Abstract:
A DC-DC converter with auto-switching between pulse width modulation (PWM) and pulse frequency modulation (PFM) is used in an organic light emitting diode (OLED) display and judges power consumption of an OLED array of the display according to display data of the display and thus determines a switching time between PWM and PFM. The converter comprises a content statistic unit, an auto-switching control unit, a PWM controller, a PFM controller and a power generator. The content statistic unit receives and accumulates the display data of the OLED display and then outputs a statistic value. The auto-switching control unit outputs a control signal according to the statistic value to control the PWM controller and the PFM controller and to switch to a PWM mode or a PFM mode.

Description:
This application claims priority of No. 098125779 filed in Taiwan R.O.C. on Jul. 31, 2009 under 35 USC 119, the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a DC-DC converter with auto-switching between pulse width modulation (PWM) and pulse frequency modulation (PFM), and more particularly to a DC-DC converter, which has auto-switching between PWM and PFM, is applied to an organic light emitting diode (OLED) display and selects a PWM mode or a PFM mode according to display data of the display. 
     2. Related Art 
     An organic light emitting diode (OLED) display displays a frame using light emitting elements. The OLED display also displays the frame according to the brightness differences generated by different currents flowing through the pixels. The current of each pixel varies with the variation of the displayed frame, the difference between the maximum power consumption and the minimum power consumption of the OLED module is significantly great. It is a great challenge to provide the stable power for the OLED module, such as the power with the small ripple, without lowering the efficiency. 
     The DC-DC converter has two modulation methods including pulse width modulation (PWM) and pulse frequency modulation (PFM). As for the efficiency, PWM is suitable for the condition of great current loading, while PFM is suitable for the small current loading. Most DC-DC converters use the single modulation method, and some DC-DC converters judge the switching between PWM and PFM according to the magnitude of the current loading. However, it is not an easy issue to switch between PWM and PFM according to the magnitude of the current loading. 
     The technology of auto-switching between the PWM and PFM operation modes using in the DC-DC converter will be described in the following. Referring to  FIG. 1 , the auto-switching DC-DC converter  10  disclosed in U.S. Pat. No. 7,173,404 includes a starter  102 , an auto PWM/PFM controller  103 , a PWM controller  106 , a PFM controller  108  and a power switch unit  110 . The load  112  in this drawing may be an OLED module of an OLED display. 
     The starter  102  transmits a starting-enabling signal to the auto PWM/PFM controller  103  to enable the auto PWM/PFM controller  103 . Thereafter, the PWM controller  106  and the PFM controller  108  respectively transmit feedback signals to the auto PWM/PFM controller  103 . The auto PWM/PFM controller  103  generates a select signal according to the feedback signals of the PWM controller  106  and the PFM controller  108 , and outputs the select signal to the PWM controller  106  and the PFM controller  108 . So, the auto PWM/PFM controller  103  enables the PWM controller  106  or the PFM controller  108  according to the select signal. The enabled controller (PWM controller  106  or PFM controller  108 ) outputs a control signal to the power switch unit  110 . Thus, the power switch unit  110  switches and outputs a DC power to the load  112  according to the control signal. Meanwhile, the load  112  also transmits another feedback signal to the PWM controller  106  and the PFM controller  108 . 
     Therefore, the auto-switching DC-DC converter  10  controls the power switching in a close loop manner. In general, the close loop control mode is more complicated. So, the cost is higher. 
     SUMMARY OF THE INVENTION 
     In view of the above-identified problems, it is therefore an object of the invention to provide a DC-DC converter, which has auto-switching between PWM and PFM and selects a PWM or PFM mode according to display data in an OLED display. 
     To achieve the above-identified object, the invention provides a DC-DC converter with auto-switching between PWM and PFM. The converter includes a content statistic unit, an auto-switching control unit, a PWM controller, a PFM controller and a power generator. The content statistic unit receives and accumulates display data of the OLED display and then outputs a statistic value. The auto-switching control unit outputs a switch control signal according to the statistic value. The switch control signal has a first state and a second state. The PWM controller receives the switch control signal, and outputs a PWM power when the switch control signal is in the first state. The PFM controller receives the switch control signal, and outputs a PFM power when the switch control signal is in the second state. The power generator respectively receives the PWM power and the PFM power and thus outputs one of the PWM power and the PFM power as a power voltage. 
     Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention. 
         FIG. 1  shows the conventional architecture of a DC-DC converter with auto-switching between PWM and PFM. 
         FIG. 2  shows the circuit block diagram of a DC-DC converter with auto-switching between PWM and PFM according to a first embodiment of the invention. 
         FIG. 3  is a schematic illustration showing a statistic table of input data of one frame. 
         FIG. 4  shows the architecture of a statistic unit according to a first embodiment of the invention. 
         FIG. 5  shows the architecture of a statistic unit according to a second embodiment of the invention. 
         FIG. 6  shows the architecture of a statistic unit according to a third embodiment of the invention. 
