Abstract:
Circuits and methods for current limited DC-to-DC converters having multiple power sources have been disclosed. One of multiple power sources is activated by a multiplexer. A current sensing unit, deployed close to the multiplexer, detects if an input current to the DC-to-DC converter has reached a current limit. In case the current limit is reached a signal is sent to the DC-to-DC converter to reduce the current drawn the DC-to-DC converter.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention relates generally to the field of DC-to-DC converters and relates more specifically to current limited DC-to-DC converters having multiple power sources. 
     (2) Description of the Prior Art 
     The performance of DC-to-DC converters is key for mobile electronic devices. DC-to-DC converters are often used to supply power for electronic devices as well as charging batteries at the same time if possible, i.e. if enough energy is available for both electronic devices and batteries. A limit to the input current of such DC-to-DC converters protects the power sources supplying the input for the DC-to-DC converter and the DC-to-DC converter itself and smoothes the output current of the converter. 
     Every modern integrated power management system has to be able to accommodate for a broad range of voltage sources (USB, 5V wall adapter, Firewire, automotive battery). Each of them comes in a variety of output specs, in particular regarding nominal output voltage and maximum current capability. The power management unit (PMU) has to guarantee that in every circumstance the load seen by the power source is within the specified ranges. This is generally done imposing a current limitation on the PMU according to the kind of power source connected to it. 
     Multiple power sources can be used to achieve a more stable operation of a DC-to-DC converter. 
     There are patents or patent publications dealing with the operation of DC-to-DC converters using multiple power sources. 
     U.S. Patent Application Publication (US 2007/0018502 to Bazinet) discloses a system for supplying power from multiple power sources to a powered device having first and second input power supplies for respectively providing power from first and second power sources. An input selector circuit is responsive to the first and second input power supplies for producing an input power supply signal provided to a power regulator, such as a DC-DC converter, for generating a regulated output power supply signal. The power regulator includes a first transistor device controlled to support conversion of the input power supply signal into the output power supply signal if the input power supply signal is provided by the first input power supply, and a second transistor device controlled to support conversion of the input power supply signal into the output power supply signal if the input power supply signal is provided by the second input power supply. 
     U.S. Pat. No. 7,132,764 to Kumar et al. proposes an apparatus and method for regulating multiple input voltages for a microelectronics device. The apparatus includes a controller configured to maintain each of the multiple input voltages supplied to one or more microelectronics devices within a range. The controller may be further configured to maintain input voltage according to a lower limit providing a wider regulation window for the power supply to operate without sacrificing performance for the microelectronics device or degrading its reliability. In one embodiment, the controller is configured to statically or dynamically adjust a gain factor for of a feedback loop in order to maintain the device input voltage according to a regulating loadline. 
     U.S. Pat. No. 6,522,190 to Malik et al. discloses a highly efficient power supply with redundant multiple input voltage sources. The power supply uses switching transistors, specifically MOSFET&#39;s, to create paths for current from one of the voltage sources to the load. The switching transistors are switched either “on” or “off” by comparators which compare the output from the voltage sources. These comparators allow the highest voltage source to provide power to the load, and keep the other switching transistors “off” that connect the common load to other voltage sources. Because the switching transistors have lower conduction losses than diodes in conventional power supplies, the power supply in accordance with the present invention is more efficient. 
     SUMMARY OF THE INVENTION 
     A principal object of the present invention is to achieve a current limited DC-to-DC converter providing a smooth input current limit. 
     A further object of the present invention is to integrate current sensing functionality into power multiplexing for multiple power supply input applications. 
     A further object of the present invention is to achieve a current limited DC-to-DC converter having an improved performance 
     Moreover a further object of the present invention is to achieve a smooth input current sensing. 
     In accordance with the objects of this invention a method for improving the performance of current limited DC-to-DC converters has been achieved. The method invented comprises, firstly, the steps of: (1) providing a DC-to-DC converter, a multiplexer switch unit connected to the DC-to-DC converter capable to activate one of multiple power sources at a point of time having integrated means to sense active input currents, and a means to detect if an input current reaches a current limit, (2) sensing available power sources, and (3) selecting one of the power sources having a voltage above a voltage threshold by closing a corresponding multiplexer switch, Furthermore the method comprises (4) sensing an input current of the DC-to-DC converter from the power source selected, (5) checking if the input current from the power source selected reaches its current limit and, if so, go to step (6), otherwise go to step (4), and (6) reducing the input current drawn by the DC-to-DC converter and go to step (4). 
     In accordance with the objects of this invention a system to convert DC-to-DC voltage having a current limit and improved performance has been achieved. The system invented comprises: a multiplexing switch unit having multiple inputs and outputs, wherein each input is connected to a power source and a first output is a power output to a DC-to-DC converter and a second output delivers a signal to the DC-to-DC converter if an input current exceeds a maximum limit, wherein the multiplexing switch unit comprises current sensing means attached to each switch of the switch unit, and wherein only one of the switches is closed forming an active power path through said multiplexer, said current sensing means, wherein the sensing means that is connected to said active power path initiates, in case a current through said current sensing unit reaches a maximum current limit, a signal to the DC-to-DC converter to reduce a current drawn by a load of the DC-to-DC converter, and said DC-to-DC converter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings forming a material part of this description, there is shown: 
         FIG. 1   a  illustrates a block diagram of the basic functions of the power supply of a DC-to-DC converter. 
         FIG. 1   b  illustrates an alternative block diagram of the basic functions of the power supply of a DC-to-DC converter. 
         FIG. 2  illustrates in more detail a preferred embodiment of a current sensing unit and related power switch. 
         FIG. 3  illustrates a flowchart of a method invented to control the input current of a DC-to-DC converter. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments disclose methods and systems to achieve a DC-to-DC converter providing a well-regulated input current and a high performance. The DC-to-DC converter of the present invention is characterized by having more than one power sources, a multiplexer connecting the most suitable power source to the DC-to-DC converter via a current sensing unit of each power path of a multiplexer. The invention is applicable to all kinds of DC-to-DC converters, such as switched mode converters and linear converters. 
     The invention can be applied to current limited switched converters as e.g. buck converters, boost converters, or buck/boost converters as ell as to other types of current limited types of DC-to-DC converters as e.g. LDO converters. A typical application of the present invention is related to non-regulated power sources that have strict maximum current limitations, i.e. a current drawn from them must not exceed a specified value. 
       FIG. 1   a  illustrates a block diagram of the basic functions of the present invention. Provided are two power sources  1  and  2 , e.g. USB and firewire. It should be noted that more than two power sources could also be used with the present invention. Furthermore  FIG. 1   a  shows a multiplexing switching unit  3  having two main switches  4  and  5  wherein each main switch is connected to a power source and to an input current sensing unit  6 . Only one power source  1  or  2  is exclusively connected via multiplexing main switches  4  or  5  and via a corresponding input current sensing unit  6  to a DC-to-DC converter  7  having an input port  9  and an output port  8 . A connection  11  is provided between the current sensing units  6  and the converter  7  to limit the load current. 
     An important feature of the invention is that a input current sensing functionality  6  is integrated in the power multiplexing block  3  for different input applications, thus providing a smoother current sensing than in the switching pass device of the DC-to-DC converter  7 , which is burdened with a lot of noise. 
     In case the current through an active power path, which is formed by a closed switch of the multiplexing switch unit  3  exceeds a defined maximum current limit the correspondent current sensing unit  6  initiates a reduction of the correspondent input current limit via a current limit control of the DC-to-DC converter  7 . 
     It should be understood that  FIG. 1   a  shows only an example of an embodiment of the present invention. The invention could be applied to more than two power sources as well. 
     It has to be noted that moving the current sensing to the input side of the DC-to-DC converter relaxes the design requirements and introduces an inherent filtering of the current sensing information and improves the behavior of the buck in current limit operation. 
       FIG. 1   b  illustrates an alternative block diagram of the basic functions of the power supply of a DC-to-DC converter. The difference to the block diagram shown in  FIG. 1   a  is that only one current sense unit  6  is required and the selection of the different power sources  1  or  2  is performed in the multiplexer unit  3 . 
       FIG. 2  illustrates in more detail a preferred embodiment of a current sensing unit and related power switch, i.e. the area  10  circled the a dotted line of  FIG. 1 , and how it can be integrated with the other components. The embodiment of  FIG. 2  shows an active power path from power source  2  i.e. switch  5  is closed and switch  4  is open. 
     The switching/current sensing unit of  FIG. 2  comprises a main pass device  20  and a sensing transistor  21 . A programmable current source  23  generates a reference current I ref . The reference current I ref  can be set by a digital control unit  24 . A voltage comparator  22  compares the drain source voltages of the main pass device  20  and of the sensing transistor  21 . The main pass device  20  is furthermore connected to the input  9  of the DC-to-DC converter. The sensing transistor  21  is matched and scaled in regard to the main pass device  20 , in a preferred embodiment it is scaled in a ratio of 10000:1. 
     It should be noted that each power path has its own main pass device  20  and sensing transistor  21 . The reference current source  23 , the comparator  22  and the digital control can be shared between all power paths. 
     Assuming that e.g. a current limit of I LIMIT =100 mA for the input current I in  is defined and that a scale factor between the main pass device and the sensing transistor is 10000:1. Then, according to the scale factor of 10000:1, a reference current I REF =10 uA is imposed across sensing transistor  21 . In this way the comparator  22  will trip when the source-drain voltages across the main switch  20  and the sense transistor  21  are be the same, i.e. when the current across the main switch is I MAIN =10000*I REF =100 mA, i.e. correspondent to the defined current limit I LIMIT . An according signal is then sent to the DC-to-DC converter  7  in order to activate a reduction of the current drawn from the input. 
     Initially a power path is selected depending on which port a power source is detected. Normally this is implemented via a voltage comparator that checks if the voltage on the port has exceeded a specified threshold. Each power source is compared to a threshold and, if higher, the corresponding switch is closed. If multiple power sources are above threshold priority is given to one of them. 
       FIG. 3  illustrates a flowchart of a method invented to improve the performance of DC-to-DC converters having multiple input power sources. A first step  30  describes the provision of a DC-to-DC converter, a multiplexer switch unit connected to the DC-to-DC converter capable to activate one of multiple power sources at a point of time having integrated means to sense active input currents, and a means to detect if an input current reaches a current limit. In the following step  31  available power sources are sensed and step  32  describes a selection of one of the power sources having a voltage above a voltage threshold by closing a corresponding multiplexer switch. In step  33  the input current of the DC-to-DC converter, provided by the power source selected, is sensed The following step  34  is a check if the input current from the power source selected has reached a defined current limit and, if so, the process flow goes to step  35 , otherwise the process flow goes back to step  33  again. In step  35  the current drawn by the DC-to-DC converter is reduced. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.