Abstract:
A laser diode driving device is disclosed. Sufficient driving current to a laser diode is maintained, even when the laser diode reaches high temperature. A laser diode light-emitting system utilizing the driving device is also disclosed. To reduce probability of the transistor entering the saturation region by parallel connecting a plurality of transistors and increasing stability of a laser diode driving circuit under high temperature.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a driving device and in particular to a driving device for compensating a laser diode and a light-emitting system utilizing the driving device.  
           [0003]    2. Description of the Related Art  
           [0004]    Normally, brightness of a light emitted from a laser diode decays while the temperature grows. Thus a driving device of the laser diode must provide sufficient amount of driving current for the laser diode to maintain a fixed level of brightness. However, under a higher temperature with the required current, transistors of the driving circuit cannot function in an active region, such that the driving circuit cannot provide sufficient driving current for the laser diode.  
           [0005]    [0005]FIG. 1 shows the circuit of a conventional driving device for a laser diode. The conventional driving circuit for a laser diode comprises a laser diode module  12 , a driving module  11 , a diode D, and a driving circuit  15 . The laser diode module  12  emits light according to a driving current Ic and outputs a brightness signal MD according to the brightness of the emitted light. The driving module  11  outputs a driving signal LDO according to the brightness signal MD. The driving circuit  15  outputs the driving current Ic for the laser diode module  12  according to the driving signal LDO.  
           [0006]    The laser diode module  12  comprises a laser diode  13 , a photo-detector  14 , and a load resistor RL. The photo-detector  14  generates the brightness signal MD according to brightness of the light emitted from the laser diode  13 .  
           [0007]    The driving circuit  15  comprises a PNP Bipolar Junction Transistor (hereafter as transistor) Q 1 , a current-limiting resistor R, and a capacitor C. The transistor Q 1  outputs the driving current Ic according to the driving signal LDO. The current-limiting resistor R helps to generate an emitter voltage V E . And the capacitor C is used to cancel noise.  
           [0008]    When the brightness of the light emitted from the laser diode  13  decays due to the increasing temperature, the brightness signal MD detected by the photo-detector  14  also decreases. The driving module  11  compares the brightness signal MD with a preset value. The driving signal LDO is decreased when the brightness signal MD is less than the preset value. And the driving current Ic increases when the transistor Q 1  is driven by a smaller driving signal LDO. Thus, a larger driving current is sent to the laser diode  13 .  
           [0009]    [0009]FIG. 2 a  shows characteristic curves illustrating the relation between driving current and temperature of a laser diode. As shown in the diagram, the laser diode requires more driving current as the temperature rises.  
           [0010]    [0010]FIG. 2 b  shows characteristic curves illustrating the relation between collector current and emitter-collector saturation voltages of the transistor. As shown in the diagram, the saturation voltage V EC(sat)  of the transistor Q 1  increases as the temperature rises.  
           [0011]    The emitter current I E  and the collector current I C  will increase when the driving signal LDO decreases. And the emitter voltage V E  is obtained by:  
           
         V 
         E 
         =V 
         P 
         −I 
         E 
         R;  
       
           [0012]    wherein V P  is voltage source, I E  is the emitter current, and R is resistance of the current-limiting resistor. The emitter voltage V E  decreases as the emitter current I E  increases. The collector voltage V C  increases and the emitter-collector voltage V EC  decreases as the collector current I C  increases.  
           [0013]    [0013]FIG. 3 a  is a schematic diagram of a transistor. FIG. 3b shows characteristic curves of the transistor. A is a saturation region, B is an active region, and C is a cut-off region. The transistor Q 1  is in the saturation region A when the saturation voltage V EC(sat)  exceeds the emitter-collector voltage V EC . The collector current I C  is not controlled by the base current I B  if the transistor Q 1  is in the saturation region A. And then the transistor Q 1  can not provide sufficient amount of the driving current for the laser diode  13  when the temperature rises.  
           [0014]    Therefore, when the temperature rises, prior arts fail to maintain the transistor working in the active region B and are unable to increase the driving current of the laser diode effectively.  
