1. Field of the Invention
The present invention relates to a DC-DC voltage converter. More particularly, the present invention relates to a DC-DC voltage converter with an output voltage lower than a reference voltage.
2. Description of Related Art
As electronic information technique advances, circuit designs of various electronic products become increasingly complex. For many circuit designs, cell voltage is converted to lower power supply voltage, for example mobile phone, PDA and other hand held equipment with power supplied by cell. For example, for portable computer, voltage converter is used in a common power supplier to serve as a basic voltage adjuster, so increasingly more DC-DC voltage converters are used in various electronic products.
The function of the DC-DC voltage converter is outputting an output voltage smaller than an input voltage, and stabilizing the voltage. Referring to FIG. 1, a well-known DC-DC voltage converter 100 is shown. During operation of the DC-DC voltage converter 100 as shown in FIG. 1, it receives a DC input voltage Vin provided by an alkaline cell or a lithium cell, and the DC input voltage Vin is converted to a lower fixed DC output voltage Vout. After the output voltage Vout is fed back by a feedback circuit 101, it is compared with a reference voltage Vref through a comparator 103, and a control signal is generated to a driving circuit 105 according to a comparing result. The driving circuit 105 sends a driving signal to a power circuit 107 according to the received control signal. The power circuit 107 is connected to the input voltage Vin and a voltage stabilizing circuit 109. The power circuit 107 controls conducted/non-conducted state between the input voltage Vin and the voltage stabilizing circuit 109 according to the received driving signal, and then outputs a stable output voltage Vout after being stabilized by the voltage stabilizing circuit 109.
However, in most of the portable electronic products, alkaline cells or lithium cells are used as the electric source. The voltage of the cell may be increasingly reduced with operation time and using time, so in order to obtain a stable output voltage, under many situations, the DC-DC voltage converter adopts boost-buck two-stage voltage converter.
In Taiwan patent No. 1281305, a dual-input voltage converter is provided, which includes a comparator for comparing the output Vout and a reference voltage Vref to generate a signal to a controller, and the controller determines to switch a first voltage converting circuit or a second voltage converting circuit. Referring to FIG. 2, a schematic view of an embodiment of this patent is shown. The first voltage converting circuit is composed of transistors SW1 and SW2, an inductor L and a capacitor Co, and the second voltage converting circuit is composed of transistors SW2 and SW3, the inductor L, and the capacitor Co. The controller 201 switches the transistors SW1 and SW2 to generate a current I to charge the capacitor Co through the inductor L to obtain an output voltage Vout. The comparator 203 compares the output voltage Vout and the reference voltage Vref. When the output voltage Vout is lower than the reference voltage Vref, an output signal Se of the comparator 203 makes the controller 201 turn off the transistor SW1, such that a voltage VDD is converted to a voltage VPP after being boosted by a boost converter 205. The controller 201 switches the transistors SW2 and SW3 to convert the voltage VPP to the output voltage Vout, such that the dual-input voltage converter 200 can be considered as a boost-buck two-stage voltage converter, and avoids the situation that the output voltage Vout is reduced due to the reduce of the input voltage Vin and thus causing an unstable output. However, the current DC-DC converter still has some defects which are described as follows.
1. Although the two-stage voltage converter can solve the situation that the reduction of the input voltage Vin results in the unstable output voltage Vout, it cannot operate under a state that the output voltage Vout is smaller than the reference voltage Vref, and it depends on the boost converter to make the output voltage Vout maintain at the state of being larger than the reference voltage Vref. However, low operating voltage enables the electronic product integrate more transistors on a single chip, so in practical application, various electronic products use low voltage increasingly. Through prediction of “International Technology Roadmap for Semiconductors (ITRS)” issued by Semiconductor Industry Association (SIA), in the period from now to the year of 2016, operating voltage and operating current of IC will respectively show falling and rising trends. It can be known that the output voltage Vout of the voltage converter required by many electronic products becomes increasingly smaller, so as to directly result in the difficulty of selecting value of the reference voltage Vref. A situation that the output voltage Vout is smaller than the reference voltage Vref occurs more frequently. It is impossible for the conventional DC-DC voltage converter to operate under the situation that the output voltage Vout is smaller than the reference voltage Vref, and it is a blind spot in the design of the DC-DC voltage converter.
2. Under the situation that the output voltage Vout is smaller than the reference voltage Vref, the dual-input voltage converter adopts the method of boosting and then bucking. However, it is necessary to add a boost converter, a voltage converting circuit, and some other related electronic components in the circuit. However, PCB space of the current DC-DC voltage converter is quite limited, which does not satisfy the developing trend of the modern electronic product of “a small size with big function”. The DC-DC voltage converter is widely used in the electronic products, and the increase of the electronic components in the DC-DC voltage converter will result in the increase of manufacturing cost.