Abstract:
The present invention discloses a photo driver circuit with power saving function and a corresponding method thereof. The photo driver circuit with power saving function comprises: a photo sensor device for sensing light and generating a corresponding photo current; a current amplifier amplifying the photo current; an output control circuit for generating a driver output signal according to the output of the current amplifier; and a power saving control circuit for activating a power saving mode in which the power saving control circuit outputs one or more control signals to shut down one or both of the current amplifier and the output control circuit.

Description:
BACKGROUND 
   1. Field of Invention 
   The present invention relates to a photo driver circuit with power saving function, in particular to such a circuit applied to applications in which power consumption is a concern. The present invention also relates to a photo driving method. 
   2. Description of Related Art 
   A photo resistor senses the intensity of ambient light and converts it into an electronic signal. Photo resistors are widely used in applications such as night lamps and automobile headlights whose operation is controlled according to the intensity of ambient light. However, photo resistors are not the most satisfactory choice in such electric products because they contain toxic material (metal sulfide), and their response to light is relatively slow. 
   On the other hand, CMOS photo sensor devices (including photo diodes, photo transistors and photo ICs) are also capable of sensing the intensity of ambient light, while they are non-toxic. 
   Regardless whether photo resistors or CMOS photo sensor devices are used, they generate larger current when the intensity of ambient light is high; thus a troublesome problem occurs when such devices are applied to illumination control. Taking the night lamp as an example, when the intensity of ambient light is high and the night lamp does not need to function, the circuit consumes large power due to high current generated in the circuit. In this regard, power-saving function is very much desired, in particular when the product is operated by the power of a battery, or in an application wherein power consumption is a concern. 
   In view of the foregoing, it is desirous, and thus an objective of the present invention, to provide a photo driver circuit with power saving function which can be applied to e.g., Christmas light strings, bicycle headlight, etc. 
   SUMMARY 
   An objective of the present invention is to provide a photo driver circuit with power saving function, to solve the problem in the prior art. 
   Another objective of the present invention is to provide a photo driving method. 
   In accordance with the foregoing and other objectives, the present invention provides a photo driver circuit with power saving function which comprises: a photo sensor device for sensing light and generating a corresponding photo current; a current amplifier amplifying the photo current; an output control circuit for generating a driver output signal according to the output of the current amplifier; and a power saving control circuit for activating a power saving mode in which the power saving control circuit outputs one or more control signals to shut down one or both of the current amplifier and the output control circuit. 
   From another aspect, the present invention provides a photo driving method which comprises the steps of: sensing light and generating a corresponding photo current; generating an amplified current according to the photo current; generating a driver output signal according to the amplified current; and activating a power saving mode in which the generation of the amplified current is stopped when the intensity of the sensed light is larger than a predetermined threshold. 
   Preferably, in the above photo driver circuit or photo driving method, a signal required for generating the driver output signal is stored in the power saving mode. The signal for example may be a digital signal stored in a digital latch circuit, or an analog signal stored in a sample-and-hold circuit. 
   Preferably, in the above photo driver circuit or photo driving method, the sensitivity of the driver output signal to the photo current is adjustable. 
   Preferably, in the above photo driver circuit or photo driving method, the activation point of the power saving mode, i.e., at what condition the power saving mode is activated, is adjustable. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description, appended claims, and accompanying drawings. 
       FIG. 1  is a schematic circuit diagram showing a first embodiment of the present invention. 
       FIG. 2  shows a preferred embodiment of the output control circuit  16 . 
       FIG. 3  shows a preferred embodiment of the power saving control circuit  18 . 
       FIG. 4  is a schematic circuit diagram showing another embodiment of the present invention. 
       FIG. 5  is a schematic circuit diagram showing yet another embodiment of the present invention. 
       FIG. 6  shows another preferred embodiment of the power saving control circuit  18  which may be used in the embodiment of  FIG. 5 . 
       FIG. 7  shows another preferred embodiment of the power saving control circuit  18  which may be adjusted in digital manner. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a schematic circuit diagram showing a first embodiment of the present invention. As shown in the figure, in this embodiment, the photo driver circuit  10  with power saving function includes a photo sensor device  12  (shown as a photo diode in the figure, but can be any other photo sensor device such as a photo transistor or a photo IC), a current amplifier  14 , an output control circuit  16 , and a power saving control circuit  18 . The photo sensor device  12  senses light and generates photo current; the current amplifier  14  amplifies this photo current and transmits it to the output control circuit  16 . In one embodiment, the output control circuit  16  generates an output driver signal OUT according to the output from the current amplifier  14 ; this output driver signal OUT for example may be used to drive a next-stage circuit (not shown) for automatic illumination control. Alternatively, the signal OUT may be other types of signals, such as a multi-bit data signal. 
