Abstract:
A voltage converter improves the efficiency thereof by connecting a boost converter and an LDO regulator with a buck converter in parallel. The boost converter boosts up a supply voltage to generate a first output voltage at a first output, and the buck converter bucks down the supply voltage to generate a second output voltage at a second output. When the second output voltage is lower than a threshold, the LDO regulator converts the first output voltage to a third voltage at said second output.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to a voltage converter and more particularly, to the efficiency improvement of a voltage converter. 
   BACKGROUND OF THE INVENTION 
   Battery is widely used for the power source in portable electronic products. However, the battery voltage will be gradually decayed with its operational time or suddenly dropped down resulted from instant increasing of load current flowing through the internal resistor of the battery. For a battery voltage will be out of a desired range, it is generally employed buck-boost converter or two-stage, i.e., boost-then-buck, voltage converter in order to maintain a stable output voltage for power supply to a load. 
     FIG. 1  shows a conventional two-stage voltage converter  10  that includes a boost converter  12  connected in series with a buck converter  14 . The boost converter  12  is connected between a supply voltage V S  provided by one or more batteries and an output  1202  to boost up the supply voltage V S  to generate an output voltage V OUT1  to supply for a load  162  connected to the output  1202 , and the buck converter  14  is connected between the output  1202  and  1402  to convert the boosted voltage V OUT1  to another output voltage V OUT2  to supply for another load  164  connected to the output  1402 . For typical applications, the supply voltage V S  is in the range of from 1.8V to 3.3V, the boosted voltage V OUT1  is about 3.3V, and the bucked voltage V OUT2  is about 1.8V. The boost converter  12  comprises an inductor L 1  connected between the supply voltage V S  and a node  1204 , a diode D 1  connected between the node  1204  and the output  1202 , a transistor Q 1  connected between the node  1204  and ground, a capacitor C 1  connected between the output  1202  and ground, and a boost controller  122  to switch the transistor Q 1  for regulating the output voltage V OUT1 . On the other hand, the buck converter  14  comprises an inductor L 2  connected between the output  1402  and a node  1404 , a diode D 2  connected between the node  1404  and ground, a capacitor C 2  connected between the output  1402  and ground, a transistor Q 2  connected between the output  1202  and the node  1404 , and a buck controller  142  to switch the transistor Q 2  for regulating the output voltage V OUT2 . However, for the two-stage voltage converter  10  boosting up the supply voltage V S  first and then bucking down the boosted voltage V OUT1 , the total efficiency to convert the supply voltage V s  to the output voltage V OUT2  will be the efficiency product of the boost converter  12  and the buck converter  14 , i.e., η Boost ×η Buck , and therefore, the total efficiency of the two-stage voltage converter  10  is decreased by such two-stage conversion. 
     FIG. 2  shows a conventional SEPIC converter  20  that comprises a boost converter  22  and a buck-boost converter  24  both connected to a supply voltage V S . As usual, the boost converter  22  is connected between the supply voltage V S  and a load  262  connected to its output  2204 , to boost up the supply voltage V S  to generate an output voltage V OUT1 , at the output  2204 . The buck-boost converter  24  is connected between the supply voltage V S  and another load  264  connected to its output  2406 , to convert the supply voltage V S  to another output voltage V OUT2  at the output  2406 . The boost converter  22  comprises an inductor L 1  connected between the supply voltage V S  and a node  2202 , a diode D 1  connected between the node  2202  and the output  2204 , a capacitor C 1  connected between the output  2204  and ground, a transistor Q 1  connected between the node  2202  and ground, and a boost controller  222  to switch the transistor Q 1  for regulating the output voltage V OUT1 . On the other hand, the buck-boost converter  24  comprises an inductor L 2  connected between the supply voltage V S  and a node  2402 , another inductor L 3  connected between a node  2404  and ground, a diode D 2  connected between the node  2404  and the output  2406 , a capacitor C 2  connected between the output  2406  and ground, another capacitor C 3  connected between the nodes  2402  and  2404 , a transistor Q 2  connected between the node  2402  and ground, and a buck controller  242  to switch the transistor Q 2  for regulating the output voltage V OUT2 . However, a buck-boost converter does not have high conversion efficiency, and the two energy-storing elements, inductors L 2  and L 3 , bring the buck-boost converter  24  to high cost and large size. 
