Patent Publication Number: US-2007114849-A1

Title: Electrical load with preferential source

Description:
FIELD OF THE INVENTION  
      The present invention relates generally to internally powered devices with optional alternative power sources.  
     BACKGROUND OF THE INVENTION  
      Battery powered devices are designed to accept power from alternative power sources. Such devices can include, for example, Personal Data Assistants (PDA), media players, digital cameras, mobile telecommunication devices, motors, and any other device or apparatus which is battery operated and can also receive power from an external power source. Such devices can be powered from a battery when other power sources are not available, for example, when the device is not connected to the external power source, or when the external power source is not available. Handheld devices are typically not connected to external power sources during travel or use. Other devices are sometimes connected to external power sources when used, for example wireless telephones employing a cradle for charging the battery. Some devices normally connected to external power sources will use power drawn from a battery when the external power source is not available, for example during a power outage and the like. Typically, the external power source is a DC power source powered from an AC electric socket to charge the device&#39;s battery and/or to power the device from the external power source. Powering devices from an external power source, or a power source other than the battery, conserve battery charge and improve longevity. Some devices include internal charging mechanisms, which allow charging the battery while powering the device from the external power source.  
      In many cases, when the device is powered from an external power source it disconnects the option of drawing power from the battery. Therefore, presently available devices will draw power from the external power source or from the battery, but not from both. In some cases an external power source has limitations on the amount of power it can supply. Thus, a device that requires more power than the external power source can supply, cannot be powered by the external power source, even if the device only occasionally has peak requirements which exceed the limits of the external power source. The Universal Serial Bus (USB) interface is an example of such an external power source. Some devices use the USB interface to connect peripherals and communicate with a computer but do not operate the device by drawing power from the USB port since their requirements exceed the port&#39;s capability. For example a USB 2.0 port is limited to providing 100 mA for a normal device and 500mA for a high power device. Typically such devices continue to use batteries even when connected to an external USB power source or require a connection to an additional external power source aside the USB connection.  
      Another example of a device that makes use of an external power source and a battery is an Uninterruptible Power Source (UPS) used as a back up power device for computing and other supported devices. When the main power source stops supplying power or fails to correctly supply a parameter of the required power, for example less than the required voltage or an unstable voltage or current frequency, the UPS will switch between the external power source and the battery and provide power to the supported device with battery power. In such a case the battery power source takes charge and provides the required power to the load instead of the external power source. When the main power source is functioning properly the battery is recharged by the main power source in parallel to the supply of power to power the load. A UPS directs the power to power the supported device either from the external power source (main) or the internal power source (battery) and does not complement the output power from the external power source with power from the internal source.  
      Likewise, in motorized vehicles during normal usage, power is generally supplied from a motor powered alternator or a rechargeable battery. Typically, excess charge from the alternator is used to recharge the battery and a deficiency of charge is complemented by the battery. Typically a control circuit is used to monitor the battery voltage and control the alternator output to prevent over charging the battery. However, it should be noted that the power source of the load automatically draws power from the available resources and is not controlled. Some batteries such as lead batteries can be charged freely, in contrast other batteries such as Li-Ion and Ni-Mh are limited in the number of charges and they require an accurate charging profile, for example taking into account the discharge level and controlling the current and voltage of the charge with dependence on time and temperature.  
      U.S. Pat. No. 4,104,539 to Hase describes a parallel redundant and load sharing regulated AC system. The system described has two power sources, a commercial (main) power source and an inverter. The two power sources share a load approximately equally. The inverter assumes the load if line quality is out of predetermined limits. On the other hand, if the inverter fails the commercial power line assumes the whole load. U.S. Pat. No. 6,236,582 to Jalaleddine describes a load share controller for balancing current between multiple supply modules. U.S. Pat. No. 4,359,679 to Regan describes a switching DC regulator and load sharing system for multiple regulators. U.S. Pat. No. 4,766,364 to Biamonte et al. discloses a parallel power system comprising a plurality of voltage regulating power supplies connected in a master slave configuration, the number of regulators being one greater than required to provide load current requirements. The master regulator generates a control signal to control the output of the individual slave regulators to provide balanced load sharing.  