   One feature of the present invention is the power saving control circuit  18  provided in the photo driver circuit  10 . The power saving control circuit  18  issues control signals  181  and/or  182  to enable or disable the current amplifier  14  and/or the output control circuit  16 ; when one or both of the current amplifier  14  and the output control circuit  16  are disabled, the photo driver circuit  10  enters the “power saving mode”. To “disable” the current amplifier  14  or the output control circuit  16  means to stop the operation of part of the amplifier circuit or the control circuit for power saving purpose; it does not mean that the whole amplifier circuit or the whole control circuit is shut down. For example, the current amplifier  14  may include multiple stages of amplifier circuits, and in this case to “disable” the current amplifier  14  can be to shut down the stages of amplifier circuits which are relatively more power-consuming, but still keep at least one stage of amplifier circuit for sensing the ambient light. 
   More specifically, supposing the circuit shown in the figure is for illumination control, and its next stage is an illumination device (such as an LED), in the case where the power saving control circuit  18  is not provided, when the intensity of ambient light is high, the photo sensor device  12  generates large current, and the current amplifier  14  amplifies this current even larger. Thus, the output control circuit  16  generates a low-level output driver signal OUT according to the output from the current amplifier  14 . On the other hand, when the intensity of ambient light is low, the photo sensor device  12  generates low or no current, and the output control circuit  16  generates a high-level output driver signal OUT according to the output from the current amplifier  14 , to activate the illumination. The output driver signal OUT can be an analog signal or a digital signal, depending on the design of the next stage circuit. 
   In the operation described above, when the intensity of ambient light is high and no illumination is required, the current amplifier  14  and the output control circuit  16  consume considerably large current, and therefore should be improved. 
   According to the present invention, a power saving control circuit  18  is provided which can disable the current amplifier  14  and/or the output control circuit  16  as required when a preset condition is met, to enter the power saving mode. The “preset condition” is also referred to as the “activation point” of the power saving mode, which for example may be a preset threshold of the intensity of light sensed by the photo sensor device  12 . There are various ways to embody the power saving control circuit  18  so that it can determine whether a preset condition is met; in one embodiment, the determination can be based on the signal  141  (which is a current signal, for example) generated by the current amplifier  14  and/or the signal  161  (which is a voltage signal, for example) generated by the output control circuit  16 . When the signal  141  meets a preset current condition, or when the signal  161  meets a preset voltage condition, or both, the power saving control circuit  18  issues a control signal, so that the photo driver circuit  10  enters the power saving mode. 
   When the photo driver circuit  10  enters the power saving mode, it can be arranged so that no current flows in the output control circuit  16 , and therefore the output driver signal OUT is naturally at low level, to turn off the illumination. However in some applications, the output driver signal OUT may need to be at high level when the photo driver circuit  10  enters the power saving mode; in this case, a latch circuit may be provided in the output control circuit  16 , to ensure that the circuit at the next stage receives a correct output driver signal in the power saving mode. FIG.  2  shows an example of such output control circuit  16 . As shown in the figure, the output control circuit  16  in this embodiment includes an N-bit level detector  162  (wherein N is an integer), a latch circuit  164  and an output stage driver circuit  166 . The N-bit level detector  162  receives the analog output signal from the current amplifier  14  and converts it into an N-bit digital signal; the converted digital signal is stored in the latch circuit  164 . The N-bit level detector  162  may simply be a comparator if N=1. The latch circuit  164  includes an enable switch EN; the content in the latch circuit  164  can be changed only when the switch EN is enabled by the control signal  181 , otherwise the content is latched. The output stage driver circuit  166  generates the output driver signal OUT according to the content in the latch circuit  164 . Thus, in the power saving mode, because the current amplifier  14  stops functioning and the output therefrom is no longer valid, the N-bit level detector  162  can be shut down, while the content in the latch circuit  164  is still correct and is capable of providing a correct output driver signal to the circuit at the next stage to the photo driver circuit  10 . 
   As stated above, in one embodiment, the power saving control circuit  18  determines whether to enter the power saving mode according to the voltage signal  161  from the output control circuit  16 . As an example, the signal  161  may be generated from the location as shown in the figure, i.e., the output node of the latch circuit  164 . In this arrangement, the signal  161  is digital and may facilitate the judgment to be made by the power saving control circuit  18 . However, depending on the circuit design of the output control circuit  16  and the power saving control circuit  18 , the signal  161  is not limited to be a digital signal, and is not limited to be taken from the location shown in the figure. If the signal  161  is analog, it can be compared with a preset reference voltage to achieve the same function required. 
