Abstract:
A charge pump provides a multiplication factor of ⅔ by using a three-phase mode of operation. In a first mode, first and second capacitors are charged from an input voltage while a third capacitor drives the output voltage based on stored charge in the third capacitor. In a second mode, the output terminal is connected to the common node of the first and second capacitors. In a third mode, the voltage potential across the second capacitor is subtracted from the sum of the input voltage and the voltage potential across the first capacitor to generate the output voltage. Operated in this manner, the first, second, and third capacitors will provide the desired ⅔× voltage multiplication. This relatively low multiplication factor can be beneficial in applications requiring 2.5V and 1.8V supplies for integrated circuits, particularly where the input voltage is provided by a lithium battery.

Description:
RELATED APPLICATIONS  
       [0001]     This is a continuation-in-part of pending U.S. patent application Ser. No. 11/264,884 filed Nov. 1, 2005, and entitled, “LED Bias Current Control Using Adaptive Fractional Charge Pump” by Sorin S. Georgescu, Anthony G. Russell and Chris Bartholomeusz. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates to the field of electronic circuits, and in particular, to an efficient, low noise fractional charge pump.  
         [0004]     2. Related Art  
         [0005]     Most portable electronic devices contain digital and analog circuits operating at 2.5 Volts or below. However, the battery power used in such devices generally provides a supply voltage that is above the operating voltage of these devices (typically around 3.6 V). For example, a modern rechargeable lithium ion or lithium polymer battery is typically rated to have a nominal output voltage of 3.7 V, but may actually provide a voltage in the range of 2.7 to 4.2 V, depending on the charge state of the battery.  
         [0006]     This variability in battery supply voltage necessitates circuitry to step down the supply voltage to the acceptable level. One of the common schemes is to use a charge pump with multiple capacitors. A charge pump can have 2 capacitors equally dividing the battery voltage.  
         [0007]     An implementation of such type of charge pump is known as a “½×” charge pump.  FIGS. 1A and 1B  are schematic diagrams of a conventional ½× charge pump  100 , which receives an input voltage V_IN 1  and provides a reduced output voltage V_OUT 1  to a load D 140 . Charge pump  100  includes an input terminal  101 , charging capacitors C 110  and C 120 , a storage capacitor C 130 , and an output terminal  102 . While not shown for clarity, charge pump  100  also includes interconnect circuitry for connecting capacitors C 110  and C 120  in the configurations shown in  FIGS. 1A and 1B .  
         [0008]     Charge pump  100  operates by switching between the two phases of operation shown in  FIGS. 1A and 1B . In  FIG. 1A , a charging phase is shown, in which capacitors C 110  and C 120  are serially connected between input terminal  101  and ground, while capacitor C 130  is connected between ground and output terminal  102  (load D 140  is always connected between output terminal  102  and ground). During this charging phase, capacitors C 110  and C 120  are charged by input voltage V_IN 1  to voltages V 11  and V 12 . Under steady state conditions, capacitors C 110  and C 120  will both be charged to half of input voltage V_IN 1  during this charging phase. Meanwhile, a voltage V 13  stored on capacitor C 130  is provided as output voltage V_OUT 1  for driving load D 140 .  
         [0009]     Then, in a discharging phase shown in  FIG. 1B , capacitors C 110  and C 120  are connected in parallel between input terminal  101  and output terminal  102 . Specifically, the positive plate (marked with a triangular indicator) of capacitor C 110  is connected to input terminal  101 , while the negative plate (unmarked) of capacitor C 110  is connected to output terminal  102 . Likewise, during the discharging phase, the positive plate (marked) of capacitor C 120  is connected to the input terminal  101 , while the negative plate (unmarked) of capacitor C 120  is connected to output terminal  102 .  
         [0010]     Because capacitors C 110  and C 120  are inverted and connected in parallel after input terminal  101 , the output voltage V_OUT 1  provided during the discharging phase shown in  FIG. 1B  is equal to the difference of input voltage V_IN 1  and the average of voltages V 11  and V 12  on capacitors C 110  and C 120 , respectively. As described above with respect to  FIG. 1A , both capacitors C 110  and C 120  are charged to half of input voltage V_IN 1  during the charging phase. Therefore, the output voltage V_OUT 1  provided during the discharging phase is simply equal to one half of input voltage V_IN 1  (i.e., 0.5*V_IN 1 ).  
         [0011]     Therefore, the output voltage range of ½× charge pump  100  is between 1.35 V and 2.1 V when provided with a lithium ion battery voltage (i.e., 2.7 V to 4.2 V) as in input voltage.  
