mode of operation, the LED controller 706 modulates a duty cycle of an active value of the LED current it i.sub.LED in conformity with the dimming level for a second set of dimming levels.

(19) To determine which of the two modes of operation is to be used in generating the LED current i.sub.LED, LED lighting system 700 first detects a dimming level for LEDs 102. When LED lighting system 700 includes dimmer 106, the LED lighting system 700 also includes a phase delay detector 710 to detect phase delays in the phase modulated signal V.sub.. The phase delay detector 710 generates a phase delay signal , and the phase delays represented by the digital phase delay signal represent dimming levels. Melanson III describes an exemplary embodiment of phase delay detector 710.

(20) In at least one embodiment, the LED lighting system 700 also includes an optional mapping system and filter 711 to map the dimming levels indicated by the phase delay signal to predetermined digital values of dimming signal D.sub.V. Melanson IV describes an exemplary mapping system and filter 711 that maps values of dimming signal D.sub.V to perceived light levels. The LED lighting system 700 receives the dimming signal D.sub.V as a dimming level input. In at least one embodiment, LED lighting system 700 omits the mapping system and filter 711, and the dimming strategy module 704 receives the phase delay signal as a direct, digital dimmer signal input having values indicating dimming levels.

(21) FIG. 8 depicts an exemplary timing diagram for the dimming controller 707 and/or the current controller 709 of LED controller 706 within controlled LED lighting system 700. Dimming controller 707 and current controller 709 can each be implemented as a time-based controller that controls FET Q.sub.1 such that the output voltage V.sub.out of controlled LED lighting system 700 has a desired average value. Because the control applied to FET Q.sub.1 by time-based dimming controller 707 or time-based current controller 709 always causes controlled LED lighting system 700 to integrate up or down (e.g., integration response), time-based dimming controller 707 or time-based current controller 709 is said to apply bang-bang control.

(22) As indicated by its name, time-based dimming controller or time-based current controller 709 implements a time-based control methodology, rather than one of the conventional magnitude-based control methodologies. Time-based dimming controller 707 or time-based current controller 709 receives a compared voltage V.sub.COMP which is a comparison of sensed signal LEDi.sub.sense indicative of a current or voltage (e.g., sense current i.sub.LEDsense) in controlled LED lighting system 700 and a target or reference signal i.sub.target(t), such as an analog or digital current or an analog or digital voltage provided from dimming strategy module 704. In the depicted timing diagram, sensed signal LEDi.sub.sense is, for example, the current i.sub.LEDsense sensed at the drain of FET Q1 going through resistor R.sub.sense, as shown in FIG. 7, and the target current i.sub.target(t) is a target current i.sub.TARGET provided from dimming strategy module 704. Of course, in alternative embodiments, sensed signal LEDi.sub.sense and the target signal i.sub.target(t) may both be voltages.

(23) While a control signal C.sub.G1 supplied to the LED system (LEDs 102) is in a first state (e.g., such as an on-state), a polarity change in a comparison of the sensed signal LEDi.sub.sense and the target/reference signal i.sub.target(t) is detected at a first time. Based on the first time, a second time is determined at which to change a state of the control signal C.sub.G1 supplied to the LED system (LEDs 102). At the determined second time, the state of the control signal C.sub.G1 supplied to the LED system (LEDs 102) is changed from the first state to a second state (e.g., such as an off-state).

(24) In FIG. 8, sensed signal LEDi.sub.sense, which is either rising or falling at all times (e.g., polarity change), has repeating cycles of period P each comprising an interval T1 in which sensed signal LEDi.sub.sense is rising and an interval T2 in which sensed signal LEDi.sub.sense is falling. Each interval T1 in turn comprises an interval A (e.g., A(0), A(1), etc.) during which sensed signal LEDi.sub.sense rises from a cycle initial value (e.g., one state) to the target signal i.sub.target(t) and a subsequent interval B during which sensed signal LEDi.sub.sense rises from the target signal i.sub.target(t) to a cycle maximum value (e.g., another state). Sensed signal LEDi.sub.sense falls from the cycle maximum value to the initial value of the next cycle during interval T2. For clarity, intervals A and B are identified with ascending numerical cycle indices (A(0), A(1), etc. and B(0), B(1), etc.).