         FIG. 7  is a schematic illustration showing an aspect of the invention. 
         FIG. 8  shows the architecture of an OLED display with the built-in DC-DC converter according to the second embodiment of the invention. 
         FIG. 9  shows the architecture of an OLED display with the built-in DC-DC converter according to the third embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be apparent from the following detailed description, which proceeds with reference to the accompanying drawings, wherein the same references relate to the same elements. 
     The DC-DC converter with the PWM/PFM auto-switching and the OLED display using the converter will be described with reference to the accompanying drawings. 
     According to the characteristic of the technology of the OLED module, the brighter pixel consumes more power. So, the power-consumption difference between the fully black frame and the fully white frame of the OLED module is very large. Based on this power-consumption property, the efficiency of the DC-DC converter may be improved by performing the proper switching between PWM and PFM so that the power consumption of the OLED display can be lowered. The typical DC-DC converter only can perform the switching between PWM and PFM according to the magnitude of the current loading at the loading end. However, the OLED display driver with the built-in DC-DC converter can obtain the contents to be displayed by the OLED display. So, the DC-DC converter of the invention judges the power consumption of the OLED array according to the display data, and thus determines the switching time between PWM and PFM. 
       FIG. 2  shows the circuit block diagram of an OLED display  20  with a built-in DC-DC converter according to a first embodiment of the invention. Referring to  FIG. 2 , the OLED display  20  includes a DC-DC converter  21  with auto-switching between PWM and PFM, a timing converter  220 , a display driving unit  222  and an OLED array  224 . The timing converter  220  receives display data, and adjusts a timing of the display data, and then outputs the display data to the display driving unit  222 . Thereafter, the display driving unit  222  drives the OLED array  224 . Because the timing converter  220 , the display driving unit  222  and the OLED array  224  are the typical architectures of the OLED display, detailed descriptions thereof will be omitted. 
     The DC-DC converter  21  with auto-switching between PWM and PFM of the invention includes a statistic unit  211 , an auto-switching control unit  212 , a PWM controller  213 , a PFM controller  214  and a power generator (or power generating unit)  215 . The statistic unit  211  receives the display data of the OLED display  20 , accumulates the display data and then outputs a statistic value. In general, the statistic unit  211  accumulates the data of each display frame and then outputs the statistic value. The auto-switching control unit  212  determines the control mode to be used according to the statistic value outputted from the statistic unit  211 , and outputs a mode control signal. When the statistic value is greater, it represents that the display frame is brighter, and the OLED display has the higher power consumption. So, the PWM mode is adopted. On the contrary, when the statistic value is smaller, it represents that the display frame is darker, and the OLED display has the lower power consumption. So, the PFM mode is adopted. The PWM controller  213  and the PFM controller  214  receive the mode control signal, and then output a PWM power and a PFM power to the power generator  215 . The power generator  215  receives the PWM power and the PFM power, then provides one of the PWM power and the PFM power to the display driving unit  222  and the OLED array  224 . 
       FIG. 3  is a schematic illustration showing a statistic table of input data of one frame. As shown in  FIG. 3 , the transversal coordinate represents the input data, and the longitudinal coordinate represents the count value, wherein the count values are summated and divided by the data quantity to obtain an average. The current load of the OLED array may be roughly predicted according to the average so that the suitable control mode may be selected. For example, the PWM mode is suitable for the condition of the higher current load, and the PFM mode is suitable for the lower current load. Although the average is generated in this statistic table, it is possible to make the judgement directly according to the statistic value in practice. 
       FIG. 4  shows the architecture of a statistic unit  40  according to the first embodiment of the invention. Referring to  FIG. 4 , the statistic unit  40  includes an adder  401  and a register  403 . The adder  401  adds each display data to the temporarily stored value of the register  403  to generate an added value, and then stores the added value back to the register  403 . The statistic unit  40  outputs the temporarily stored value of the register  403  as the statistic value after the vertical sync signal is enabled, and then clears the register  403 . Thus, the auto-switching control unit  212  can compare the statistic value with a threshold value, and outputs the mode control signal. That is, when the statistic value is greater than the threshold value, the mode control signal is the high level signal so that the PWM mode is adopted. When the statistic value is smaller than the threshold value, the mode control signal is the low level signal so that the PFM mode is adopted. 
       FIG. 5  shows the architecture of a statistic unit  50  according to a second embodiment of the invention. Referring to  FIG. 5 , the statistic unit  50  includes a multiplier  505 , a look-up-table  507 , an adder  401  and a register  403 . In practice, the pixel value of the display data of the OLED array is not directly proportional to its actually consumed current. Thus, the look-up-table  507  is utilized to record the weighting of the actually consumed current of each pixel value in this embodiment. So, the statistic unit  50  utilizes the multiplier  505  to multiply the display data by a weighting and then outputs the multiplied product to the adder  401  so that the statistic unit can reflect the actual state much more. The architectures of the adder  401  and the register  403  are the same as those of the statistic unit  40  of the first embodiment, so detailed descriptions thereof will be omitted. 
