Patent Publication Number: US-11398778-B2

Title: Charge pump structure with regulated output voltage

Description:
This patent application is a national phase filing under section 371 of PCT/EP2018/078413, filed Oct. 17, 2018, which claims the priority of European patent application 17198261.4, filed Oct. 25, 2017, each of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to a charge pump structure with a regulated output voltage. 
     BACKGROUND 
     Charge pumps are utilized to generate output voltage levels above an available supply voltage. They have the advantage of being inductorless and operating with comparably low interference problems. A usual charge pump generates a fixed multiple of some supply voltage. 
     There are various methods to generate higher voltages from a low supply voltage. All of them can be amended by a regulation structure, which makes the generated voltage potentially configurable and stabilized. This is typically done using a voltage divider to compare the output voltage to some stabilized reference. Overvoltage protection is typically not inherent, but must be basically added as a separate function just in case. 
     The charge pumps define their output voltage by design structure as a multiple of their supply voltage. This may be a safe procedure, regarding overvoltage, but is problematic for a configurable ASSP chip which has to be operable in a whole range of supply voltages, for example 2.5 V to 3.6 V. 
     With the given condition of a wide range of a supply voltage Vsup, for example 2.5 V&lt;Vsup&lt;3.6 V, a standard charge pump structure cannot be used. For example a desired output voltage of Vout=15 V at Vsup=2.5V could be generated by a standard charge pump having a multiplication factor k=6 so that the charge pump generates an output voltage Vout=k*Vsup=6*Vsup. However, at another input supply voltage of Vsup=3.6 V this would yield an output voltage Vout=k*Vsup=6*3.6 V=21.6 V, which in this example would be above a safety limit of the process of 18 V. 
     Obviously, an ASSP chip which requires both a wide input voltage range as well as a configurable output voltage, and in addition has to obey technological safety limits, needs to implement regulation, stabilisation and limitation mechanisms. 
     A simple solution may be to operate the charge pump with a fixed multiplication factor and to regulate the input supply voltage Vsup down to a fixed value—in the example above, this would mean to reduce Vsup=3.6 V to Vsup=2.5 V by a standard linear voltage regulator. But this would be a highly inefficient and still not very flexible solution: The power, given by the 1.1 V voltage drop multiplied by the input current, would be lost, limiting efficiency to below 70%. 
     Another possibility to generate a configurable high output voltage is based on using an inductance-based switched-mode converter. These power converters are known for their high interference level and their comparably bulky components. In particular in gas meter applications many users refuse inductance-based power converters to avoid possibly high induced voltage peaks or even sparks in failure case. 
     There is a need to provide a charge pump structure with regulated output voltage to generate a stabilized, but configurable output voltage, wherein an upper level of the output voltage is limited. 
     SUMMARY 
     Embodiments provide a charge pump structure and provide a stabilized and configurable output voltage. 
     The charge pump structure with regulated output voltage comprises an output terminal to provide the output voltage, a charge pump to generate the output voltage as a function of an input supply voltage and a control circuit to limit a level of the output voltage. The control circuit is configured to generate a control voltage, wherein the level of the output voltage is controlled by the control voltage such that the output voltage does not exceed a threshold value. 
     The charge pump structure generates a configurable output voltage by controlling a standard charge pump based on a configurable control voltage. The charge pump to generate the output voltage may be configured as a main charge pump. The control circuit may comprise another charge pump to generate the control voltage as a function of a reference voltage. The other charge pump for generating the control voltage from the reference voltage may be configured as the reference charge pump of the charge pump structure. The reference charge pump is configured as a powerless charge pump operating from a stabilized reference where the powerless charge pump is configurable through changing its structure and thus its multiplication factor. 
     The proposed structure solves the problem by combining two standard charge pumps, where the main charge pump pumps power to the desired output voltage level, while the other one, i.e., the reference charge pump, is essentially powerless, but acts as a reference to the main charge pump. In this proposed structure, the main charge pump never reaches its potential maximum voltage given by the multiplication factor and the input supply voltage of the main charge pump. The main charge pump rather stops pumping at the desired output voltage level. The reference voltage and the control circuit comprising the reference charge pump inherently act as an overvoltage limiter. 
     In particular, the output terminal of the charge pump structure is coupled to a current branch of the control circuit, the current branch having a variable resistance. The current branch is located between the output terminal and a ground potential. The control voltage controls the variable resistance of the current branch such that the variable resistance has a lower level, when the output level of the output voltage exceeds a threshold value. The charge provided at the output terminal gets unloaded via the current branch having the low resistance. On the other hand, the control voltage controls the variable resistance of the current branch such that the variable resistance has a level higher than the lower level, when the output level of the output voltage is below the threshold value. 