(25) In accordance with the present invention, time-based dimming controller 707 or time-based current controller 709 can control FET Q.sub.1 to implement any of a number of time-based control methodologies. For example, time-based dimming controller 707 or time-based current controller 709 can implement constant period control so that period P is constant (and intervals T1 and T2 vary between cycles), or constant on-time control so that interval T1 is constant (and period P and interval T2 vary between cycles), or constant off-time control so that interval T2 is constant (and period P and interval T1 vary between cycles). A desired methodology may be selected, for example, to reduce electromagnetic interference (EMI) with surrounding circuitry.

(26) The simplest control methodology, which also enables an immediate lock to the target signal i.sub.target(t), is a constant on-time or constant off-time approach in which one of intervals T1 or T2 is of constant duration and the other interval (and period P) varies in duration. In a constant off-time control methodology, time-based dimming controller 707 or time-based current controller 709 controls FET Q.sub.1 such that the interval A of interval T1 during which the sensed signal LEDi.sub.sense is less than the target signal i.sub.target(t) and the interval B of interval T1 during which the sensed signal LEDi.sub.sense is greater than the target signal i.sub.target(t) are equal. According to this constant off-time control methodology, the duration of interval B for each cycle is determined in accordance with the following equation 1:
B(N)=[B(N1)+A(N)]/2,(Equation 1)
where N is the cycle index. Thus, for example, utilizing Equation 1, time interval B(1) is equal to the average of time intervals B(0) and A(1). Interval T2 is, of course, fixed in duration.

(27) The constant on-time control methodology employs the same equation as the constant off-time approach, except that in the constant on-time approach, interval T1 is of constant duration, interval A is the portion of interval T2 in which the sensed signal LEDi.sub.sense exceeds the target signal i.sub.target(t), and interval B is the portion of interval T2 in which the sensed signal LEDi.sub.sense is less than the target signal i.sub.target(t). Time-based current controller 709 again controls FET Q.sub.1 such that intervals A and B are of equal duration.

(28) With reference now to FIG. 9, a current-lime plot 900 and a control signal time plot 902 are shown. An LED current time period for an LED current i.sub.LED includes both active time periods and inactive time periods. An exemplary active time period of an LED current time period utilized by controlled lighting system 700 for dimming control is shown in current-time plot 900. A Pulse Width Modulation (PWM) switching control signal C.sub.G1 is shown in control signal time plot 902 plotted over the same active period (e.g., from 0 to 120 microseconds). The PWM switching control signal C.sub.G1 controls the current levels of the LED current i.sub.LED over the active time period for controlling the dimming levels of the LEDs 102. The active time period is generally defined as an LED current pulse (e.g., LED current pulse 901), that is, from when the current level of the LED current i.sub.LED ramps up to and fluctuates at an average high current value i.sub.high (e.g., 0.45 Amp) and through and until the time when the current level of the LED current i.sub.LED ramps down to a low current value i.sub.low (e.g., 0 Volts).

(29) As shown in FIG. 9, the PWM switching frequency f.sub.SW for the PWM control signal C.sub.G1 is different than the PWM dimming frequency rate f.sub.DIM for the LED current i.sub.LED for controlling dimming levels of LEDs 102 over time. Thus, for the operations of LED lighting system 700, two different PWM operating frequencies are respectively being utilized for the control signal C.sub.G1 and the dimming control (e.g., control of the levels of LED current i.sub.LED). Exemplary operating frequencies for PWM dimming frequency rate f.sub.DIM widely ranges from 100 Hz to 20 kHz. The PWM dimming frequency rate f.sub.DIM is provided by duty cycle modulating the LED current time period at a rate of greater than fifty (50) Hz. Exemplary operating frequencies for PWM switching frequency f.sub.SW range from 50 kHz to 250 kHz.