         [0012]     As portable devices become increasingly advanced while at the same time shrinking in size, power efficiencies must continually be improved. While ½× charge pump  100  can provide a reduced supply voltage of half the battery voltage, the battery voltage can vary significantly, thereby resulting in significant variation in the reduced supply voltage. For example, the output voltage range of ½× charge pump  100  is between 1.35 V and 2.1 V when provided with a nominal 3.7 Volt lithium ion battery having a voltage range of 2.7 V to 4.2 V as an input voltage. In this case, the desired nominal output voltage is about 1.85 V. Thus, the output voltage provided by ½× charge pump  100  may be significantly below the desired nominal output voltage. In this case, the available battery charge is small and the efficiency is also small. For this reason, ½× charge pump  100  is not ideally suited for use in all applications.  
         [0013]     It would therefore be desirable to have a charge pump capable of applying a multiplication factor greater than ½× and less than 1× to an input voltage. It would also be desirable to have a system and method for stepping down a supply voltage that maximizes power efficiency while minimizing die area requirements.  
       SUMMARY OF THE INVENTION  
       [0014]     Accordingly, the present invention provides a charge pump that applies a ⅔× voltage scaling factor, rather than the conventional 1/1× or ½× scaling factors. As a result, an optimum output voltage can be achieved for a given input voltage, which can beneficially improve power efficiency in situations where conventional charge pumps provide excessive or insufficient voltage multiplication.  
         [0015]     In one embodiment, a ⅔× charge pump can include first, second, and third capacitors, with the third capacitor connected between the output terminal of the charge pump and ground. The first and second capacitors are connected in three different connections to the input terminal of the charge pump during three different phases of operation to provide the ⅔× multiplier function.  
         [0016]     In a charging phase, the first and second capacitors are connected in series between the input terminal and ground, so that the output terminal is driven by the charge stored on the third capacitor. In a first discharging phase, the output terminal is connected to the common node of the first and second capacitors connected in series, so that the voltage provided at the output terminal is the difference of the input voltage and the voltage across the first capacitor.  
         [0017]     Finally, in a second discharging phase, the first and second capacitors are connected between the input terminal and the output terminal, with the first capacitor inverted relative to the input terminal, and the second capacitor having the same orientation as during the charging phase, but connected between the first capacitor and the output terminal. Therefore, the output voltage provided during the second discharging phase is equal to the sum of the input voltage and the voltage potential across the first capacitor, minus the voltage potential across the second capacitor.  
         [0018]     By operating the charge pump in this manner, the average voltages on the first and second charge pumps will be one third and two thirds, respectively, of the input voltage, thereby causing the average output voltage provided by the charge pump to be equal to 0.66 times the input voltage.  
         [0019]     The invention will be more fully understood in view of the following description and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIGS. 1A and 1B  are schematic diagrams of the operation of a conventional ½× charge pump.  
         [0021]      FIGS. 2A, 2B , and  2 C are schematic diagrams of the operation of a reduced area ⅔× charge pump, in accordance with an embodiment of the invention.  
         [0022]      FIG. 2D  is a schematic diagram of an exemplary switch configuration for the charge pump of  FIGS. 2A-2C .  
         [0023]      FIG. 3  is a block diagram of an electronic device that incorporates the charge pump of  FIGS. 2A-2C .  
         [0024]      FIGS. 4 and 5  are schematic diagrams of discharge phases of a ⅔× charge pump in accordance with alternate embodiments of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0025]     Conventional charge pumps can generate output voltages that are higher or lower than necessary for many applications. Excess voltage gain must then be attenuated, which results in wasted power (and reduced battery life for devices incorporating conventional charge pumps). Insufficient voltage gain results in low operating efficiency. By providing a charge pump that applies a ⅔× voltage scaling factor, rather than the conventional 1/1 or ½ scaling factors, an optimum output voltage can be achieved for a given input voltage, which can beneficially improve power efficiency in situations where conventional charge pumps provide either excessive or insufficient voltage.  