      Typically, prior art devices use either the internal power source (e.g. battery) or the external power source. If the external source cannot supply the full power requirements it is not used. None of the prior art references disclose an apparatus and method for using external power sources first and complementing the supply of power to the powered device from the internal power source, when necessary. There is therefore a need in the art for a device with a regulator circuit that provides a regulated supply of power at its output for powering a load.  
     SUMMARY OF THE INVENTION  
      An aspect of an embodiment of the invention relates to a device with a regulator circuit that provides a regulated supply of power at its output. The regulator circuit inputs power from at least two power sources. One of the sources is an internal power source (e.g. a battery), which is optionally able to provide sufficient power to generally power the device by itself. The second power source is external to the device. The regulator circuit monitors the power at the output to ensure a continuous supply satisfying the load. The regulator circuit provides preference in using the power supplied by the external power source, to conserve the power of the internal power source. The internal power source complements the power supplied by the external power source to enable provision of the required power at the output of the regulator circuit.  
      In an exemplary embodiment of the invention, the external power source provides sufficient power to power the regulated output therefore substantially no power is used from the internal power source when the external power source is available.  
      In an exemplary embodiment of the invention, the external power source does not provide any power therefore all of the power to power the regulated output is provided by the internal power source.  
      In an exemplary embodiment of the invention, the external power source is able to provide only some of the power required to power the regulated output and the rest is provided by the internal power source.  
      In an exemplary embodiment of the invention, the power output by the regulator circuit is not affected by the source of the power.  
      In some embodiments of the invention, the internal power source is a disposable battery. Alternatively, the internal power source is a rechargeable battery.  
      There is thus provided according to an exemplary embodiment of the invention, a regulator circuit, embedded in a device, which is adapted to draw power from a power source internal to the device and a power source external to the device, the regulator circuit comprising: 
      a first circuit segment for regulating power supplied by the internal power source;     a second circuit segment for regulating power supplied by the external power source;     an output circuit segment that monitors the output of the regulator circuit and supplies regulated power to the device; and     wherein responsive to the monitoring the regulator circuit preferentially draws power from the second circuit segment and complements the drawn power with power from the first circuit segment to maintain a regulated power supply at said output.    

      In an exemplary embodiment of the invention, the regulator circuit powers the device from the internal power source when an external power source is not connected. Optionally, the external power source is unable to provide sufficient power to power the device in some cases.  
      In an exemplary embodiment of the invention, the second circuit segment provides regulated voltage with a current limit. Optionally, the regulator circuit has a different reference voltage when providing power only from the external power source and when providing power in conjunction with power from the internal power source.  
      In an exemplary embodiment of the invention, the output circuit segment provides power of a substantially constant voltage regardless of the power source used to supply the power. Optionally, the output circuit segment provides power of different voltages dependent on the sources providing the power.  
      In an exemplary embodiment of the invention, fluctuations in the voltage provided by said output circuit segment are less than a predetermined percent of the magnitude of the provided voltage. Optionally, the power supplied by the external power source is direct current.  
      In an exemplary embodiment of the invention, the regulator circuit comprises multiple outputs drawing current from the same sources. Optionally, the multiple outputs provide different voltage levels.  
      In an exemplary embodiment of the invention, power from the internal source is used to complement the power from the external source only if the sums of the currents drawn by the multiple outputs exceed the current limit of the external source. Optionally, the internal power source is a disposable battery.  
      In an exemplary embodiment of the invention, the monitoring is performed by comparing the output voltage to a reference voltage. Optionally, the output provides direct current.  
      There is thus additionally provided according to an exemplary embodiment of the invention, a method of powering a device from an internal and external power source while conserving charge of the internal power source, comprising:  
      regulating the power supplied by the internal power source and the external power source;  
      monitoring the power supplied for powering the device;  
      attempting to draw all the power required by the device from the external power source;  
      compensating for unavailable power required by the device from the external power source by drawing power from the internal power source; and  
      combining the power from the external power source with the power from the internal power source to supply power to power the device.  
      In an exemplary embodiment of the invention, the external power source is unable to supply all the power required to power the device. Optionally, the external power source provides a constant voltage regardless of the external load.  
      In an exemplary embodiment of the invention, the output provides power of different voltages dependent on the power sources providing the power. Optionally, fluctuations in the voltage provided by said output are less than a predetermined percent of the magnitude of the provided voltage.  