     FIG. 3  shows an embodiment of the power saving control circuit  18 . In this embodiment, the signal  141  is an analog current signal which for example may be taken from the current amplifier  14  as shown in  FIG. 1 , or directly from the output of the photo sensor device  12 . The signal  161  is a single-bit digital signal which for example may be taken from the location as shown in  FIG. 2 . If the latch circuit  164  in  FIG. 2  outputs a multi-bit signal, the single-bit digital signal  161  may be one of the digits of the multi-bit signal, or a logic operation may be performed on the digits to generate the signal  161 . As shown in the figure, a current comparator ICP  184  compares the signal  141  with a predetermined reference current Iref. When the signal  141  is larger than the predetermined reference current Iref, it means that the intensity of the ambient light is higher than a predetermined brightness threshold, and the comparator  184  outputs a high level signal. And, when the signal  161  is high, it means that the output from the N-bit level detector  162  is equal to or higher than a predetermined voltage threshold (for example, in the case N=1, the output from the N-bit level detector  162  is a high level signal “1”). When both inputs of a logic circuit  186  is high, it outputs low level control signals  181  and  182 , to disable the current amplifier  14  and the output control circuit  16 . When one of the inputs of a logic circuit  186  is low, it outputs high level control signals  181  and  182 , to enable the current amplifier  14  and the output control circuit  16 . 
   The power saving control circuit  18  in the above embodiment determines whether to enter the power saving mode according to both the signals  141  and  161 , and sends out both control signals  181  and  182  to concurrently enable or disable the current amplifier  14  and the output control circuit  16 . In another embodiment, the determination can be based on either one of the signals  141  and  161 . In yet another embodiment, only one of the control signals  181  and  182  is issued. Furthermore, it can be arranged so that the control signals  181  and  182  are independently issued according to different determination basis. The embodiment to base the determination on both signals  141  and  161  provides the benefit that it filters out noises in the signal  141  due to fluctuation in the intensity of the ambient light, so that the photo driver circuit  10  will not keep switching between the power saving mode and the normal operation mode. 
     FIG. 4  shows another embodiment according to the present invention, wherein an adjustment circuit  20  is provided to adjust the level of the output signal from the current amplifier  14 . The purpose to provide the adjustment circuit  20  is to adjust the ratio of the input signal level of the output control circuit  16  to the current generated by the photo sensor device  12 , so that the sensitivity of the photo driver circuit  10  to the intensity of ambient light (or, the sensitivity of the output driver signal OUT to the intensity of ambient light) can be adjustable. In this embodiment, the adjustment circuit  20  is a variable resistor connected externally to the photo driver circuit  10  via a pin P 1 , so that a user may adjust the sensitivity. The adjustment circuit  20  may be embodied in various other ways in various other locations, such as adjusting the amplification rate of the current amplifier  14 . In this embodiment, because the output from the current amplifier  14  is a current signal, the voltage at the node A can be adjusted by adjusting the resistance of the variable resistor  20 . Thus, the ratio of the input signal level of the output control circuit  16  to the current generated by the photo sensor device  12  is correspondingly adjusted. Of course, depending on whether the input signal of the output control circuit  16  is a current signal or a voltage signal, the internal circuitry of the output control circuit  16  should be correspondingly designed. 
   Moreover, as shown in  FIG. 5 , the magnitude of the signal  141  received by the power saving control circuit  18  also can be adjusted to adjust the activation point of the power saving mode. In the embodiment of  FIG. 5 , an adjustment circuit  142  is provided, which is a variable resistor connected externally to the photo driver circuit  10  via a pin P 2 , so that a user may adjust the magnitude. In accordance thereto, the internal circuitry of the power saving control circuit  18  may be as shown in  FIG. 6 , in which the current signal  141  is converted to a voltage signal at node B by the adjustment circuit  142 . A voltage comparator VCP  185  compares the voltage at the node B with a predetermined reference voltage Vref, to decide the level of one input to the logic circuit  186 . As such, the adjustment circuit  142  provides a weighting; the photo current generated by the photo sensor device  12  is timed by this weighting, and then compared with a reference value. The above shows an example to adjust the activation point of the power saving mode; note that the activation point of the power saving mode can be adjusted in various other ways. For example, it can be adjusted by adjusting the reference current Iref in  FIG. 3 . Furthermore, the adjustment can be made in a digital manner instead of the aforementioned analogous manner, as shown in  FIG. 7 . 
   The present invention can be applied to products which receive power from a battery, or to applications wherein power consumption is a concern, such as Christmas light strings, bicycle headlights, etc. 
   The spirit of the present invention has been explained in the foregoing with reference to its preferred embodiments, but it should be noted that the above is only for illustrative purpose, to help those skilled in this art to understand the present invention, and not for limiting the scope of the present invention. Within the same spirit, various modifications and variations can be made by those skilled in this art. For example, the function of “high” and “low” of the signals in the embodiments can be arranged otherwise; the positive and negative inputs of the comparators ICP  184  or VCP  185  can be interchanged, with the logic circuit  186  correspondingly modified. As another example, in the output control circuit  16 , the latch circuit can be replaced by a sample-and-hold circuit, to store a signal in an analogous manner. In view of the foregoing, it is intended that the present invention cover all such modifications and variations, which should interpreted to fall within the scope of the following claims and their equivalents.