         [0026]      FIGS. 2A, 2B , and  2 C are schematic diagrams of an embodiment of a ⅔× charge pump  200  for receiving an input voltage V_IN 2  and providing a lower output voltage V_OUT 2  to a load D 240  (depicted as an LED for exemplary purposes). Charge pump  200  includes an input terminal  201 , charging capacitors C 210  and C 220 , a storage (output) capacitor C 230 , and an output terminal  202 . Charge pump  200  also includes interconnect circuitry  205  (e.g., wiring, switches, control logic) for wiring (i.e., providing the electrical paths between) capacitors C 210 , C 220 , and C 230  in the configurations shown in  FIGS. 2A, 2B , and  2 C. An exemplary switching configuration for interconnect circuitry is described below with respect to  FIG. 2D .  
         [0027]     Charge pump  200  operates by switching between the three phases of operation shown in  FIGS. 2A, 2B , and  2 C. In  FIG. 2A , a charging phase is shown, in which capacitors C 210  and C 220  are serially connected between input terminal  201  and ground. Meanwhile, capacitor C 230  is connected between ground and output terminal  202  (load D 240  is always connected between output terminal  202  and ground). Note that “ground” can refer to any supply voltage lower than input voltage V_IN 2 , such that capacitor C 230  and load D 240  are connected between output terminal  202  and a lower supply voltage terminal (not shown for clarity). In alternate implementation, this charging phase can be omitted as charging can be done also during a subsequent discharge phase.  
         [0028]     During the charging phase, capacitors C 210  and C 220  are charged by input voltage V_IN 2  to voltages V 21  and V 22 , while a voltage V 23  stored on capacitor C 230  is provided as output voltage V_OUT 2  for driving load D 240 . Note that because capacitors C 210 , C 220 , and C 230  are always either charging or discharging, voltages V 21 , V 22 , and V 23  are actually average voltages. However, so long as the different operational phases are short enough, the actual changes in voltages V 21 , V 22 , and V 23  during each phase will be relatively small. Therefore, for descriptive and analytical purposes, voltages V 21 , V 22 , and V 23  can be considered to be essentially constant.  
         [0029]     In the first discharging phase shown in  FIG. 2B , capacitor C 210  and capacitor C 220  remain connected in series between input terminal  201  and ground. However, the common node of capacitors C 210  and C 220  is connected to output terminal  202 . Under these conditions, the potential across capacitor C 210  generated during the charging phase is therefore subtracted from the input voltage V_IN 2  to generate output voltage V_OUT 2  during the first discharging phase shown in  FIG. 2B . Thus, during the first discharging phase, output load D 240  is driven by, and storage capacitor C 230  is charged by, the difference of input voltage V_IN 2  and voltage V 21  on capacitor C 210  (i.e., V_OUT 2 =V_IN 2 −V 21 ).  
         [0030]     Then, in the second discharging phase shown in  FIG. 2C , capacitors C 210  and C 220  are connected in series between input terminal  201  and output terminal  202 , with the orientation of capacitor C 210  being inverted compared to the previous discharge phase of  FIG. 2B . Specifically, the positive plate (marked with a triangular indicator) of capacitor C 210  is connected to the positive plate of capacitor C 220 , while the negative plate of capacitor C 210  is connected to the input terminal  201 . The negative plate of capacitor C 220  is connected to the output terminal  202 . Therefore, during the second discharging phase depicted in  FIG. 2C , output voltage V_OUT 2  is equal to the sum of input voltage V_IN 2  and the voltage V 21  across capacitor C 210 , minus the voltage V 22  across capacitor C 220  (i.e., V_OUT 2 =V_IN 2 +V 21 −V 22 ). This output voltage V_OUT 2  then drives load D 240  and charges storage capacitor C 230 . The process then switches back to the charging phase of  FIG. 2A  and continues cycling in this manner to provide the desired charge pumping action.  
         [0031]     Note that unlike conventional charge pumps (e.g., ½× charge pump  100  of  FIGS. 1A-1B ), ⅔× charge pump  200  includes three distinct operational phases (as described with respect to  FIGS. 2A-2C ). Those three phases cause capacitors C 210  and C 220  to exhibit different nominal voltage potentials (i.e., voltages V 21  and V 22  will not be equal), and that difference in voltage levels determines the nominal value for output voltage V_OUT 2 .  
         [0032]     To calculate the nominal values for voltages V 21  and V 22 , Kirchoff&#39;s Second Law (conservation of voltage) can be used to generate voltage equations for the three phases of operation. Those equations can then be solved for voltages V 21  and V 22  to determine the relationship between those two voltages. For the charging phase ( FIG. 2A ), Kirchoff&#39;s Second Law states that: 
 