      In an exemplary embodiment of the invention, the internal power source is a disposable battery. Alternatively, the internal power source is a rechargeable battery.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings. Identical structures, elements or parts, which appear in more than one figure, are generally labeled with a same or similar number in all the figures in which they appear, wherein:  
       FIG. 1  is a schematic illustration of a device with a regulator circuit, according to an exemplary embodiment of the invention;  
       FIG. 2  is a schematic diagram of a power regulating circuit, according to an exemplary embodiment of the invention;  
       FIG. 3  is a timing diagram, according to an exemplary embodiment of the invention;  
       FIG. 4  is a schematic diagram of an alternative power regulating circuit, according to an exemplary embodiment of the invention;  
       FIG. 5  is a timing diagram, based on an alternative regulating circuit according to an exemplary embodiment of the invention;  
       FIG. 6  is a schematic diagram of a power regulating circuit providing multiple outputs for multiple loads, according to an exemplary embodiment of the invention; and  
       FIG. 7  is a schematic diagram of a current regulator for a power regulating circuit with multiple outputs, according to an exemplary embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention discloses a new and novel apparatus and method for conserving the power in an internal power source of a device when an external power source is available, by using whatever power is available from the external power source first and complementing the supply of power to the device from the internal power source, if and when necessary. The invention discloses a device with a regulator circuit that provides a regulated supply of power at its output for powering a load. The regulator circuit inputs power from at least two power sources. One of the sources is an internal power source, which is optionally able to provide sufficient power to power the device by itself. Such an internal power source can be a battery, optionally a rechargeable battery. The second power source is external to the device, such as for example, an AC power source coupled to an AC/DC converter, power provided by a USB output of a device or solar cells, which sometimes can contribute to powering the device . The regulator circuit monitors the power at the output to ensure a continuous supply satisfying the load. The regulator circuit gives preference in using the power supplied by the external power source, to conserve the power of the internal power source. The internal power source is used to complement the power supplied by the external power source to enable provision of the required power at the output of the regulator circuit.  
       FIG. 1  is a schematic illustration of a device  100  with a power regulating circuit  120 , according to an exemplary embodiment of the invention. In an exemplary embodiment of the invention, device  100  is a portable electronic device, for example an MP3 player, a cordless telephone base unit, a digital camera, a portable personal computer, or other devices. Optionally device  100  comprises a logic circuit  140 , for providing device functionality. In an exemplary embodiment of the invention, device  100  comprises an internal power source  130  to provide electric current to power the device. In an exemplary embodiment of the invention, device  100  is adapted to accept power from an external power source in addition to internal power source  130 . Optionally, power regulating circuit  120  accepts electrical power from internal power source  130  and the external power source simultaneously to provide regulated power to logic circuit  140 . In an exemplary embodiment of the invention, power regulating circuit  140  provides preference to drawing power from the external power source over internal power source  130 , wherein the internal power source complements for inability of the external power source to provide the power requirements of device  100 . In some embodiments of the invention, the external power source provides AC current or DC current. Optionally, AC current is converted to DC current before being handled by power regulating circuit  120 . In some embodiments of the invention, internal power source  130  is a disposable battery. Alternatively, internal power source  130  can be a rechargeable battery that is recharged external to device  100 . In some embodiments of the invention, the internal power source is a mechanical or chemical source (e.g. based on fuel). In some embodiments of the invention, internal power source  130  is recharged by excess electric power when device  100  is attached to an external power source. Optionally, internal power source  130  provides a constant DC voltage. In an exemplary embodiment of the invention, device  100  is powered by internal power source  130  while a user is using device  100  as a portable device. Optionally, when the user is using device  100  in a supporting environment, for example at home or in an office, device  100  can be attached to an external power source, for example plugged into an electric socket (mains), to power the device and conserve battery charge. In some embodiments of the invention, device  100  requires a connection (constantly, periodically, frequently or non-frequently) to an external device (e.g. a computing device), for information exchange (e.g. transferring or backing up data), for functionality of the device, for example an Internet connection for an IP phone, or other reasons. Optionally, during the time that device  100  is connected to an external device, device  100  is optionally adapted to draw electric power from the external device through the connection, in order to conserve the charge of internal power source  130 . In an exemplary embodiment of the invention, as shown in  FIG. 1 , device  100  is connected by a cable  150  to an external device, for example a computer  160 . In some embodiments of the invention, device  100  is connected to the USB port of the computer or the serial or parallel port. Alternatively, device  100  may be connected to a 1394 port, PS2 port (e.g. the mouse port or the keyboard port or to other standard or non-standard ports of computer  160 .) Typically, many computer ports supply a voltage line, for example +5V, to power attached devices. In some cases the amount of power (e.g., current) drawn from this port is limited, such as in the case of USB were a current limit depends on the state of operation and the type of device, for example 100 mA for a ‘standard’ devices or 500 mA for a high-power devices.  