       FIG. 6  shows the architecture of a statistic unit  60  according to a third embodiment of the invention. Referring to  FIG. 6 , the statistic unit  60  includes a look-up-table  607 , an adder  401  and a register  403 . In practice, the pixel value of the display data of the OLED array is not directly proportional to its actually consumed current. So, the look-up-table  507  is utilized to record relative values of the actually consumed currents corresponding to different pixel values in this embodiment. Thus, the statistic unit  60  adopts the look-up-table  607  to convert the pixel values of the display data into the relative values of the actually consumed current, and then outputs the relative values to the adder  401 . The architectures of the adder  401  and the register  403  are the same as those of the statistic unit  40  of the first embodiment, so detailed descriptions thereof will be omitted. 
     Furthermore, the auto-switching control unit  212  of the invention may be implemented in two aspects. In the first aspect, the judgement is made according to a single threshold value. That is, the auto-switching control unit  212  only sets one threshold value, and switches to the PWM mode when the statistic value is greater than or equal to the threshold value; or otherwise switches to the PFM mode when the statistic value is smaller than the threshold value. In the second aspect, the judgement is made according to two threshold values. In order to prevent to switch between different modes frequently, the judgement method similar to the Smitt trigger may be adopted.  FIG. 7  is a schematic illustration showing this aspect of the invention. That is, two threshold values, such as the first threshold value and the second threshold value shown in  FIG. 7 , are set. When the current mode is the PFM mode, then the mode cannot be switched to the PWM mode until the statistic value exceeds the second threshold value. When the current mode is the PWM mode, then the mode cannot be switched to the PFM mode until the statistic value is smaller than the first threshold value. Thus, if the second switching method based on the two threshold values is adopted, the auto-switching control unit  212  maintains the current mode when the statistic value ranges between the first threshold value and the second threshold value. 
     In addition, the statistic value of the frame cannot be outputted from the statistic unit until the statistic unit has completely received the data of one frame. Thus, PWM/PFM switching of the OLED display with the built-in DC-DC converter according to the first embodiment lags behind the displaying of the contents by the time of one frame. For example, when the frame rate is 60 Hz, then the switching time needs 1/60 seconds to select PWM/PFM. According to the continuity of the typical frames, the delay of 1/60 seconds cannot affect the switching performance of the DC-DC converter of the invention. 
       FIG. 8  shows the architecture of an OLED display  80  with the built-in DC-DC converter according to the second embodiment of the invention. Referring to  FIG. 8 , the OLED display  80  includes a DC-DC converter  21  with auto-switching between PWM and PFM, a timing converter  820 , a display driving unit  822 , a memory  826  and an OLED array  224 . The DC-DC converter  21  of this embodiment is the same as the DC-DC converter  21  of the OLED display  20  of the first embodiment, so detailed descriptions thereof will be omitted. However, the DC-DC converter  21  of this embodiment receives the data outputted from the memory  826 . The OLED display  80  of this the embodiment mainly has the added memory  826 . The memory  826  receives and stores the display data. The timing converter  820  outputs a control signal to the memory  826  and the display driving unit  822  so that the memory  826  outputs the data to the display driving unit  822 , and the display driving unit  822  drives the OLED array  224  according to the received data. Because the timing converter  820 , the memory  826 , the display driving unit  822  and the OLED array  224  have the architectures that can be found in the typical OLD display, detailed descriptions thereof will be omitted. 
       FIG. 9  shows the architecture of an OLED display  90  with the built-in DC-DC converter according to the third embodiment of the invention. Referring to  FIG. 9 , the OLED display  90  includes a DC-DC converter  21  with auto-switching between PWM and PFM, a timing converter  920 , a display driving unit  922 , a memory  926  and an OLED array  224 . The DC-DC converter  21  of this embodiment is the same as the DC-DC converter  21  of the OLED display  80  of the second embodiment, so detailed descriptions thereof will be omitted. The difference between the OLED display  90  of this embodiment and the OLED display  80  of the second embodiment resides in that the data of the display driving unit  922  is provided by the timing converter  920 . Thus, the DC-DC converter  21  receives the data outputted from the timing converter  920 . Because the timing converter  920 , the memory  926 , the display driving unit  922  and the OLED array  224  have the architectures that can be found in the typical OLD display, detailed descriptions thereof will be omitted. 
     Because the DC-DC converter  21  of the invention directly controls the switching between PWM and PFM directly according to the display data without feeding the current load (i.e., the consumed current of the OLED array) back to the DC-DC converter, the architecture of the invention is simpler than that of the prior art, in which the feedback method is adopted. 
     While the invention has been described by way of examples and in terms of preferred embodiments, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications. For example, an additional set of memories may further be added to the OLED display to serve as the buffer for the display data so that the DC-DC converter can firstly read the display data in the buffer and the PWM/PFM mode may be adjusted immediately. 
     While the invention has been described by way of examples and in terms of preferred embodiments, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications.