Abstract:
An automatic switch method and apparatus are used to set the output current of a USB by slowly increasing the output current under monitoring the voltage on the USB. If the USB could normally operate until the output current reaches a threshold value, a higher level is set for the output current, otherwise a lower level is set instead. The process may be performed periodically to dynamically determine the output current for the USB.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention is related generally to a universal serial bus (USB) and, more particularly, to an automatic switch method and apparatus for a USB.  
       BACKGROUND OF THE INVENTION  
       [0002]     Currently, there are two types of USB ports, 100 mA type and 500 mA type. When using USB port for a battery charger, if the USB port is a 100 mA type but 500 mA is sinking to a charged battery, the voltage on the USB port would be loading to fail. Contrarily, if the USB port is a 500 mA type but only 100 mA is sinking to the charged battery, the charge time will be long and the charge efficiency is low. However, it is difficult for the end users and system designers to know or predict which USB type will be used.  
         [0003]     On the other hand, when using a multi-ports USB for battery charging, the charge current setting and charge efficiency is a trade-off. If the charge current setting is low, the charge time will be long so that the charge efficiency is low. If the charge current setting is high in order for short charge time and high charge efficiency, the power supply will be easily overloading when other device also uses other port of the USB at the same time.  
         [0004]     Therefore, it is desired an automatic switch method and apparatus for a USB.  
       SUMMARY OF THE INVENTION  
       [0005]     An object of the present invention is to provide an automatic switch method and apparatus for a USB to identify the type of the USB.  
         [0006]     Another object of the present invention is to provide an automatic switch method and apparatus for a USB to dynamically set the output current of the USB under monitoring the voltage on the USB.  
         [0007]     In an automatic switch method and apparatus, according to the present invention, a voltage monitor circuit monitors a voltage on the USB to determine a voltage state signal, and a current control circuit controls an output current of the USB based on the voltage state signal. Preferably, the current control circuit includes a current switch connected between the USB and system, and a current setting controller for controlling the current switch to set the output current for the USB based on the voltage state signal. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0008]     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:  
         [0009]      FIG. 1  shows an automatic switch apparatus for a USB according to the present invention;  
         [0010]      FIG. 2  shows variations of the USB voltage and USB current when the USB shown in  FIG. 1  is a lower current type;  
         [0011]      FIG. 3  shows variations of the USB voltage and USB current when the USB shown in  FIG. 1  is a higher current type;  
         [0012]      FIG. 4  shows variations of the USB voltage and USB current of the circuit shown in  FIG. 1  during its operation; and  
         [0013]      FIG. 5  shows a flowchart of setting the USB current according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]      FIG. 1  shows an automatic switch apparatus  10  for a USB according to the present invention, in which a voltage monitor circuit  12  monitors the voltage V on the USB for generating a voltage state signal S 1 , and a current control circuit  14  is connected between the USB and a system  16  to control the output current Io of the USB based on the voltage state signal S 1 . In the voltage monitor circuit  12 , two resistances R 1  and R 2  are connected between the USB voltage V and ground GND to serve as a voltage divider for dividing the USB voltage V to produce a detected voltage Vd, and a comparator  122  compares the detected voltage Vd with a reference voltage VREF to determine the voltage state signal S 1 . In the current control circuit  14 , a PMOS transistor  144  is used as a current switch connected between the USB and system  16 , and a current setting controller  142  controls the PMOS transistor  144  based on the voltage state signal S 1  to set the USB current Io flowing therethrough.  
         [0015]     When the system  16  is connected to the USB through the automatic switch apparatus  10  at beginning, the current setting controller  142  controls the PMOS transistor  144  for the output current Io to slowly increase, and during this time period, the voltage monitor circuit  12  monitors the voltage V on the USB. As the output current Io increases, the USB voltage V may gradually decrease. As shown in  FIG. 2 , if the USB voltage V decreases to reach a threshold voltage Vth, i.e., the detected voltage Vd reaches the reference voltage VREF, before the output current Io reaches a threshold current, for example 500 mA, it is indicated that the USB is a lower current type, for example a 100 mA type. Therefore, the automatic switch apparatus  10  will set the output current Io at a lower level, for example 100 mA. On the contrary, as shown in  FIG. 3 , if the USB voltage V is always higher than the threshold voltage Vth until the output current Io reaches 500 mA, it is indicated that the USB is a 500 mA type or higher current type. Therefore, the automatic switch apparatus  10  will set the output current Io at a higher level, for example 500 mA. In other words, the automatic switch apparatus  10  may automatically set a suitable level for the output current Io of the USB depending on the identified type of the USB.  
         [0016]     During the USB operates, the automatic switch apparatus  10  continuously detects the voltage V on the USB. Referring to  FIGS. 4 and 5 , once the USB voltage V decreases to or lower than the threshold voltage Vth for some reasons, for example in the circumstances that the USB is a multi-ports USB and there is one or more devices are connected to other port or ports of the USB to drain so much current therefrom to cause the USB voltage V to drop down, as indicated at time t 1  of  FIG. 4 , the automatic switch apparatus  10  will reduce the output current Io to a predetermined value, for example 0 mA, as indicated by step  20  of  FIG. 5 . Then, in step  22 , the automatic switch apparatus  10  slowly increases the output current Io under monitoring the USB voltage V. If the USB voltage V decreases to reach the threshold voltage Vth, as indicated at time t 2  in  FIG. 4 , before the output current Io reaches 500 mA, it goes to step  24  to reduce the output current Io until the USB voltage V increases to reach the minimum voltage Vmin for normal operation, as shown in  FIG. 4 . After a time period, the step  22  repeats. If the USB voltage V is never down to the threshold voltage Vth until the output current Io reaches 500 mA in the step  22 , for example at time t 3  in  FIG. 4 , it goes to step  26  to set the output current Io at 500 mA and then continuously monitors the USB voltage V in step  28 . Once the USB voltage V is down to the threshold voltage Vth again, for example at time t 4  in  FIG. 4 , it goes to the step  24  or step  20 . If the USB voltage V remains higher than the threshold voltage Vth in the step  28 , the setting for the output current Io will not change, i.e., always at 500 mA.  
         [0017]     The automatic switch apparatus  10  can not only automatically identify the type of the USB and sets a suitable level for the USB current Io accordingly, but also dynamically set another suitable level for the USB current Io according to the variations of the USB voltage V in the operation of the USB. Therefore, the USB will never breakdown resulted from overload and high efficiency operation may be maintained simultaneously.  
         [0018]     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.