       FIG. 2  is a schematic diagram of a power regulating circuit  200 , according to an exemplary embodiment of the invention. As shown in  FIG. 2  power regulating circuit  200  accepts power from two sources: an external power source  215  and internal power source  130 . A circuit segment  210  accepts the power from external power source  215  and provides it to a load  240 , for example logic circuit  140 . Optionally, circuit segment  210  is a constant voltage current limiting circuit, for providing current to load  240 . In an exemplary embodiment of the invention, a circuit segment  220  comprises internal power source  130  for providing power to load  240 . In an exemplary embodiment of the invention, a circuit segment  230  comprises a comparator  235  that compares between a voltage related to the voltage supplied by the output of power regulator circuit  200  (e.g., Vout or a value proportional to Vout, such as via a resistor divider) and a reference voltage (Vref 13  c) of power regulating circuit  200 , in order to control division of the supply of power by the two circuit segments ( 210 ,  220 ) of power regulating circuit  200 . In an exemplary embodiment of the invention, Vref 13  c is defined so that comparator  235  will be activated when Vout drops below the desired voltage by a preset amount. Optionally, Vref 13 c is defined to be Vref 13  c=Vk−G*(Vout 13 min−Vref), where Vk is the voltage at the output of an amplifier  255 , which amplifies voltage by the value G. Optionally, when Vout=Vref, Vout 13  min is the allowed drop of the output. Optionally, if amplifier  255  has a high value, the accuracy of Vref 13  c is not critical and may be set to a voltage within the dynamic range between Vk and Vref.  
      The function of power regulating circuit  200  will be clear from a discussion of its voltage output relative to the current provided to a load.  FIG. 3  is a timing diagram  300  illustrating the function of power regulating circuit  200  of  FIG. 2 , according to an exemplary embodiment of the invention. Power regulating circuit  200  of  FIG. 2  is initially in a first mode (M=0), designated by the output M of comparator  235 . In an exemplary embodiment of the invention, load  240  draws an increasing current from power regulating circuit  200  as shown by line  322  of graph  320  in timing diagram  300 . Initially, circuit segment  210  provides the full current demand of load  240  from the current provided by external power source  215 . Graph  310  in timing diagram  300  shows the voltage as a function of time responsive to the current demands shown in graph  320 . As the current demand of load  240  rises and draws near to the current limitation of circuit segment  210  (designated by Imax in  FIG. 3 ), the voltage provided by circuit segment  210  begins to drop to allow the supply of the current demand. In an exemplary embodiment of the invention, comparator  235  receives on one input a voltage proportional to the difference between Vout and a voltage V 1 . Voltage V 1  controls the output voltage of circuit segment  210 . Optionally, voltage V 1  depends on the output of comparator  235 . Initially M=0 and V 1  is equal to Vref. When M=1, V 1  is equal to Vref+dv. It should be noted that the value of dv is selected based on the output voltage deviation allowed for Vout. Additionally, the value of dv is selected to be larger than normal voltage variations at the output due to small variations in the load, noise or other disturbances. In the shown embodiment, at time t 1 , when the current demand reaches and begins to exceed Imax, circuit segment  210  responds by lowering its output voltage (due to the current limit blocking a current increase from circuit segment  210 ). As a result, at time t 1 +dt, the output voltage provided by circuit segment  210  drops below Vout 13  min, comparator  235  is triggered, and the output M of comparator  235  is set to one (M= 1 ) causing circuit segment  230  to change to a second mode. In an exemplary embodiment of the invention, when changing to the second mode (M=1) circuit segment  230  raises the value of V 1  by dv, in order to assure that power regulator circuit  200  will remain in the second state (M=1) until the current demands of load  240  are reduced below Imax and the voltage provided by circuit segment  210  goes back to its initial value. In the second mode, circuit segment  220  is enabled to provide current from internal power source  130  to compensate the shortage in the current supplied from external power source  215 . In the shown embodiment, the power supplied by circuit segment  220  raises the voltage on load  240  (Vout) substantially back to the initial voltage on load  240  before circuit segment  210  reduced its contribution due to the current limit. At this stage a DC-DC converter  250  in circuit segment  220  operates as a voltage regulator, were its control loop attempts to keep the voltage on load  240  stable substantially at the value of Vref. Initially, when changing to the second mode circuit segment  210  provides a current at the level of Imax by operating as a current source, since its reference voltage is raised to Vref+dv. In an exemplary embodiment of the invention, device  100  is designed to be able to be powered by internal power source  130  to allow portability or use without the need to be connected to an external power source. Therefore any power requirement by load  240  can be supplied by circuit segment  220  if not satisfied by the provision of circuit segment  210 , unless internal power source  130  is depleted and needs to be recharged or replaced.  