 V _IN2= V 21+ V 22  [Eqn. 1]
 
         [0033]     For the first discharging phase ( FIG. 2B ), Kirchoff&#39;s Second Law states that: 
 
 V _OUT2= V _IN2− V 21  [Eqn. 2]
 
 As described above with respect to  FIG. 2B , the orientation of capacitor C 210  with respect to input terminal  201  during the first discharging phase is inverted from the charging phase to the first discharging phase. Therefore, the voltage potential stored across capacitor  210  during the charging phase is subtracted from the input voltage V_IN 2  during the first discharging phase. 
 
         [0034]     Finally, for the second discharging phase ( FIG. 2C ), Kirchoff&#39;s Second Law states that: 
 
 V _OUT2= V _IN2+ V 21− V 22  [Eqn. 3]
 
 As described above with respect to  FIG. 2C , capacitor C 210  is connected with a reversed orientation with respect to input terminal  201  during the second discharging phase. Therefore, the voltage potential (V 21 ) across capacitor C 210  is added to input voltage V_IN 2 . However, during the second discharging phase, the orientation of capacitor C 220  is same as the orientation during the charging phase. Therefore, the voltage potential (V 22 ) across capacitor C 220  is subtracted from the input voltage V_IN 2  during the second discharging phase. 
 
         [0035]     Substituting Equation 2 into Equation 3 yields: 
 
 V _IN2− V 21= V _IN2+ V 21− V 22  [Eqn. 4]
 
 which reduces to the following: 
 
 V 22=2* V 21  [Eqn. 5]
 
 Thus, the voltage potential across capacitor C 220  (i.e., voltage V 22 ) is twice the magnitude of the voltage potential across capacitor C 210  (i.e., voltage V 21 ). Substituting Equation 5 into Equation 1 then yields: 
 
 V 21=(⅓)* V   —   IN 2  [Eqn. 6]
 
 Finally, substituting Equation 6 into Equation 2 yields the following for output voltage V_OUT 2 : 
 
 V _OUT2=(⅔)* V _IN2  [Eqn. 7]
 