     The variable resistance may be configured as a controllable current path of a transistor. The charge pumped to the output terminal by the main charge pump is reduced by switching the transistor in the conductive state. That means that, in the regulated charge pump structure, the voltage level of the main charge pump never reaches its potential maximum voltage, and the charge pump structure stops pumping at the desired output voltage level. 
     The main charge pump may be configured as an activatable charge pump that may be operated in an activated operation state to generate the pumped output voltage of the regulated charge pump structure and to be operated in a deactivated operation state to interrupt the generation of the pumped output voltage. The control circuit may generate the activation signal to activate/deactivate the main charge pump. The control circuit is configured to generate the activation signal with a level to stop the generation of the output voltage, when the level of the output voltage exceeds the threshold value. 
     The regulated charge pump structure may comprise an inherent overvoltage protection circuit. The inherent overvoltage protection circuit is configured to reduce the pumped output voltage, when it is detected that the pumped output voltage exceeds the threshold value and the control circuit fails to generate the activation signal to stop the generation of the output voltage, or any other failure prevents to deactivate the main charge pump, even if the control circuit has generated the activation signal. 
     In comparison to configurable inductance-based switched-mode converters, the proposed charge pump structure does not actively use any inductances. In comparison to standard charge pump structures, it provides a configurable output voltage that is not directly referred to the input supply voltage, but to an independently stabilized reference voltage. The proposed structure thus generates a regulated output voltage level having an upper value/threshold value independent of the supply voltage. In contrast to a typical state-of-the-art implementation, it does not use lossy voltage dividers or other inherently lossy regulation structures on the high power side. This makes it particularly suitable for highly efficient low-power applications. When using the proposed charge pump structure in common chip technologies, the area consumption of the charge pump structure is reduced and the charge pump structure causes a smaller power dissipation in comparison to a standard charge pump structure. In addition, it features an integrated overvoltage protection. In particular, the powerless generation of the control voltage allows integrating a passive overvoltage protection in an electronic circuit, the overvoltage protection being permanently effective. 
     Additional features and advantages are set forth in the detailed description that follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the embodiments as described in the written description and claims hereof, as well as the appended drawings. It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework to understand the nature and character of the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a first embodiment of a charge pump structure with regulated output voltage; 
         FIG. 1B  shows a second embodiment of a charge pump structure with regulated output voltage; 
         FIG. 2A  shows an embodiment of a main charge pump of the charge pump structure with regulated output voltage; 
         FIG. 2B  shows another embodiment of a main charge pump of the charge pump structure with regulated output voltage; 
         FIG. 2C  shows another embodiment of a main charge pump of the charge pump structure with regulated output voltage; and 
         FIG. 3  shows an embodiment of a reference charge pump of the charge pump structure with regulated output voltage. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Embodiments of a charge pump structure with regulated output voltage and its components are now be described more fully hereinafter with reference to the accompanying drawings. The embodiments of the charge pump structure and its components may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that the disclosure will fully convey the scope of the embodiments of the charge pump structure and its components to those skilled in the art. The drawing is configured to clearly illustrate the different embodiments of the charge pump structure and its components. 
     The basic problem to be solved is to generate a regulated, fixed voltage from a wide range of (input) supply voltages Vsup, for example 2.5 V&lt;Vsup&lt;3.6 V. This must be achieved without using inductors, in a low power environment and at high efficiency. 
       FIG. 1  shows an embodiment of a regulated charge pump structure  1  which provides a stabilized, but configurable output voltage Voutmain having an upper limit/threshold value which is independent from the level of an input supply voltage Vsup. The regulated charge pump structure comprises an output terminal OUT to provide the output voltage Voutmain, a charge pump  10  to generate the output voltage Voutmain as a function of an input supply voltage Vsup. The input supply voltage Vsup is applied to a supply voltage input terminal V 10  of the main charge pump  10 . The regulated charge pump structure further comprises a control circuit  30  to limit a level of the output voltage Voutmain. The control circuit  30  is configured to generate a control voltage Voutref. The level of the output voltage Voutmain is controlled by the control voltage Voutref such that the output voltage Voutmain does not exceed a threshold value. 