      In an exemplary embodiment of the invention, DC-DC converter  250  of circuit segment  220  is in an enabled state before circuit segment  220  contributes current to load  240 , for example in order to provide Vref or other control voltages. Optionally, when power regulator circuit  200  changes to the second mode, circuit segment  220  enables its output (e.g. enabling transistors controlling the output), and immediately provides current without a delay that is common in voltage regulator feedback loops (such a delay is typically used to prevent oscillations of the circuit on one hand, but limit the response to sharp changed at the output voltage on the other hand.  
      In an exemplary embodiment of the invention, at time t 2 , load  240  changes in a way that reduces current consumption as shown by line  324  in graph  320  of diagram  300 . Optionally, the current supplied by circuit segment  220  which is compensating the current supplied by circuit segment  210  begins to decrease until circuit segment  210  is able to supply the current on its own (as noted before circuit  210  operates as a current source in this mode, thus provide a fixed amount of current to the load). In an exemplary embodiment of the invention, as the current drawn by load  240  is reduced to the level of Imax, the voltage on load  240  (Vout) begins to rise, since circuit segment  220  provides a substantially constant voltage contribution (e.g. Vref) and includes elements that prevent a reverse current flow from load  240  into elements of circuit segment  220  (e.g. a reverse current sense circuit that blocks the transistors gates). Optionally, at time t 3  circuit segment  210  provides a constant current (Imax), that is above the consumption of load  240 , causing an increase in the output voltage value (by up to dv above Vref), to which the reference voltage on comparator  235  is set for.  
      Optionally at time t 3 +dt Vout and V 1  are very close in value (e.g., both are substantially at Vref+dv) triggering comparator  235  setting M back to zero (M=0) and returning circuit segment  230  to the first mode. Optionally, the switching point is determined by Vref, offset dv and an optional threshold value of the comparator for switching back to the first mode. Optionally, using a comparator with hysteresis the changing value is set close to V 1 =Vref+dv. When changing to the first mode circuit segment  220  is disabled and V 1  returns back to its initial value (Vref). In an exemplary embodiment of the invention, circuit  200  provides a voltage output with a substantially constant value, for example approximately 5V, 3.3V, 3.0V, 1.8V or 1.5V or other values. Optionally, the value of dv used in various places in the circuit may vary from case to case in the circuit, however in all the circuit dv is generally of the same magnitude and is relatively small in comparison to Vref and Vout, for example less than a predetermined percent of these voltage values (e.g. 10%, 5%, 2% or 1%). In an exemplary embodiment of the invention, if no power is available from external power source  215 , circuit segment  230  would change to mode M=1, since there is a difference between the output voltage of circuit segment  210  (0V) and the reference voltage. Optionally, being in mode M=1 enables circuit segment  220 , so that internal power source  130  provides power for load  240 . In an exemplary embodiment of the invention, external power source  215  provides the full power requirement of device  100  so that substantially no power is required from internal power source  130 . In some embodiments of the invention, external power source  215  provides unstable power, for example a source that has fluctuations in the available current or a source that goes on and off.  