   Moreover, as shown in  FIG. 1  and  FIG. 2 , other transient loadings  160  and  260 , such as photoflash and motor, also connected to the supply voltage V S  would generate surge current It that causes the supply voltage V S  suddenly dropped down because of the surge current It flowing through the internal resistor of the battery, and thereby the supply voltage V S  may be lower than the output voltage V OUT2 , as shown by curve  406  in  FIG. 4 , to further degrade the efficiency thereof. 
   Although both the voltage converters  10  and  20  shown in  FIG. 1  and  FIG. 2  may maintain the output voltage V OUT2  stably at desired level, their conversion efficiencies are only around 80%, as shown in  FIG. 5  by curve  52  for the two-stage voltage converter  10  and by curve  54  for the SEPIC converter  20 . 
   Therefore, it is desired an efficiency improved voltage converter. 
   SUMMARY OF THE INVENTION 
   One object of the present invention is to provide a voltage converter in which the efficiency is improved by a combination of linear mode and switch mode converters. In a voltage converter, according to the present invention, a boost converter is connected between a supply voltage provided by one or more batteries and a first output, a buck converter is connected between the supply voltage and a second output, and a low dropout (LDO) regulator is connected between the first output and the second output. The boost converter boosts up the supply voltage to generate a first output voltage at the first output, and the buck converter bucks down the supply voltage to generate a second output voltage at the second output. When the supply voltage is lower than a threshold, the LDO regulator converts the first output voltage to a third voltage at the second output. A shutdown circuit is further included in the buck converter to turn off the buck converter to prevent reverse current to flow toward to the battery. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  shows a conventional two-stage voltage converter; 
       FIG. 2  shows a conventional SEPIC converter; 
       FIG. 3  shows an embodiment according to the present invention; 
       FIG. 4  shows the variation of the output voltage VOUT 2  of the voltage converter  30  upon a transient loading; 
       FIG. 5  shows the relations between power conversion efficiency and supply voltage for the voltage converter according to the present invention and the conventional voltage converters; and 
       FIG. 6  shows an embodiment for the buck converter according to the present invention to prevent reverse current to flow toward to the battery. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3  shows an embodiment according to the present invention, in which linear mode and switch mode converters are combined together to improve the efficiency thereof. A voltage converter  30  comprises a boost converter  32  connected with a supply voltage V S  to boost up the supply voltage V S  to generate an output voltage V OUT1 , at its output  3202  to supply for a load  382  connected to the output  3202 , a buck converter  34  connected with the supply voltage V S  to buck down the supply voltage V S  to generate another output voltage V OUT2  at its output  3402  to supply for another load  384  connected to the output  3402 , and an LDO regulator  36  connected between the outputs  3202  and  3402  to convert the output voltage V OUT1  to yet another output voltage V OUT3  at the output  3402  connected with the load  384  when the output voltage V OUT2  is lower than a threshold. The boost converter  32  comprises an inductor L 1  connected between the supply voltage V S  and a node  3204 , a diode D 1  connected between the node  3204  and the output  3202 , a capacitor C 1 , connected between the output  3202  and ground, a transistor Q 1  connected between the node  3204  and ground, and a boost controller  322  to switch the transistor Q 1  for regulating the output voltage V OUT1 . On the other hand, the buck converter  34  comprises an inductor L 2  connected between the output  3402  and a node  3404 , a diode D 2  connected between the node  3404  and ground, a capacitor C 2  connected between the output  3402  and ground, a transistor Q 2  connected between the supply voltage V S  and the node  3404 , and a buck controller  342  to switch the transistor Q 2  for regulating the output voltage V OUT2 . 
   In normal operation, the LDO regulator  36  does not work, and the voltage supplied to the load  384  is V OUT2  provided by the buck converter  34 . However, when the output voltage V OUT2  is lower than the threshold because of power consumption of the battery or transient loading such as photoflash and motor, the LDO regulator  36  operates and provides the output voltage V OUT3  supplied to the load  384 . For typical applications, the supply voltage V S  is in a range of from 1.8V to 3.3V, the output voltage V OUT1  is about 3.3V, the output voltage V OUT2  is about 1.8V, the output voltage V OUT3  is about 1.75V, and the threshold is substantially equal to the output voltage V OUT3 , about 1.75V. 
