Abstract:
A dimming circuit and method for a LED provide a first driving voltage or a second driving voltage according to a dimming signal provided by a functional IC to enable or disable the LED. The values of the first and second driving voltages are controlled so that overstressing of the LED is avoided while the functional IC is capable of working even when the LED is off. The LED&#39;s life time is thus prolonged.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional of co-pending application Ser. No. 13/081,131, filed on Apr. 6, 2011, for which priority is claimed under 35 U.S.C. §120; and this application claims priority of Application No. 201010146432.5 filed in P.R. China on Apr. 14, 2010 under 35 U.S.C. §119, the entire contents of all of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention is related generally to a dimming circuit and method and, more particularly, to a dimming circuit and method for LEDs. 
     BACKGROUND OF THE INVENTION 
     In LED dimming systems, conventionally the LED is turned on and off between ground and its forward voltage to fulfill dimming function. The abrupt change of voltage may arise of the danger of overstressing the LED and other peripheral components. For a system whose power is LED&#39;s output, it will temporarily shutdown during the LED&#39;s off period. This causes limits when designing such circuits. In further detail, as shown in  FIG. 1 , a conventional LED dimming circuit includes a boost integrated circuit (IC)  10  to boost a battery voltage Vbat into a driving voltage Vo for a LED and a functional IC  12  connected to the anode of the LED for dimming control. Dimming is realized through a switch M serially connected to the LED, for which the functional IC  12  provides a dimming signal Dpwm to switch the switch M in order to adjust the average current Iled of the LED, thereby achieving dimming control for such as bright, dim and flashing. Circuits and operations for the boost IC  10  and the functional IC  12  have been mature and need not to be discussed in detail herein. When the functional IC  12  turns off the switch M to cut off the current Iled, since no path to ground exists, the output VOUT of the boost IC  10  will endure a very high voltage due to the continuously charged capacitor Cout connected at the output VOUT, and thereby push the boost IC  10  into its over voltage protection mode. When the functional IC  12  turns on the switch M again, the charge stored in the capacitor Cout will rush into the LED, and the LED will endure a large voltage before the output voltage Vo drops to the LED&#39;s normal forward voltage again. In this way, although the functional IC  12  can work when the LED is off, the boost IC  10 , the functional IC  12  and the LED are overstressed by a very high voltage and this causes quality concerns. For those functional ICs sensitive to power, this method may even cause errors during dimming period. 
       FIG. 2  shows another possible solution for a battery powered LED flashlight dimming system, in which the functional IC  12  is powered separately, e.g. by another battery Vbat 2 . When the LED is on, the functional IC  12  enables the boost IC  10  to boost the battery voltage Vbat 1  into a driving voltage Vo equal to the normal forward voltage of the LED. When the LED is off, the functional IC  12  disables the boost IC  10 , and thus the driving voltage Vo will not increase to the extent that the boost IC  12  enters its over voltage protection mode. By doing this, not entering the over voltage protection mode makes the whole system safer and prolongs the utility time of the LED. However, this approach also has two drawbacks. (1) For low battery power, e.g. 0.9V, most functional ICs are unable to work under such low supply voltage. This limits the application of the solution. (2) The LED is dimmed between the normal forward voltage Vf and a ‘low’ voltage (i.e. Vbat 1 −VD). The voltage drop during dimming is not minimized and the LED is still overstressed by some unnecessary abrupt voltage change. For example, assuming that Vbat 1 =1.5V, VD=0.7V and Vf=3.6V, the LED will be overstressed by an abrupt voltage change ΔV=Vf−(Vbat 1 −VD)=3.6V−(1.5V−0.7V)−2.8V when it is switched from on to off, or from off to on. This abrupt voltage change ΔV increases with the decrease of the battery voltage Vbat 1 . The abrupt voltage change will shorten the LED&#39;s life time. 
     Therefore, it is desired a dimming circuit and method for LEDs that prolongs the LED&#39;s life time while maintains a certain low voltage when the LED is off to support other functional circuits. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide a dimming circuit and method for LEDs. 
     Another objective of the present invention is to provide a dimming circuit and method that prevent LEDs from large abrupt voltage change when being dimming. 