      Optionally, many alternative circuits can be implemented, for example without changing the base voltage supplied to amplifier  255  between two modes.  FIG. 4  is a schematic diagram of an alternative power regulating circuit  400 , according to an exemplary embodiment of the invention. Similar to power regulating circuit  200 , power regulating circuit  400  comprises a circuit segment  410 , which provides power from an external source which is voltage regulated with a current limit; a circuit segment  420 , which provides power from internal power source  130 , and a circuit segment  430 , which compares the voltage supplied by circuit segment  410  with a reference voltage provided by internal power source  130 . In an exemplary embodiment of the invention, circuit segment  430  compares between voltages related to Vout and the reference voltage of power regulator circuit  400 , in order to control division of power supply from the two circuit segments ( 410 ,  420 ). In power regulating circuit  400  the reference voltage V 1  used to drive circuit segment  410  and indirectly also comparator  235 , is set at a constant value, which is optionally a little higher (by a small value dv as described above) than the reference voltage of power regulating circuit  400 . In contrast in power regulating circuit  200  the reference voltage used by comparator  235  alternates between Vref and Vref+dv. Optionally, comparator  235  in circuit  400  can be connected to the output of amplifier  255  as in circuit  200  instead of the input to amplifier  255 . Optionally, in such a case the reference voltage of the comparator would be set as described above for circuit  200  (Vref 13  c).  
       FIG. 5  is a timing diagram  500 , according to an exemplary embodiment of the invention. In an exemplary embodiment of the invention, power regulating circuit  400  is initially in a first mode, designated as M=0. In an exemplary embodiment of the invention, load  240  draws current from circuit  400  at an increasing amount as shown by line  522  of graph  520  in timing diagram  500 . Initially, circuit segment  410  provides the full current demand of load  240  from the current provided by external power source  215 . Graph  510  in timing diagram  500  shows the voltage as a function of time responsive to the current demands shown in graph  520 . As current demands of load  240  rise and draw near to the current limit of circuit segment  410  (designated by Imax in  FIG. 5 ), the voltage provided by circuit segment  410  begins to drop as the current limit circuit takes control. In an exemplary embodiment of the invention, at time t 1  in graph  510 , the output voltage provided by circuit segment  410  drops by a preset value (dv), which is defined by the comparator threshold. Comparator  235  is triggered by the voltage drop (dv), causing the output M of comparator  235  to be set to one (M=1) and circuit segment  400  changes to a second mode. In an exemplary embodiment of the invention, in the second mode circuit segment  420  is enabled to provide current from internal power source  130  to compensate the shortage in current supplied from external power source  215 . Optionally, voltage of the power supplied by circuit segment  420  is driven at the reference voltage level of power regulating circuit  400 , thus keeping the voltage output at the level of the reference voltage of power regulating circuit  400 . In an exemplary embodiment of the invention, at time t 2 , load  240  begins to reduce current consumption as shown by line  524  in graph  520  of diagram  500 . The current supplied by circuit segment  420  which is compensating the current supplied by circuit segment  410  begins to decrease until circuit segment  410  is able to provide the required current on its own. At this point the current supplied by circuit segment  420  is zero. In an exemplary embodiment of the invention, as the current drawn by load  240  is reduced further, the voltage on load  240  (Vout) begins to rise, since circuit segment  420  does not sink any current (e.g. by utilizing reverse current detectors as discussed above which disconnect its output transistors), and circuit segment  210  provides a constant current Imax. As a result the voltage supplied by circuit segment  210  rises by up to the value of dv. Once the output voltage reaches Vout=V 1 =Vref+dv as the current drawn from it is reduced below Imax, circuit segment  210  returns to voltage regulation mode of operation, wherein the voltage is stabilized at Vout=V 1 . At that time (noted as t 3 ) the voltage from circuit segment  410  is back at its initial value (reference voltage of power regulating circuit  400  (Vref)+dv). The reduction of the difference in voltage between Vout and the reference value of comparator  235  as defined by the hysteresis of the comparator (which prevents it from vibrating), un-triggers comparator  235  setting M back to zero (M=0). When M is set back to zero, circuit segment  430  returns to the first mode and the output drive of circuit segment  420  is disabled. Note that in the exemplary embodiment, as discussed above, in state M=0, the DC-DC circuit  420  is enabled but the output transistors are disabled. In this state the output is disabled but the control loop of the regulating circuit is active and is at a “working point” of its control loop. This guarantee fast response of the circuit to a change from state M=0 to M=1, without delays associated with its feedback loops. Optionally the comparator  235  may be connected to the output of amplifier G in  410 , similar to the connection shown in  FIG. 2 , with proper adjustment to its thresholds. In an exemplary embodiment of the invention, if no power is available from external power source  215 , circuit segment  430  would change to mode M=1, since there is a big difference between the output voltage of circuit segment  410  (0V) and the comparators reference voltage. Optionally, being in mode M=1 enables circuit segment  420 , so that internal power source  130  provides power for load  240 .  
