1303916 九、發明說明： 【發明所屬之技術領域】 ^本發明係有關一種電壓轉換器，特別是關於一種維持 定電流輸出的電壓轉換器及其控制方法。 ~ 【先前技術】 $ _現今幾乎所有的可攜式消費性電子產品都配有液晶 _ 顯不螢幕，例如行動電話、MP3隨身聽和PDA，由於液晶 本身不發光，這些顯示器都需要背光照明，使用者才能看 ，，上顯示的圖形和文字，白光LED是目前最常採用的照 明叙置，而由於LED的亮度和流經的順向電流成正比，因 此對LED驅動器而言，輸出電壓或電流的恒定是十分重要 的。 所有專為驅動led而設計的驅動器都以提供定電流為 ^ 解决方法可分為電感式或是電荷泵浦式這兩大類^ 種方法各有其優缺點。電荷泵浦式驅動器是利用離散 將電源從輸人端傳送至輸出端，因為不需使用任何電 ，2，所以是相當受歡迎的供電方式，但缺點在於它只能提 、t、，限的輪出電壓範圍，絕大多數電荷泵浦元件的轉換比 最夕/、此達到輸入電壓的兩倍，這表示輸出電壓不可能高 於輸入電壓的兩倍，因此若想利用電荷泵浦驅動一顆以上 的LEf，就必須採用並聯的方式。電感式驅動器的體積小， ' ；S大夕以升壓轉換器為主要架構，適合使用在電池 守]長的/肖費性電子產品上。但此種驅動器有輸出電 5 1303916 壓必須大於輸入電壓的限制，因此，被驅動的LED必須串 聯，才能使負載需要的輸出端電壓一定高於電池供應的輸 入端電壓，此外，當輸出電壓過高時，流經負載的電流仍 會增大，使得LED亮度改變，這個缺點在以電阻做為調節 ^ 器的驅動器中特別明顯，例如美國國家半導體（National ^ Semiconductor ; NS)推出的驅動器晶片，型號LM3519，其 調控器係利用電晶體的電阻特性調控輸出給LED的電流， 但此一產品的輸出電壓無法低於輸入電壓，故當輸入電壓 • 過高時，輸出電壓也隨之升高，造成負載得到的電壓高於 其所需的電壓，負載上的電流也因此增加，又如NS另一 產品，型號LP3933的光源管理晶片，此一產品能夠提供 數個定電壓輸出，但輸出電壓仍必須大於輸入電壓。因 此，美國專利第6166527號提出一種降升壓切換式調節 器，在電容、電感、輸入端以及輸出端之間設置多個開關， 藉由控制各開關的開閉’切換輸出端電壓高於或低於輸入 I 端電壓，但此種電路需要多個開關，成本提高，且轉換效 率低。 因此’一種具南轉換效率、能負擔輸入電壓高於負載 " 所需電壓、維持定電流輸出的電壓轉換器，乃為所冀。 【發明内容】 本發明的目的之一，在於提出一種提供定電流輸出的 電壓轉換器。 本發明的目的之一，在於提出一種輸入電壓能夠大 6 1303916 於、小於或等於負載所需電壓的電壓轉換器。 本發明的目的之一，在於提出一種高轉換效率的電壓 轉換器。 根據本發明，一種電壓轉換器包括一電感串聯一負载 和一電流源，該電感和輸出端之間有一第一開關，和接地 鈿之間有一第二開關，一電容連接至該輸出端，一升壓控 制電路控制該第-及第二開關的開閉，一電流源控制電路 偵測该電流源的跨壓並根據該跨壓的變動發出一偵測信 號給該升壓控制電路以及發出—控制信號給該電流源，言: 升壓控制電路根據該偵測信號切換該第一及第二開關的 開閉’該控制信號驅動該電流源，使該負載上的電流維持 定值。變化地，該第一開關被一二極體取代。 【實施方式】 ά圖1係本發明之第一實施例，電感12和負載22、電 二源^串聯’電容2 4連接在輸出端16和接地端G N D之 曰 1迅感12和輸出端16間設有電晶俨14,4拉丄上 間設有電晶體18，電曰體:二=:和接地端_ 關使用，實施例中都做為開 實施例中為-電晶體】8 :閉’電流源30於本 體8，在其他實施例中，電晶體28彔 W其他電路代替’例如電流鏡，電；體二 測電流源30上的佟颅^ 利电路26偵 tr, νίηι^Β, , ° 從輸入端!。經電==，電晶體14導通，電流 感12直接泌到輸出端16，故輪出電壓 7 1303916 ν_幾乎等於輸入電壓vin，由於電晶體有飽和特性，因此 電晶體28源/汲極之間的壓差高於一飽和電壓後，電晶體 28的電流就維持在一定值，不再隨電壓上升而增大，此一 電流稱為飽和電流，故即使電晶體2 8源/>及極之間的電壓 ^ 繼續升高，流經負載22的電流，即電晶體28的飽和電流， : 仍維持定值，因此，輸入電壓Vin供給負載22所需的電壓 後，多餘的電壓便被電流源30吸收。當輸入電壓Vin過低 而無法支持負載22時，電流源控制電路26偵測到電晶體 • 28上的跨壓低於最小跨壓值，便發出一偵測信號到升壓控 制電路20以及一控制信號到電晶體28，升壓控制電路20 根據偵測信號導通電晶體18，截止電晶體14，讓電感12 儲能，再截止電晶體18並導通電晶體14，讓電感12釋出 所儲存的能量對電容24充電，因而得到大於輸入電壓Vin 的輸出電壓V〇ut，另外，該控制信號驅動電晶體28，以維 持流經負載22的電流。 ^ 圖2係本發明之第二實施例，將圖1的電晶體14以 二極體34取代，升壓控制電路20只控制電晶體18的開 閉，其他條件和圖1之實施例相同。當輸入電壓vin夠高 — 時，電晶體18開路，電流從輸入端10經二極體34直接 • 流向輸出端16,此時輸出電壓Vm幾乎等於輸入電壓Vin， 做為電流源30的電晶體28吸收多餘的電壓，使負載22 的電流維持定值。當輸入電壓Vin過低，無法支持負載22 所需時，電流源控制電路26偵測到電流源30上的跨壓低 於最小跨壓值，便發出一偵測信號到升壓控制電路20以 8 1303916 及控制k號到電流源30，升壓控制電路56根據偵測信 號導通電晶體18，讓電感12儲能，再截止電晶體18，電 感12釋出儲存的能量對電容24充電，因而得到大於輸入 電壓vin的輸出電壓vout，控制信號則驅動電晶體28，以 維持流經負載22的電流。 圖3顯示一電流鏡電路，可以用來取代圖i和圖2中 的電晶體28，作為電流源3〇。 由於本發明的電壓轉換器使用較少開關便可實現輸 =電壓能夠大於、小於或等於負載所需電壓的目的，故能 得到比習知電壓轉換器更好的轉換效率。 【圖式簡單說明】 圖1係本發明之第_實施例； 圖2係本發明之第二實施例；以及 圖3係一電流鏡電路。 【主要元件符號說明】 10 輸入端 12 電感 14 電晶體 16 輸出端 18 電晶體 20 升壓控制電路 22負載 1303916 24 電容 26 電流源控制電路 28 電晶體 30 電流源 34 二極體1303916 IX. Description of the invention: [Technical field to which the invention pertains] The present invention relates to a voltage converter, and more particularly to a voltage converter for maintaining a constant current output and a control method therefor. ~ [Prior Art] $ _ Today almost all portable consumer electronics products are equipped with LCD _ display screens, such as mobile phones, MP3 players and PDAs, these displays require backlighting because the LCD itself does not emit light. The user can see, the graphics and text displayed on it, white LED is the most commonly used lighting description, and because the brightness of the LED is proportional to the forward current flowing through it, the output voltage or Constant current is very important. All drivers designed to drive LEDs provide constant current as a solution. The solution can be divided into two types: inductive or charge-pumped. Each has its own advantages and disadvantages. The charge-pumped driver uses discrete to transfer power from the input end to the output. Because it does not need any electricity, 2, it is a very popular power supply, but the disadvantage is that it can only mention, t, and limit. The range of the voltage of the turn-off voltage, the conversion of most charge-pumped components is equivalent to the eve, which is