(30) In FIG. 9, while control signal C.sub.G1 turns on and has a high value (e.g., 1), LED current i.sub.LED ramps up during 0 microsec. to 12.5 microsec. in accordance to ramp-up slope R.sub.UP1 and charges up to 0.45 Amp. When control signal C.sub.G1 turns off and has a low value (e.g., 0), LED i.sub.LED starts decreasing from 0.45 Amp through 0.4 Amp and reaches 0.35 Amp at which point control signal C.sub.G1 turns back on and has a high value (e.g., 1). The current level of LED current i.sub.LED continues to fluctuate in this manner (e.g., between 0.45 Amp and 0.35 Amp) in accordance with the pulses (e.g., turning on and off FET Q.sub.1 of LED lighting system 700) of control signal C.sub.G1. The fluctuations of the LED current level span from 12.5 microsec. through 92.5 microsec.

(31) From 92.5 microsec. to 120 microsec., the current level of LED current i.sub.LED ramps down in accordance with ramp-down slope R.sub.DN1 from 0.4 Amp to 0 Amp since control signal C.sub.G1 is turned off and stays at 0. The actual charge amount Q.sub.Actual for LED current pulse 901 is calculated as follows:
Q.sub.Actual=Q1+Q2Q3Equation 2

(32) The following charge amounts are determined by the following area calculations:
Q1=*(X1*Y)=*((12.50)*0.4)=2.5 CoulombsEquation 3
Q2=Z1*Y=(92.512.5)*0.4=32 CoulombsEquation 4
Q3=*(X2*Y)=*((12092.5)*0.4)5.5 CoulombsEquation 5

(33) Thus, the total actual charge amount Q.sub.Actual for LED current pulse 901 is:
Q.sub.Actual=2.5+32+5.5=40 CoulombsEquation 6

(34) However, due to discrete limitations (e.g., discrete time/steps) of charge quantization, the total actual charge amount Q.sub.Actual for an active time period (e.g., LED current pulse 901) may differ from what a total desired charge amount Q.sub.Desire is. Thus, the total desired charge amount Q.sub.Desire that is desired to be delivered to LEDs 102 is calculated as follows:
Q.sub.Desire=Q1+Q2+Q3+/Q.sub.errorEquation 7
The quantization error charge amount Q.sub.error may be a deficient charge amount or an excess charge amount depending on what the total desired charge amount Q.sub.Desire is relative to what the total actual charge amount Q.sub.Actual that can actually be delivered. If the quantization error charge amount Q.sub.error is a deficient charge amount, then the quantization error charge amount Q.sub.error is compensated by adding the equivalent charge amount in during a next or future time period (e.g., future LED current pulse) of LED current time period. For example, if the actual charge amount is 40 Coulombs, but 41 Coulombs is the desired charge amount Q.sub.Desire and cannot be achieved due to charge quantization limitations, then the quantization error charge amount Q.sub.error is a deficiency of 1 Coulomb (e.g., Q.sub.error=Q.sub.ActualQ.sub.Desire=40 Coulombs41 Coulombs=1 Coulomb). In this case, 1 Coulomb is added in during a next of future time period to compensate the actual charge amount Q.sub.Actual for the desired charge amount Q.sub.Desire. On the other hand, if the quantization error charge amount Q.sub.error is an excess charge amount, then the quantization error charge amount Q.sub.error is compensated by subtracting an equivalent charge amount from a next or future time period (e.g., future LED current pulse) of LED current time period. For example, if the actual charge amount is 40 Coulombs, but 39 Coulombs is the desired charge amount Q.sub.Desire and cannot be achieved due to charge quantization limitations, then the quantization error charge amount Q.sub.error is an excess amount of 1 Coulomb (e.g., Q.sub.error=Q.sub.ActualQ.sub.Desire=40 Coulombs39 Coulombs=+1 Coulomb). In this case, 1 Coulomb is subtracted from a next of future time period to compensate the actual charge amount Q.sub.Actual for the desired charge amount Q.sub.Desire.