         SUMMARY OF THE INVENTION  
         [0015]    It is therefore an object of the present invention to provide a laser diode light-emitting system reducing probability of the transistor entering the saturation region by parallel connecting a plurality of transistors and increasing stability of a laser diode driving circuit under high temperature.  
           [0016]    According to the above mentioned object, the laser diode light-emitting system disclosed in the present invention comprises a laser diode module, a driving module, and a plurality of bipolar junction transistors. The laser diode module receives driving current, emits light, and outputs a brightness signal corresponding to the brightness of the light. The driving module changes a voltage level of driving signal according to voltage level of the brightness signal. The plurality of bipolar junction transistors are connected in parallel and coupled to a voltage source, providing the driving current to the laser diode module, wherein bases of the BJTs are coupled to the driving signal and wherein a value of the driving current is changed according to the voltage level of the driving signal.  
           [0017]    When the bipolar junction transistors are PNP-type, the collector outputs the driving current to the laser diode module, and the emitter is coupled to the voltage source. The driving signal is directly proportional to the brightness signal and inversely proportional to the driving current.  
           [0018]    When the bipolar junction transistors are NPN-type, the emitter outputs the driving current to the laser diode module, and the collector is coupled to the voltage source. The driving signal is inversely proportional to the brightness signal and directly proportional to the driving current.  
           [0019]    Furthermore, the present invention provides a laser diode driving device outputting a driving current to a laser diode module, wherein when receiving the driving current, the laser diode module emits light and outputs a brightness signal corresponding to the brightness of the light. The laser diode driving device comprises a plurality of bipolar junction transistors and a driving module. The bipolar junction transistors are connected in parallel and coupled to a voltage source, providing the driving current to the laser diode module. The driving module changes a voltage level of a driving signal according to a voltage level of the brightness signal.  
           [0020]    The present invention provides a laser diode driving circuit comprising a laser diode module, a driving module, and a plurality of current paths. The laser diode module receives a driving current to emit light and outputs a brightness signal corresponding to the brightness of the light. The driving module changes a voltage level of a driving signal according to a voltage level of the brightness signal. Each of the current paths is controlled by the driving signal, wherein a sum of currents on all current paths is the driving current, wherein the driving current is changed according to the voltage level of the driving signal, and wherein the current on each current path is in an active region. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    The present invention can be more fully understood by reading the subsequent detailed description and examples with reference made to the accompanying drawings, wherein:  
         [0022]    [0022]FIG. 1 shows a circuit of a conventional laser diode driving device;  
         [0023]    [0023]FIG. 2 a  shows driving current to temperature characteristic curves of the laser diode;  
         [0024]    [0024]FIG. 2 b  shows comparing I C  to V EC(sat)  characteristic curves of the transistor;  
         [0025]    [0025]FIG. 3 a  is a schematic diagram of the transistor;  
         [0026]    [0026]FIG. 3 b  shows V EC  to I C  characteristic curves of the transistor;  
         [0027]    [0027]FIG. 4 is a circuit diagram of an embodiment according to the present invention;  
         [0028]    [0028]FIG. 5 is a schematic diagram showing the working point of the transistor. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    [0029]FIG. 4 is a circuit diagram of the embodiment according to the present invention. For the sake of illustrating the present invention concisely, parts that are similar or identical to those with regard to the prior art or conventional art are identified with the same reference numerals, and their explanations are omitted. As shown in the diagram, a laser diode light-emitting system comprises a laser diode module  12 , a driving module  11 , a current-limiting resistor R, a capacitor C, a diode D, and a plurality of current paths. The laser diode module  12  receives a driving current I C  to emit light and output a brightness signal MD corresponding to the brightness of the light. The driving module  11  changes a voltage level of a driving signal LDO according to a voltage level of the brightness signal MD. Each current path is controlled by the driving signal LDO. And a sum of the currents on all paths is the driving current, wherein the driving current is changed according to the voltage level of the driving signal LDO.  
         [0030]    Each current path comprises a NPN-type bipolar junction transistor or a PNP-type bipolar junction transistor. In this embodiment, there are two PNP-type bipolar junction transistors Q 1 , Q 2 , connected in parallel and coupled to a voltage source V P . These two transistors is used or providing the driving current I C  to the laser diode module  12 . Bases of the transistors Q 1 , Q 2  are coupled to the driving signal LDO, wherein a value of the driving current I C  is changed according to the voltage level of the driving signal LDO.  
         [0031]    The current-limiting resistor R is coupled between the voltage source V P  and the emitters of the transistors Q 1 , Q 2 . The capacitor C is coupled between the voltage source V P  and the bases of the transistors Q 1 , Q 2  for noise cancellation. The diode D is used to rectify the direction of current.  
         [0032]    The laser diode module  12  comprises a laser diode  13 , a photo-detector  14 , and a load resistor RL. The photo-detector  14  detects the brightness of the light emitted from the laser diode  12 . The load resistor RL transforms the detected brightness into a brightness signal MD.  
         [0033]    In FIG. 4, the brightness of the light emitted from the laser diode  13  decays while the temperature grows. Therefore, the brightness signal MD detected by the photo-detector  14  is also decreased. The driving module  11  decreases the voltage of the driving signal LDO in order to increase the base current I B , such that the driving current I C  is increased correspondingly. In the embodiment, the functions of the parallel connected transistors Q 1  and Q 2  are the same.  
         [0034]    Accordingly, a total amount of the base current I B  is the sum of all the base currents of the transistors Q 1  and Q 2  (I B =I B1 +I B2 ). A total amount of the collector current I C  is the sum of the collector currents of the transistors Q 1 , Q 2  (I C =I C1 +I C2 ). And a total amount of the emitter current I E  is the sum of the emitter currents of the transistors Q 1 , Q 2  (I E =I E1 +I E2 ). Since the transistors Q 1 , Q 2  are working under the same circumstance and have the same function and working point, current through each terminal of the transistors Q 1  equals that of the transistors Q 2 .  
         [0035]    Because the amount of total emitter current I E  equals (1+h FE )×I B , the amount of total emitter current I E  and the amount of total collector current I C  are increased when the base current I B  increases. Furthermore, because the transistors Q 1 , Q 2  are connected in parallel, the amount of total emitter current I E  is evenly distributed between the emitter current of the transistors Q 1 , Q 2 . Thus, even the amount of total emitter current I E  suddenly increases, the emitter currents I E1 , I E2  remain relatively stable.  
         [0036]    Also the current at the collector of each transistor Q 1 , Q 2  does not change rapidly. Thus the voltage between the emitter and the collector of each transistor remains stable and exceeding a saturation voltage V EC(sat) . Therefore, each of the transistors is kept in an active region, whereby the driving current of the laser diode  13  can be under control.  
         [0037]    The saturation voltage V EC(sat)  may increase as the temperature or the collector current increases. The present invention eases the increasing of the saturation voltage V EC(sat) .  
         [0038]    Moreover, when NPN-type bipolar junction transistors are used for the current paths, the emitters output the driving current to the laser diode module  12 , the collectors are coupled to the voltage source V P , and the bases are coupled to the driving signal. The driving signal is inversely proportional to the brightness signal and directly proportional to the driving current.  
         [0039]    [0039]FIG. 5 is a collector currents to emitter-collector voltages schematic diagram showing the working point of the transistors. A is a saturation region and B is an active region. Point W 1  is the working point of the transistor in the prior art when higher collector current is needed. Utilizing the present invention decreases the collector current of each transistor and increases the voltage between emitter and collector of the transistor. Thus the working point is moved to the active region B as Point W 2  in the diagram.  
         [0040]    The present invention reduces the probability of the transistor entering the saturation region. By parallel connecting transistors, collector current for each transistor is decreased such that the voltage between the emitter and collector of the transistor is less than saturation voltage V EC(sat) . Additionally, the present invention changes the working point of the transistor from the saturation region to the active region.  
         [0041]    While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.