twice the input voltage, which means that the output voltage cannot be higher than twice the input voltage, so if you want to use the charge pump drive More than one LEf must be connected in parallel. The inductive driver is small in size, and the S-Xi is based on a boost converter. It is suitable for use in battery-operated/long-cost electronic products. However, such a driver has an output voltage of 51303916. The voltage must be greater than the input voltage limit. Therefore, the driven LEDs must be connected in series in order to make the output voltage required by the load be higher than the input voltage of the battery. In addition, when the output voltage is over When high, the current flowing through the load will still increase, causing the brightness of the LED to change. This shortcoming is particularly noticeable in the case of resistors as regulators, such as the driver chip from National Semiconductor (NS). Model LM3519, its controller uses the resistance characteristics of the transistor to regulate the current output to the LED, but the output voltage of this product can not be lower than the input voltage, so when the input voltage is too high, the output voltage will also rise. The voltage generated by the load is higher than the required voltage, and the current on the load is also increased. For example, another product of NS, the light source management chip of model LP3933, can provide several constant voltage outputs, but the output voltage is still Must be greater than the input voltage. Therefore, U.S. Patent No. 6,166,527 proposes a step-down switching regulator in which a plurality of switches are provided between a capacitor, an inductor, an input terminal, and an output terminal, by controlling the opening and closing of each switch to 'switch the output terminal voltage to be higher or lower. The voltage at the input terminal I, but this circuit requires multiple switches, the cost is increased, and the conversion efficiency is low. Therefore, it is a kind of voltage converter that has a south conversion efficiency, can withstand an input voltage higher than the load required, and maintains a constant current output. SUMMARY OF THE INVENTION One object of the present invention is to provide a voltage converter that provides a constant current output. One of the objects of the present invention is to provide a voltage converter having an input voltage that can be as large as 1,1303916, less than or equal to the voltage required by the load. One of the objects of the present invention is to provide a voltage converter with high conversion efficiency. According to the present invention, a voltage converter includes an inductor in series with a load and a current source, a first switch between the inductor and the output, and a second switch between the ground and a ground, a capacitor connected to the output, The boost control circuit controls the opening and closing of the first and second switches, and a current source control circuit detects a cross voltage of the current source and sends a detection signal to the boost control circuit according to the change of the cross voltage and issues a control The signal is applied to the current source, and the boost control circuit switches the opening and closing of the first and second switches according to the detection signal. The control signal drives the current source to maintain a constant current on the load. Alternatively, the first switch is replaced by a diode. [Embodiment] FIG. 1 is a first embodiment of the present invention. An inductor 12 and a load 22, and an electric two-source series-connected capacitor 2 are connected between the output terminal 16 and the ground terminal GND. There are electro-crystal rafts 14 and 4, and a transistor 18 is arranged on the upper side of the cymbal. The electric sputum body: two =: and the grounding terminal _ is used in the embodiment, and in the embodiment, it is an electro-transistor in the embodiment. 