     The control circuit  30  of the regulated charge pump structure comprises another charge pump  20  to generate the control voltage Voutref as a function of a reference voltage Vref. The charge pump  10  is configured as the main charge pump which pumps power to the desired output voltage level. The other charge pump  20  is configured as a reference charge pump. The reference charge pump  20  is configured as a powerless charge pump. The reference voltage Vref is a regulated voltage which may be available in a chip comprising the charge pump structure  1 . The reference voltage Vref may be generated by a linear voltage regulator at a reference voltage input terminal V 20  of the reference charge pump  20 . The reference voltage Vref is multiplied in a powerless way by the reference charge pump  20  to provide the control voltage Voutref, i.e., Voutref=Vref*k. According to a possible embodiment, the multiplication factor k of the reference charge pump  20  can be variably set. 
     The control circuit  30  of the regulated charge pump structure comprises a current branch  31  having a variable resistance. The current branch  31  is arranged between the output terminal OUT and a ground potential GND. The variable resistance of the current branch  31  is controlled by the control voltage Voutref. 
     The control circuit  30  comprises a transistor  32  to provide a controllable current path having the variable resistance. The transistor  32  is arranged in the current branch  31  between the output terminal OUT and the ground potential GND. The transistor  32  comprises a control terminal G 32  to apply the control voltage Voutref to control the variable resistance of the controllable current path  31 . 
     The transistor  32  is configured to be operated such that the controllable current path  31  has a high resistance, when the level of the output voltage Voutmain of the charge pump  10  is below a threshold value. The transistor  32  is configured to be operated such that the controllable current path  31  has a low resistance, when the level of the output voltage Voutmain of the charge pump  10  is above the threshold value. In this case the high charge provided by the main charge pump  10  is unloaded via the controllable current path  31 . 
     The transistor  32  is configured to be operated in a non-conductive state, when the level of the output voltage Voutmain of the charge pump  10  is below the threshold value. The transistor  32  is configured to be operated in a conductive-state, when the level of the output voltage Voutmain of the charge pump  10  is above the threshold value. 
     According to an embodiment of the regulated charge pump structure, the threshold value is the sum of a threshold voltage of the transistor  32  and a level of the control voltage Voutref generated by the reference charge pump  20 . 
     According to an embodiment of the regulated charge pump structure  1 , the charge pump  10  is configured as an activatable charge pump. The control circuit  30  is configured to operate the charge pump  10  in the activated and deactivated state. The activatable charge pump to is configured to generate the output voltage Voutmain, when the charge pump  10  is operated in the activated operation state. The activatable charge pump  10  is configured to stop the generation of the output voltage Voutmain, when the charge pump  10  is operated in the deactivated operation state. 
     According to an embodiment of the regulated charge pump structure  1 , the control circuit  33  is configured to generate an activation signal CHP_READY with a first level such that the charge pump  10  is operated in the activated state, when the level of the output voltage Voutmain of the charge pump  10  is below a threshold voltage, for example the supply voltage Vsup. The control circuit  30  is configured to generate the activation signal CHP_READY with a second level such that the charge pump  10  is operated in the deactivated state, when the level of the output voltage Voutmain of the charge pump  10  is above the threshold voltage, for example the supply voltage Vsup. 
     According to an embodiment of the regulated charge pump structure, the charge pump structure may comprise a comparator circuit  33 . The comparator circuit  33  has an input side  133  being coupled to the current branch  31  and an output side O 33  to generate the activation signal CHP_READY to activate the main charge pump  10  to generate the output voltage Voutmain. The current branch  31  comprises at least one resistor  34  being located between the transistor  32  and the ground potential. 
     According to an embodiment of the regulated charge pump structure, the regulated charge pump structure  1  comprises an overvoltage protection circuit  40  to reduce the output voltage Voutmain provided at the output terminal OUT. The overvoltage protection circuit  40  comprises a controllable resistor  41  being arranged between the output terminal OUT and the ground potential GND. The overvoltage protection circuit  40  is configured such that the controllable resistor  41  is controlled to have a high resistance, when the level of an input voltage Voff of the comparator circuit  33  is below a threshold voltage, for example the supply voltage Vsup. The overvoltage protection circuit  40  is configured such that the controllable resistor  41  is controlled to have a low resistance, when the level of the input voltage Voff of the comparator circuit  33  is above a threshold voltage, for example the supply voltage Vsup. 
     According to an embodiment of the regulated charge pump structure  1 , the controllable resistor  41  of the overvoltage protection circuit  40  is configured as another transistor having a controllable current path with a variable resistance. The other transistor  41  has a control terminal G 41  to control the variable resistance of the controllable current path of the other transistor  41 . The control terminal G 41  of the transistor  41  is connected to the current branch  31  between the transistor  32  and the ground potential GND. Each of the transistors  32  and  41  can be a FET or a bipolar transistor. 
     The functioning of the regulated charge pump structure  1  is described in the following. 