      In an exemplary embodiment of the invention, power regulating circuit  200  provides a more uniform voltage output than power regulating circuit  400 . Optionally, a device with less tolerance to variation in voltage would prefer power regulating circuit  200  over power regulating circuit  400 . In some embodiments of the invention, circuit  400  is more simplistic than power regulating circuit  200 . In some embodiments of the invention, the voltage difference provided by power regulating circuit  400  is selected to be significant in order to provide a different voltage when powering device  100  from an internal power source and an external power source. In some embodiments of the invention, other circuit layouts are used to control usage of power from available power sources. In some embodiments of the invention, circuit  210  (shown as a linear regulator) is a DC-DC converter circuit. In other embodiments, DC-DC converter  250  is a linear regulator.  
      In some embodiments of the invention, it is desirable to power two or more different loads from the same internal power source and same external power source.  FIG. 6  is a schematic diagram of a power regulating circuit  600  providing multiple outputs for multiple loads, according to an exemplary embodiment of the invention. In an exemplary embodiment of the invention, power regulating circuit  600  may be implemented for example as shown in  FIG. 6 , to provide more than one output port (e.g. Vout 13  a, Vout 13  b) allowing different voltage levels on each port. Optionally, circuit  600  is based on circuits  200  and  400  and created by duplicating the comparator circuit segment sharing a common enhanced internal source and external source. In an exemplary embodiment of the invention, each port allows a load to draw power from circuit  600 , wherein the total power provided by circuit  600  from the “DC IN” is divided between the loads without pre-allocating the available power between the loads. As described above regarding circuits  200  and  400 , circuit  600  preferentially draws power from the external power source (DC IN) and compensates the power output with power from the internal power source, when the total power drawn by both loads exceeds the maximal power that the external power source can provide (IoutA+IoutB&gt;Imax).  
       FIG. 7  is a schematic diagram of a circuit segment  700 , representing an implementation of a current regulator for a power regulating circuit with multiple outputs as shown in  FIG. 6 , according to an exemplary embodiment of the invention. Optionally, circuit segment  700  provides current to two outputs with different voltages (VoutA and VoutB) as shown in the  FIG. 7 . In an exemplary embodiment of the invention, a circuit segment  710  serves as a current source common to both outputs, and circuit segments  720  and  730  serve as voltage regulators for the outputs. The serial connection of circuit segment  710  to circuit segments  720  and  730  implements a minimum function such that VoutA and VoutB are essentially the minimum between the current limit and the voltage limit.  
      It should be noted that  FIG. 6  and  FIG. 7  give the general layout for a regulator circuit providing for multiple loads that draw power from an internal and external source, a complete implementation of such a circuit is easily provided by one skilled in the art.  
      In an exemplary embodiment of the invention, one skilled in the art would be able to apply the above described methods and apparatus to other circuits and power sources dealing with alternating currents. The reference to regulator circuits in the above description refers to all types of, circuits that control a DC output based on an input. These circuits include among others DC-DC regulators and linear regulators.  
      It should further be appreciated that the above described methods and apparatus may be varied in many ways, including omitting or adding steps, changing the order of steps and the type of devices used. It should be appreciated that different features may be combined in different ways. In particular, not all the features shown above in a particular embodiment are necessary in every embodiment of the invention. Further combinations of the above features are also considered to be within the scope of some embodiments of the invention. Section headings are provided for assistance in navigation and should not be considered as necessarily limiting the contents of the section. It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined only by the claims, which follow.