Abstract:
By monitoring the temperature of an LED and the temperature of a controller chip for the LED, a thermal foldback control is employed to gradually decrease the driving current of the LED before the LED or the controller chip is over heated, so as to prevent an over temperature protection from being triggered to shutdown the controller chip to cause the LED to undesirably flicker.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is related generally to a light-emitting diode (LED) driver and, more particularly, to a thermal foldback control circuit and method for an LED. 
       BACKGROUND OF THE INVENTION 
       [0002]    The controller chip for LED drivers is typically equipped with over temperature protection (OTP) which will shutdown the controller chip once the temperature of the controller chip increases over a setting value, to prevent the controller chip from being burnt out. The thermal foldback control is a protection mechanism for an LED, which will decrease the driving current of the LED when the temperature of the controller chip increases over a setting value, to avoid the temperature of the controller chip still increasing. The existing design of LED lamps concentrates on the security and heat dissipation of the LEDs, and thus puts the LEDs in a space with good air convection to reduce the surface temperature of the LEDs, while puts the driver circuit in another closed space. Therefore, the controller chip in the driver circuit is hard to dissipate its heat and sometimes becomes hotter than the LEDs, which will trigger the OTP mechanism to push the controller chip into a thermal protection mode, thereby turning off the driving current of the LEDs and causing the LEDs to undesirably flicker. 
       SUMMARY OF THE INVENTION 
       [0003]    An object of the present invention is to provide a thermal foldback control circuit and method that will gradually adjust the driving current of an LED depending on temperature variation. 
         [0004]    Another object of the present invention is to provide a simple circuit and method that simultaneously monitor the temperature variations of an LED and its controller chip to adjust the driving current of the LED, thereby automatically regulating the temperatures of the LED and the controller chip. 
         [0005]    According to the present invention, a thermal foldback control circuit and method for an LED monitors the temperature of the LED and the temperature of the controller chip of the LED simultaneously, and find out the higher one thereof to determine the level for thermal balance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    These and other objects, 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: 
           [0007]      FIG. 1  is a circuit diagram of an embodiment according to the present invention; 
           [0008]      FIG. 2  is a circuit diagram of a first embodiment for the thermal programming circuit shown in  FIG. 1 ; 
           [0009]      FIG. 3  is a circuit diagram of a second embodiment for the thermal programming circuit shown in  FIG. 1 ; and 
           [0010]      FIG. 4  is a circuit diagram of an embodiment using multiple threshold signals. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]      FIG. 1  is a circuit diagram of an embodiment according to the present invention, in which an LED array  10  is coupled with a power source Vcc through a resistor R 1 , and a controller chip  12  is coupled to the LED array  10  to control the driving current lied thereof. The controller chip  12  includes a bipolar junction transistor (BJT)  14  serially coupled to the LED array  10  to provide the collector current Ic as the driving current lied, and a driver  16  coupled to the base of the BJT  14  to control the collector current Ic. Similar to prior arts, the controller chip  12  has a dimming pin DIM coupled to the driver  16  to receive a dimming input for controlling the illumination of the LED array  10 . The controller chip  12  further includes a thermal foldback control circuit  18  that provides a thermal regulation signal for the driver  16  to adjust the collector current Ic. A negative temperature coefficient resistor NTCR detects the temperature of the LED array  10  to generate an external thermal detection signal V TO  for the controller chip  12 . When the detected temperature increases, the resistance of the negative temperature coefficient resistor NTCR decreases, thereby decreasing the external thermal detection signal V TO . The thermal foldback control circuit  18  has an external thermal detection input  20  for receiving the external thermal detection signal V TO , a thermal programming circuit  22  for detecting the temperature of the controller chip  12  itself to generate an internal thermal detection signal V TI , and a multi-sense circuit  24  triggers the thermal regulation signal according to the external thermal detection signal V TO  and the internal thermal detection signal V TI . In the multi-sense circuit  24 , a multi-input comparator  26  has two positive inputs for receiving the external thermal detection signal V TO  and the internal thermal detection signal V TI , respectively, to be compared with a threshold signal V TH  received from a negative input to generate a comparison signal for a voltage generator  28  to generate an overheat detection voltage Vt, and a hysteresis comparator  30  compares the overheat detection voltage Vt with a feedback voltage Vf fed back from the LED array  10  to trigger the thermal regulation signal. Due to the driving current Iled flowing through the resistor R 1 , the feedback voltage 