     The main charge pump  10  is fed by the variable input supply voltage Vsup, while the powerless reference charge pump  20  operates from a stabilized input reference voltage Vref, for example 1.8 V. Through its configurable number of stages, the reference charge pump  20  permanently provides its configured and stabilized output/control voltage Voutref, for example, 6*Vref=14.4 V. 
     The control circuit  30  for the main charge pump  10  is built from the transistor  32 , the resistor  34 , for example a resistor having a high resistance or a current source at nA level, and a comparator  33 . The transistor  32  features some threshold voltage Vth, for example 0.6 V. As long as the output voltage Voutmain of the main charge pump  10  does not exceed the threshold value of Voutref+Vth (=15 V in the example), no current flows through the transistor  32 . Through its cyclic operation, the main charge pump  10  keeps increasing the level of the output voltage Voutmain, since the output voltage has also not reached its maximum achievable voltage level. 
     Around a level Voutmain=Voutref+Vth, the transistor  32  starts conducting (current indicated by the arrow at the current branch  31 ). Then the voltage Voff at the comparator input  133  increases rapidly, such that soon the comparator  33  toggles its output signal CHP_READY. This in turn switches off the main charge pump  32 . This way the output voltage gets stabilized to Voutmain=Voutref+Vth. The charge pump structure  1  operates in a cyclic way, so this simple regulation does not constitute an uncontrolled oscillating system. The main charge pump  10  starts again when the level of the output voltage Voutmain has dropped sufficiently due to some load current, and the activation signal CHP_READY was reset to start the main charge pump  10 . 
     According to the embodiment of the charge pump structure  1  illustrated in  FIG. 1A , the inherent overvoltage protection circuit  40  simply comprises the transistor  41 . It is important to keep the overvoltage protection circuit  40  simple to make it fail safe. If for any reason an essentially higher level of the output voltage (Voutmain&gt;Voutref+Vth) appears at the output terminal OUT, the voltage potential Voff at the input side  133  of the comparator circuit  33  increases further quickly and turns on the transistor  41 , i.e., switches the transistor  41  in a conductive state, after exceeding the input supply voltage Vsup. The overvoltage gets unloaded into the lower voltage Vsup, as current indicated by the arrow in the current path between the output terminal OUT and the transistor  41 . 
     This structure gains its inherent safety from the fact that any voltage above Voutref+Vth gets shorted towards Vsup. Since the reference charge pump  20  is constructed to generate only a limited maximum of the control voltage Voutref, no overvoltage can appear, even though the main charge pump  10  may be capable of much higher output voltages (depending on the possibly high supply voltage Vsup). 
       FIG. 1B  shows another embodiment of the regulated charge pump structure  1 , wherein the same components as illustrated in  FIG. 1A  are denoted with the same reference numbers. The main charge to, the reference charge pump  20  and the control circuit  3   o  are the same and have the same functionality as shown in  FIG. 1A . The regulated charge pump structure  1  of  FIG. 1B  comprises an overvoltage protection circuit  40  to reduce the output voltage Voutmain provided at the output terminal OUT, wherein the overvoltage protection circuit is different from the overvoltage protection circuit  40  of  FIG. 1A . The overvoltage protection circuit  40  of  FIG. 1B  comprises a diode  42  being arranged between the input side  133  of the comparator circuit  33  and the input supply voltage Vsup. 
     As shown according to the embodiment of the charge pump structure of  FIG. 1B , the functionality of the transistor  41  may be implemented even simpler by adding the diode  42  between the input side  133  of the comparator circuit  33  (anode of diode  42 ) and the input supply voltage Vsup (cathode of diode  42 ) instead. Then the unloading current would flow through the current branch  31  and the transistor  32 . However, in a real implementation it is advantageous to choose transistor  32  as a comparably small FET, while transistor  41  can be selected independently to be capable of high currents. 
     For comparison, the common alternative of using a resistive voltage divider to measure and compare the output voltage Voutmain to some stabilized reference should be discussed. This is surely possible und would replace the reference charge pump  20 . However, in a low power design the current drawn even by a high resistance voltage divider easily exceeds the current consumption of the reference charge pump  20 . Since the current drawn from the output voltage Voutmain appears at the input terminal V 10  multiplied by the multiplication factor of the main charge pump  10 , a resistor network of even 15 MΩ would cause 6 μA additional current consumption while the reference charge pump  20  consumes only 0.5 μA at the chosen example values of Voutmain=15V. 