         [0036]     Note that the same result can be derived by substituting Equations 5 and 6 into Equation 3. In either case, charge pump  200  provides a voltage multiplication factor of ⅔.  
         [0037]      FIG. 2D  shows ⅔× charge pump  200  including an embodiment of interconnect circuitry  205  that includes switches S 205 ( 1 ), S 205 ( 2 ), S 205 ( 3 ), S 205 ( 4 ), S 205 ( 5 ), S 205 ( 6 ), and S 205 ( 7 ). Switches S 205 ( 1 ), S 205 ( 2 ) and s 205 ( 6 ) are connected in series between input terminal  201  and output terminal  202 , with the positive plate of capacitor C 210  being connected to the junction between switches S 205 ( 1 ) and S 205 ( 2 ). Switch S 205 ( 3 ) is connected between input terminal  201  and the negative plate of capacitor C 210 , while switch S 205 ( 4 ) is connected between the negative plate of capacitor C 220  and the output terminal  202 . Switch S 205 ( 5 ) is connected between the negative plate of capacitor C 210  and the positive plate of capacitor C 220 , and switch S 205 ( 6 ) is connected between the positive plate of capacitor C 220  and output terminal  202 . Finally, switch S 205 ( 7 ) is connected between the negative plate of capacitor C 220  and ground.  
         [0038]     Thus, during the charging phase, switches S 205 ( 1 ), S 205 ( 5 ), and S 205 ( 7 ) are closed, while the remainder of switches S 205  are open, thereby allowing charging of capacitors C 210  and C 220  as shown in  FIG. 2A . Then, during the first discharging phase, switches S 205 ( 1 ), S 205 ( 5 ), S 205 ( 7 ) and S 205 ( 6 ) are closed, and the remainder of switches S 205  are opened, thereby connecting capacitors C 210  and C 220  as shown in  FIG. 2B . Note that switch S 205 ( 7 ) can be either open or closed during the first discharging phase, as grounding the negative plate of capacitor C 220  during this phase will have no effect on the average charge stored on capacitor C 220 . Finally, during the second discharging phase, only switches S 204 ( 3 ), S 205 ( 2 ), and S 205 ( 4 ) are closed, thereby connecting capacitors C 210  (non-inverted) and C 220  (inverted) between input terminal  201  and output terminal  202 , as shown in  FIG. 2C .  
         [0039]     Note further that various other switching configurations can be used to provide additional voltage multiplication factors. For example, by changing the second discharge phase to connect the positive plate of capacitor C 220  directly to input terminal  201  and the negative plate of capacitor C 220  to output terminal  202 , a ½× multiplication factor is obtained. In this case, capacitor C 220  obtains the same charge and voltage as capacitor C 210  during the first discharge phase. Thus, during the first discharge phase, V_OUT 2  is equal to V_IN 2  minus V 21 ; and during the second discharge phase, V_OUT 2  is equal to V_IN 2  minus V 22 . It therefore follows that V 21  is equal to V 22 , which is equal to V_IN 2 /2.  
         [0040]     Note that due to switch resistance within charge pump  200 , output voltage V_OUT 2  may not precisely reach ⅔ of input voltage V_IN 2 . For example, if the combined switch resistance (open loop) across charge pump  200  is 1 ohm during each operational phase, a 100 mA load (D 240 ) and an input voltage V_IN 2  equal to 3 V will result in an output voltage V_OUT 2  equal to 2.5 V (i.e., 2.5 V=(⅔*3.9 V)−(1Ω*0.1 A)), rather than the ideal output voltage value of 2.6 V (i.e., 2.6 V=⅔*3.9V). Therefore, reducing the switch resistance within charge pump  200  can allow output voltage V_OUT 2  to more closely approach the ideal ⅔ multiple of input voltage V_IN 2 . Note that this does not change the fact that charge pump  200  is a ⅔× charge pump, since the rating of a charge pump is based on operation under ideal conditions (i.e., no losses due to switch resistance, no load, and steady state operation). In general, any circuit incorporating charge pump  200  will operate properly so long as output voltage V_OUT 2  provided by charge pump  200  is substantially equal to ⅔ times input voltage V_IN 2  (e.g., voltage V_OUT 2  is within 5% of ⅔ times voltage V_IN 2 ).  