     According to the present invention, a dimming circuit and method for a LED select a first driving voltage setting signal or a second driving voltage setting signal according to a dimming signal provided by a functional IC, to determine the output voltage supplied to the LED being a first driving voltage or a second driving voltage. The output voltage is also supplied to the functional IC, and each of the first driving voltage and the second driving voltage is as large as enough to drive the functional IC. 
     By controlling the values of a first driving voltage and a second driving voltage to turn on and off a LED, overstressing of the LED is avoided while the functional IC is capable of working even when the LED is off. The LED&#39;s life time is thus prolonged. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objectives, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a circuit diagram of a conventional battery powered LED flashlight dimming system; 
         FIG. 2  is a circuit diagram of another conventional battery powered LED flashlight dimming system; 
         FIG. 3  is a first embodiment according to the present invention; 
         FIG. 4  is an embodiment for the selector, the voltage setting circuit and the power source shown in  FIG. 3 ; 
         FIG. 5  is a second embodiment according to the present invention; and 
         FIG. 6  is an embodiment for the selector, the voltage setting circuit and the power source shown in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the present invention, the dimming circuit and method are directed to control the driving voltages of enabling and disabling a LED, so as to prevent the LED switched between dark and light from large abrupt voltage change, for example, from ground to the LED&#39;s forward voltage. The term “disable” refers to a state where a LED is not bright in human eyes. Taking a LED having a forward voltage of 3.6V and power of 3 W for example, when the applied voltage is 2.5V, the current Iled of the LED is completely cut off, so 2.5V can be set as the driving voltage of disabling the LED, and 3.6V is the driving voltage of enabling the LED. In addition, the driving voltage supplied to the LED can be also supplied to a functional IC and other circuits. Since the driving voltage still has a value as high as 2.5V when the LED is disabled, the functional IC and other circuits can normally work even when the LED is dark. 
     As shown in  FIG. 3 , a LED dimming system according to the present invention includes a functional IC  12  to provide a dimming signal Dpwm, a voltage setting circuit  22  to provide two driving voltage setting signals EA 1  and EA 2 , a selector  20  to select one of the driving voltage setting signals EA 1  and EA 2  according to the dimming signal Dpwm for a power source  24  to determine its output voltage for the LED and the functional IC  12  is the driving voltage Vo 1  or Vo 2 . Each of the driving voltages Vo 1  and Vo 2  is as large as enough to drive the functional IC  12 . When the dimming signal Dpwm is high, the selector  20  sends out the driving voltage setting signal EA 1 , and the output voltage of the power source  24  is the driving voltage Vo 1  which enables the LED. When the dimming signal Dpwm is low, the selector  20  sends out the driving voltage setting signal EA 2 , and the output voltage of the power source  24  is the driving voltage Vo 2  which disables the LED. The driving voltage Vo 2  may be set by an off voltage setting signal Sset provided to the voltage setting circuit  22 , and thus the disable voltage Vo 2  of the LED is preset externally or in the system. Instead of abrupt voltage change between ground and the LED&#39;s forward voltage, the LED is turned on and off between a certain pre-programmed low voltage and its forward voltage. For example, for a Vf=3.6V, P−3 W LED, its current Iled is totally off when 2.5V is applied thereto. Thus the LED can be dimmed through Vo 2 =2.5V as an off voltage and Vo 1 =3.6V as an on voltage, with a voltage change ΔV=Vo 1 −Vo 2 =3.6V−2.5V=1.1V. In this manner, overstressing of the LED is avoided and other circuits whose power is the LED&#39;s output is able to work even during the LED&#39;s off period when dimming the LED. The power source  24  may be any circuit which can supply power to illuminate the LED, for example a buck, boost, linear driver etc. Moreover, the power source  24  is not necessarily connected to the anode of the LED, and may be connected to the cathode of the LED. 