     FIG. 4  shows the variation of the output voltage V OUT2  of the voltage converter  30  upon a transient loading such as photoflash and motor. In this diagram, the voltage level of 1.8V designated by curve  402  is the buck setting, and another voltage level of 1.75V designated by curve  404  is the LDO setting. Under steady state, the output voltage V OUT2  of the buck converter  34  is maintained at 1.8V, which is larger than 1.75V of the LDO setting and thus, the LDO regulator  36  does not work. Upon a transient loading to induce a surge current I t  flowing through the internal resistor of the battery, as shown by curve  406 , the supply voltage V S  drops down violently, resulting in 100% of buck converter duty and falling down of the output voltage V OUT2  eventually, as shown by curve  408 . Once the output voltage V OUT2  under 1.75V of the LDO setting, the LDO regulator  36  is triggered to convert the output voltage V OUT1  to the output voltage V OUT3  at the output  3402  of the buck converter  34  and eventually, the LDO regulator  36  substitutes for the buck converter  34  to supply power for the load  384  to maintain the normal operation of the load  384 . When the supply voltage V S  is recovering such that the output voltage V OUT2  of the buck converter  34  reaches 1.75V of the LDO setting, the LDO regulator  36  stops working, and the buck converter  34  takes the role back to supply power for the load  384 . After the transient event, the battery voltage V S  is recovered to its original level, and the output voltage V OUT2  of the buck converter  34  is maintained at 1.8V again. Most of operational time the battery voltage V S  is above 1.8V, and the power conversion is performed by the buck converter  34 , instead of the LDO regulator  36 . As a result, the average efficiency of the voltage converter  30  is improved because of the efficient buck converter  34 , even though the LDO regulator  36  has poor efficiency. 
   Another situation the battery voltage V S  under desired range is occurred when the battery power is almost exhausted out. For comparison and more detailed illustration,  FIG. 5  shows the relations between conversion efficiency and supply voltage for the voltage converter  30  according to the present invention and the conventional voltage converters  10  and  20 . Curve  50  represents the efficiency to convert the supply voltage V S  to the output voltage V OUT2  by the voltage converter  30  according to the present invention, curves  52  and  54  represent for those by the conventional two-stage voltage converter  10  and SEPIC converter  20 , respectively. When the supply voltage V S  is within the range of from 1.8V to 3.0V, the conversion efficiency for the output voltage V OUT2  according to the present invention is about within the range of from 90% to 97%, which is much larger than the range around 80% for the conventional two-stage voltage converter  10  and SEPIC converter  20 . Due to the low efficient LDO regulator  36 , the efficiency to generate the output voltage V OUT3  according to the present invention drops rapidly to about 50% when the supply voltage V S  is lower than 1.8V. However, the battery voltage V S  under 1.8V is occurred when the battery power is almost exhausted out. Therefore, the total efficiency of the voltage converter  30  according to the present invention is still higher than the conventional voltage converters  10  and  20  about 5% to 10%. 
   Referring to  FIG. 3 , when the voltage on the node  3404  is higher than the supply voltage V s , there will be a reverse current to flow toward to the battery. To prevent this reverse current I b ,  FIG. 6  provides an embodiment for the buck converter  34  that further includes a shutdown circuit  344  to monitor the voltage drop across the transistor Q 2 . For example, the shutdown circuit  344  includes a comparator  3442  that has a non-inverting input connected to the node  3404 , and an inverting input coupled to the supply voltage V S  with an offset V D  of about 50mV inserted therebetween to compensate the cutoff voltage of the transistor Q 2 . When the voltage on the node  3404  is higher than the supply voltage V S  with a difference V D , the shutdown circuit  344  generates a shutdown signal SD to turn off the transistor Q 2  by the buck controller  342 , by which reverse current I b  from the node  3404  through the transistor Q 2  to the battery is prevented. 
   While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.