         [0000]        Vf=Vcc−Iled×R 1,  [Eq-1]
 
         [0000]    which is a function of the driving current Iled. The threshold signal V TH  of the multi-input comparator  26  is provided by a temperature independent voltage source  32 . The voltage generator  28  includes a voltage-controlled current source  34  to determine an overheat detection current It according to the output of the multi-input comparator  26 , and a resistor R 2  coupled between the power source Vcc and the positive input of the hysteresis comparator  30  such that the overheat detection current It flows through the resistor R 2  to generate a voltage drop to produce the overheat detection voltage 
         [0000]        Vt=Vcc−It×R 2,   [Eq-2]
 
         [0000]    which is a function of the overheat detection current It. In one embodiment, the overheat detection current It has an initial value of zero. When the temperature of the external LED array  10  is over threshold or the internal temperature of the controller chip  12  is over threshold, the comparison signal generated by the multi-input comparator  26  will make the current source  34  to increase the overheat detection current It .and thereby decrease the overheat detection voltage Vt. As a result, the hysteresis comparator  30  will trigger the thermal regulation signal to make the driver  16  to reduce the collector current Ic, while the LED array  10  will remain operation. Each time the thermal regulation signal is triggered, the driver  16  will reduce the collector current Ic by a predetermined difference. Unless the controller chip  12  reaches the upper limit for OTP, the system won&#39;t trigger the OTIC mechanism to shutdown the controller chip  12 . Therefore, either the LED array  10  or the controller chip  12  has an over temperature, the collector current Ic will decrease without stopping the LED array  10  working. In virtue of the multi-input comparator  26 , it can find. out which. one of the temperature of the external. LED array  10  and the internal temperature of the controller chip  12  is higher, and then determine the level for temperature balance. Such a detection scheme is more flexible and allows users with more setting capacity to control the temperature protection of the LED lamps. 
         [0012]      FIG. 2  is a circuit diagram of a first embodiment for the thermal programmingcircuit  22  which employs a simple BJT circuit to generate the internal thermal detection signal V TI . A BJT  36  has a base-emitter voltage V BE  of about 0.7V at room temperature, and as the temperature increases, V BE  goes down and thus increases the collector current Ic, thereby decreasing the internal thermal detection signal V TI . The internal thermal detection signal V TI  can have its initial value set by adjusting a setting resistor Rset. The higher the resistance of the setting resistor Rset is, the lower the initial value of the internal thermal detection signal V TI  is, which in turn means the lower the threshold temperature of the controller chip  12  itself is. As shown in  FIG. 3 , by additionally providing a capacitor C at the output of the thermal programming circuit  22 , the stability of the internal thermal detection signal V TI  can be improved. For downsizing the controller chip  12 , in the embodiment of  FIG. 3 , the setting resistor Rset of the thermal programming circuit  10  is arranged outside the controller chip  12 . However, in other embodiments, the setting resistor Rset may be integrated into the controller chip  12 . 
         [0013]    The thermal foldback control circuit  18  of the present invention is advantageous because it can automatically find out the higher one of the temperature of the external LED array  10  and the internal temperature of the controller chip  12  for the adjustment of the driving current Iled in a manner that the driving current Iled is gradually decreased along the elevated temperature of either the controller chip  12  or the LED array  10 , so as to mitigate the increasing temperature and thus regulate the temperatures of the controller chip  12  and the LED array  10 , thereby reducing the risk of stopping the LED array  10  working caused by the overheated controller chip  12 . 
         [0014]    In the embodiment of  FIG. 1 , only a threshold signal VTH serves for temperature monitoring in the controller chip  12  and of the external LED array  10 . However, in another embodiment, as shown in  FIG. 4 , the LED array  10  and the controller chip  12  may be monitored by two separate threshold signals VTH 1  and VTH 2 , which are compared by using two comparators  38  and  40 . In this case, an OR gate  42  generates the comparison signal for controlling the voltage generator  28  according to the comparison results of the comparators  38  and  40 . 
         [0015]    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.