       FIG. 4  is an embodiment for the selector  20 , the voltage setting circuit  22  and the power source  24  shown in  FIG. 3 . In this embodiment, the power source  24  is an asynchronous boost power supply, which includes a pulse width modulation (PWM) comparator  28  to compare a ramp signal Sramp with from the output of the selector  20  to generate a PWM signal Spwm, a flip-flop  26  to switch a transistor M according to the PWM signal Spwm and a clock CLK so as to generate the driving voltage Vo 1  or Vo 2 . The voltage setting circuit  22  includes an error amplifier  30  to amplify the difference between the driving voltage Vo 1  or Vo 2  and a reference voltage Vref 2  so as to generate the driving voltage setting signal EA 2 , where the reference voltage Vref 2  may be adjusted by the off voltage setting signal Sset, a current sense resistor Rfb serially connected to the LED to detect the current Iled of the LED so as to generate a feedback signal Vfb, and an error amplifier  32  to amplify the difference between the feedback signal Vfb and a reference voltage Vref 1  so as to generate the driving voltage setting signal EA 1 . The selector  20  includes a switch SW 1  controlled by the dimming signal Dpwm. When the dimming signal Dpwm is high, the switch SW 1  transmits the driving voltage setting signal EA 1  to the PWM comparator  28 , so that the power source  24  regulates its output voltage at Vo 1  such that Vfb=Vref 1 , and the current Iled is regulated at Vref 1 /Rfb. When the dimming signal Dpwm is low, the switch SW 1  transmits the driving voltage setting signal EA 2  to the PWM comparator  28 , so that the power source  24  regulates its output voltage at the preset low voltage Vo 2 =Vref 2 . 
       FIG. 5  is an embodiment of an automatic off voltage detect system according to the present invention, which has two phases, phase  1  is only lasted for a short time after the system starts, and after phase  1  is finished, the system moves to phase  2 . In addition to the functional IC  12 , the selector  20  and the power source  24  as that of  FIG. 3 , this embodiment further includes a current clamping circuit  40  and an automatic voltage detector  42 . In phase  1 , under control of the current clamping circuit  40 , the power source  24  supplies the LED with its predefined off current, e.g. less than 100 uA, and the automatic voltage detector  42  detects and records the forward voltage of the LED to determine a driving voltage setting signal Vp. Phase  2  is normal operation, in which the power source  24  supplies the LED with its normal operation current or voltage. Upon the PWM dimming signal Dpwm, the LED dimming circuit turns on and off the LED between the pre-detected forward voltage Vo 2  and its normally operation forward voltage Vo 1 . In phase  2 , the automatic voltage detector  42  does not detect the forward voltage of the LED anymore, and the selector  20  selects one of the driving voltage setting signals Vref and Vp according to the dimming signal Dpwm, for the power source  24  to provide the driving voltage Vo 1  or Vo 2  for the LED and the functional IC  12 . Each of the driving voltages Vo 1  and Vo 2  is as large as enough to drive the functional IC  12 . 
       FIG. 6  is an embodiment for the selector  20 , the power source  24  and the automatic voltage detector  42  shown in  FIG. 5 . In this embodiment, the power source  24  is a linear voltage regulator that includes an error amplifier  44 , a transistor M, a current source Is and switches SW 3  and SW 4 . The error amplifier  44  controls the transistor M according to the difference between its two inputs, to regulate the current to of the transistor M. The switch SW 3  is connected between the transistor M and the LED, and controlled by a signal φ 2  coming from the current clamping circuit  40 . The switch SW 4  is connected between the current source Is and the LED, and controlled by a signal φ 1  coming from the current clamping circuit  40 . The automatic voltage detector  42  includes a sample-and-hold circuit established by a capacitor Cs and a switch SW 2 . The switch SW 2  is controlled by the signal φ 1 . The selector  20  includes a switch SW 1  controlled by the dimming signal Dpwm to transmit either the recorded voltage Vp or the reference voltage Vref as the driving voltage setting signal to the error amplifier  44 . In phase  1 , the signal φ 1  turns on the switches SW 2  and SW 4 , and the signal φ 2  turns off the switch SW 3 , so that the current source Is supplies a small current, e.g. 10 μA, to the LED, and the LED generates a voltage being recorded in the capacitor Cs as the voltage Vp. In phase  2 , the signal φ 1  turns off the switches SW 2  and SW 4 , and the signal φ 2  turns on the switch SW 3 , so that the current source Is stops supplying the small current to the LED, and the automatic voltage detector  42  stops sampling the voltage of the LED. Upon the dimming signal Dpwm, the switch SW 1  is switched to transmit the driving voltage setting signal Vref or Vp to the error amplifier  44  that regulates the current Io according to the difference between the voltage of the LED and the driving voltage setting signal Vref or Vp, so that the output voltage of the power source  24  supplied to the LED is switched between the driving voltage Vo 1  and Vo 2 . 
     While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.