8: The 'current source 30 is connected to the body 8. In other embodiments, the transistor 28 彔W other circuits are replaced by 'for example, a current mirror, and the body is measured by a current circuit 30 on the current source 30. Tr, νίηι^Β , , ° from the input!! After electricity ==, the transistor 14 is turned on, and the current sense 12 is directly secreted to the output terminal 16, so the turn-off voltage 7 1303916 ν_ is almost equal to the input voltage vin. Since the transistor has saturation characteristics, the transistor 28 source/drain After the voltage difference is higher than a saturation voltage, the current of the transistor 28 is maintained at a certain value and no longer increases with the voltage rise. This current is called a saturation current, so even the transistor 28 source/> The voltage between the poles continues to rise, and the current flowing through the load 22, that is, the saturation current of the transistor 28, remains at a constant value. Therefore, after the input voltage Vin is supplied to the voltage required by the load 22, the excess voltage is Current source 30 is absorbed. When the input voltage Vin is too low to support the load 22, the current source control circuit 26 detects that the voltage across the transistor 28 is lower than the minimum voltage value, and sends a detection signal to the boost control circuit 20 and a control. The signal is applied to the transistor 28. The boost control circuit 20 conducts the transistor 18 according to the detection signal, turns off the transistor 14, allows the inductor 12 to store energy, and then turns off the transistor 18 and conducts the transistor 14 to allow the inductor 12 to release the stored energy. The capacitor 24 is charged, thereby obtaining an output voltage V〇ut that is greater than the input voltage Vin. Additionally, the control signal drives the transistor 28 to maintain the current flowing through the load 22. Figure 2 is a second embodiment of the present invention. The transistor 14 of Figure 1 is replaced by a diode 34. The boost control circuit 20 controls only the opening and closing of the transistor 18. Other conditions are the same as in the embodiment of Figure 1. When the input voltage vin is high enough, the transistor 18 is open, and current flows directly from the input terminal 10 through the diode 34 to the output terminal 16, at which time the output voltage Vm is almost equal to the input voltage Vin, which acts as a transistor for the current source 30. 28 absorbs excess voltage and maintains the current of load 22 at a constant value. When the input voltage Vin is too low to support the load 22, the current source control circuit 26 detects that the voltage across the current source 30 is lower than the minimum voltage value, and sends a detection signal to the boost control circuit 20 to 8 1303916 and control k to current source 30, boost control circuit 56 conducts crystal 18 according to the detection signal, allows inductor 12 to store energy, and then cuts off transistor 18, and inductor 12 releases the stored energy to charge capacitor 24, thus obtaining Above the output voltage vout of the input voltage vin, the control signal drives the transistor 28 to maintain the current flowing through the load 22. Figure 3 shows a current mirror circuit that can be used in place of transistor 28 in Figures i and 2 as a current source. Since the voltage converter of the present invention achieves the purpose that the input voltage can be greater than, less than, or equal to the voltage required by the load using fewer switches, better conversion efficiency than conventional voltage converters can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a first embodiment of the present invention; Fig. 2 is a second embodiment of the present invention; and Fig. 3 is a current mirror circuit. [Main component symbol description] 10 Input 12 Inductor 14 Transistor 16 Output 18 Transistor 20 Boost Control Circuit 22 Load 1303916 24 Capacitor 26 Current Source Control Circuit 28 Transistor 30 Current Source 34 Dipole