         [0041]     By providing a ⅔× voltage multiplication factor, charge pump  200  can beneficially provide enhanced power efficiency over conventional ½× charge pumps (i.e., charge pump  100  shown in  FIGS. 1A and 1B . For example,  FIG. 3  shows a block diagram of a battery-powered device  300  that includes a battery  310 , ⅔× charge pump  200 , and load circuit  240 . Device  300  could, for example, be a cell phone, a personal digital assistant, a portable multimedia device, a digital camera, a video camera, or any other device. Battery  310  can be any type of battery, such as a lithium ion or lithium polymer rechargeable battery providing a nominal voltage VBATT of 3.7 V, with an actual output voltage range between 3.0 V and 4.2 V (other types (and any number) of batteries, such as nickel metal hydride (NiMH) rechargeable or alkaline or lithium primary (non-rechargeable) batteries, among others, could also be used). Note that the particular arrangement (order) of elements within device  300  is purely exemplary, and various other arrangements will be readily apparent.  
         [0042]     In alternate embodiments of the present invention, a ⅔ voltage multiplication factor can also be achieved by modifying the first discharging phase of  FIG. 2B  in the manner described below in connection with  FIGS. 4 and 5 .  
         [0043]      FIG. 4  is a circuit diagram illustrating the connection of capacitors C 210  and C 220  in a first discharging phase in accordance with an alternate embodiment of the present invention. The configuration of  FIG. 4  replaces the configuration of  FIG. 2B  in this embodiment. As illustrated in  FIG. 4 , capacitor C 210  is connected in series between the input terminal  201  and the output terminal  202  (while capacitor C 220  is disconnected from both of these terminals  201 - 202 ). Under these conditions, the output voltage V_OUT 2  has a value of ⅔ V_IN 2  because capacitor C 210  is connected with the same orientation as in the charging phase of  FIG. 2A . As a result, the average output voltage V_OUT 2  remains at a value of ⅔ V_IN 2 .  
         [0044]      FIG. 5  is a circuit diagram illustrating the connection of capacitors C 210  and C 220  in a first discharging phase, in accordance with yet another embodiment of the present invention. The configuration of  FIG. 5  replaces the configuration of  FIG. 2B  in this embodiment. As illustrated in  FIG. 5 , capacitor C 220  is connected in series between the ground terminal and the output terminal  202 , with an orientation that is opposite the orientation of the charging phase of  FIG. 2A . Capacitor C 210  is de-coupled from the output terminal  202 . As a result, capacitor C 220  discharges to the output terminal  202 , thereby causing the output voltage V_OUT 2  to have a value of ⅔ V_IN 2 . As a result, the average output voltage V_OUT 2  remains at a value of two thirds V_IN 2 .  
         [0045]     In accordance with yet another embodiment of the present invention, the charging phase of  FIG. 2A  is eliminated, and circuit  300  operates by switching between the configurations of  FIGS. 2B and 2C . In this embodiment, capacitors C 210  and C 220  are charged while circuit  300  is in the configuration of  FIG. 2B . At this time, the output voltage V_OUT 2  achieves a value of ⅔ V_IN 2  (as described above in connection with  FIG. 2B ). When circuit  300  switches to the configuration of  FIG. 2C , the output voltage V_OUT 2  remains at an average voltage of ⅔ V_IN 2  (as described above in connection with  FIG. 2C ).  
         [0046]     Although the present invention has been described in connection with several embodiments, it is understood that this invention is not limited to the embodiments disclosed, but is capable of various modifications that would be apparent to one of ordinary skill in the art. For example, charge pump  200  could include control logic to allow configuration and operation of capacitors C 210  and C 220  to provide different voltage multiplication factors. Furthermore, the operation of the charge pump  200  can include more phases, for example, a discharge phase from capacitor C 220  alone connected to output node  202  and/or a discharge phase with capacitor C 210  alone connected between input node  201  and output node  202 . Thus, the invention is limited only by the following claims.