     The proposed structure described above can be built with a wide variety of individual charge pump structures illustrated in  FIGS. 2A, 2B, 2C and 3 . The charge pumps typically multiply the input supply voltage Vsup by some fixed factor, using so called “flying capacitors” which are charged to the supply voltage and then switched to be in series with the supply voltage or other multiples of it. The output voltage is generally stored and smoothed by a large capacitor at the final output (not shown). 
       FIG. 2A  shows an embodiment of a charge pump comprising the supply voltage sources V 1  to provide the supply voltage Vsup. One of the supply voltage sources V 1  is coupled to a capacitor C 11  via a controllable switch S 11 . The capacitor C 11  is coupled to a ground potential by a controllable switch S 12 . The charge pump further comprises another supply voltage source V 1  to provide the supply voltage Vsup which is coupled by a controllable switch S 13  to a capacitor C 12 . The capacitor C 12  is coupled to the ground potential GND by the controllable switch S 14 . The charge pump generates a level of 3*Vsup of the output voltage using two flying capacitors loaded to the supply voltage Vsup in series, as illustrated by the arrows. 
       FIG. 2B  shows another embodiment of a charge pump comprising supply voltage sources V 2  to provide a supply voltage Vsup. The charge pump further comprises controllable switches S 21 , S 22 , S 23 , S 24  and S 25  and capacitors C 21  and C 22 . The charge pump generates a level of 3*Vsup of the output voltage using two flying capacitors, where the first capacitor C 21  is loaded to Vsup and the second capacitor C 22  is loaded to 2*Vsup. The current paths of the various operational cycles are illustrated in  FIG. 2B  by the arrows. During a first operational cycle the capacitor C 21  is loaded to the level of the supply voltage Vsup. The capacitor C 22  is loaded during a second operational cycle to 2*Vsup. The level of 3*Vsup is generated at the output of the charge pump during a third operational cycle. 
       FIG. 2C  shows another embodiment of a charge pump comprising the supply voltage sources V 3 , the controllable switches S 31 , S 32 , S 33 , S 34  and S 35 . The charge pump further comprises the capacitors C 31  and C 32 . The charge pump generates a level of 4*Vsup using two flying capacitors C 31  and C 32 , where the second stage uses the first as Vsup, thus doubling its output voltage to 2*Vsup. During a last operational cycle an output voltage of 4*Vsup is generated, as shown by the arrow in  FIG. 2C . 
       FIG. 3  sketches one stage of a charge pump that can be employed in integrated circuits at low power levels. The charge pump is configured as a symmetric charge pump stage with, for example, Vsup=1.8 V. This structure is well suitable for chip integration through its insensitivity to parasitic capacitances PC 1 , PC 2 , PC 3  and PC 4  which contribute in this structure to the desired function. The charge pump further comprises cross-coupled transistors T 41  and T 42  and other transistors T 43  and T 44 . The charge pump further comprises capacitors C 41 , C 42  and Cout. A first and a second supply voltage source V 4  and V 5  generate square-wave voltages, as illustrated in  FIG. 3 . Such stages can be stacked, and each stage basically increases its input voltage by Vsup. 
     According to a possible embodiment, the reference charge pump  20  shown in  FIGS. 1A and 1B  can be employed as a configurable 1-to-8-stage symmetric charge pump as shown in  FIGS. 2A, 2B and 2C , operating from a stabilized reference voltage Vref=1.8 V. Using a design similar to  FIGS. 2A, 2B and 2C , its output/control voltage Voutref is further increased by one more reference voltage Vref and, configurable as LSB, by another stabilized voltage around 0.9 V. Taking the threshold voltage Vth=0.6 V into consideration, this yields a 4-bit configurable range of output voltages of Voutmain=4.2 V . . . 17.7 V, in steps of 0.9 V. In reality these values are slightly different, considering other parasitic effects. This powerless reference charge pump needs only very small capacitances and is fully integrated. 
     According to a possible embodiment, the main charge pump  10  may be built from three doubling stages as sketched in  FIG. 2B . It is thus capable to generate an output voltage of Voutmain=2*2*2*Vsup=20 V . . . 28.8 V. These values may be somewhat reduced in a real implementation. Using the doubling stages concept, the number of large external capacitors for power pumping is reduced to 3 plus the obligatory final storage capacitor at the output. 
     The main charge pump  10  also features a configurable multiplication factor, for fast start-up times and optimized overall efficiency. However, the output voltage is fully controlled by the reference charge pump  20 , as long as the main charge pump  10  is configured for a higher maximum output voltage. 
     In summary, the implemented structure aims at high efficiency in a low-power system and at a high level of circuit protection. It provides a regulated and stabilized output voltage and avoids the additional current typically caused by a regulation using voltage dividers.