(35) The process for modifying charge amounts delivered at a future time (e.g., modifying the charge amounts for future LED current pulses) as discussed for FIG. 9 can be appropriately repeated for subsequent LED current pulses. In the same manner for subsequent LED current pulses, charge amounts would be respectively compensated by adding to or subtracting from charge amounts of future LED current pulses depending upon whether the error charge amount is respectively a deficient or excess charge amount relative to the desired charge amount.

(36) The dimming controller 707 can be a pulse width modulation (PWM) dimming controller or can be a delta-sigma dimming controller. Referring now to FIG. 10, a time plot 1000 of LED current shows an exemplary train of LED current pulses (e.g., current pulses 1002, 1004, 1006, and 108) that are indicative of active time periods of an LED current i.sub.LED. The time plot 1000 also shows the inactive periods generally equally spaced apart between the LED current pulses since the dimming controller 707 is a pulse width modulated dimming controller. Error charge amounts that occur during an earlier active time period is compensated during a subsequent or future active time period. In other words, an error charge amount that occurs during current pulse 1002 is compensated during current pulse 1004, and an error charge amount occurring during current pulse 1004 is compensated during current pulse 1006. An error charge amount occurring during current pulse 1006 is compensated during current pulse 1008.

(37) Referring now to FIG. 11, a time plot 1100 of LED current shows an exemplary train of LED current pulses (e.g., current pulses 1102, 1104, 1106, 1108, 1110, 1112, 1114) that are indicative of active periods of an LED current i.sub.LED. The time plot 1100 also shows the inactive periods non-uniformly spaced apart between the LED current pulses since the dimming controller 707 is a delta-sigma modulated dimming controller. Again, error charge amounts that occur during an earlier active time period is compensated during a subsequent or future active time period. In other words, an error charge amount that occurs during current pulse 1102 is compensated during current pulse 1104, and an error charge amount occurring during current pulse 1104 is compensated during current pulse 1106 and so on and so forth. As shown in FIG. 11, the pulses 1102, 1104, 1106, 1108, 1110, 1112, 1114 may be the same or different in duration and may or may not be generally uniformly spaced part.

(38) The use of a delta-sigma modulated dimming controller 707 instead of a PWM dimming controller 707 for controlling the LED current i.sub.LED in FIG. 7 provides the characteristic of broadening the signal spectrum, which minimizes the potential for audible tones. A simple second order modulator, with reasonable dither level, may be implemented for the delta-sigma modulator, and such an implementation is generally sufficient and relatively inexpensive to implement. For example, the switching frequency f.sub.SW may be 200 kHz while the delta-sigma dimming frequency rate f.sub.DIM may be 20 kHz. In this case, a minimal chance for any audio noise production exists. However, a switching current control with rapid response, as it is turned on and off at a fast rate, is needed. Thus, a time-based dimming controller 707 and a time-based current controller 709 as discussed earlier for FIG. 7 provide such a fast switching response.

(39) Thus, the actual charge amount delivered to the LEDs 102 is calculated and accumulated. The charge accumulation is compared to the desired charge amount. Modification and compensation of the total charge amount delivered to the LEDs 102 can be continuously and constantly performed, which can at least compensate for error charge amounts. Regardless of the characteristics of the start-up and start-down of the LED controller 706, the LED lighting system 700 will properly compensate and allows for a much faster PWM switching rate f.sub.SW. Such a feature allows for smooth dimming of LEDs 102 by LED lighting system 700.

(40) Exemplary pseudo-code for PWM operation of dimming control 707 is provided as follows:

(41) Dim level D, 0-1

(42) Qint charge accumulation, initialized to 0

(43) PWM period PP

(44) Full-scale current Itarget

(45) Current control sample period PCC

(46) Instantaneous LED current LEDI

(47) At PP rate, Qint=Qint+D*Itarget

(48) At PCC,

(49) Qint=QintPCC*LEDI

(50) If Qint>0, turn on LED controller

(51) If Qint<=, turn off LED controller

(52) Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.