Abstract:
Systems and methods to achieve a charge pump for generating from a single input supply voltage Vdd in three modes efficient output supply voltages having a value of 2×Vdd, ½ Vdd, and inverted Vdd. The charge pump requires 8 switches and one flying capacitor only.

Description:
This application is related to US patent applications:
     U.S. patent application Ser. No. 12/589,020, filed on Oct. 16, 2009, and   U.S. patent application Ser. No. 12/589,021, filed on Oct. 16, 2009, which are owned by a common assignee, and are herein incorporated by reference in entirety.   

     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention relates generally to DC-to-DC converters and relates more specifically to charge pumps having doubler, inverter or half the input voltage. 
     (2) Description of the Prior Art 
     Generating energy efficient reduced supply voltages is key in modern audio systems to be able to generate lower supply voltages when low power consumption for audio playback is required. Amplifiers adjusting their supply voltages dependent upon the output signal are called “Class-G” amplifiers. Current solutions have a step-down and invert function with a single flying capacitor. 
     It is a challenge for the designers of charge pumps generating voltages including step-down, invert and doubler voltages requiring a minimum number of switches with one flying capacitor. 
     There are known patents dealing with charge pumps generating symmetrical voltages. 
     U.S. Patent (U.S. Pat. No. 5,550,728 to Ellis) discloses charge pump structures having a reservoir capacitor and a pump capacitor embedded in a switch network. Each of the switches in the network is formed as an MOS transistor having a gate, which defines an array of spaced apertures and a plurality of sources and drains each disposed beneath a different one of the apertures. 
     U.S. Patent Application Publication (US 2009/0039947 to Williams) discloses a multiple output DC-to-DC voltage converter using a new time-multiplexed-capacitor converter algorithm and related circuit topologies. One embodiment of this invention includes a flying capacitor, a first output node, a second output node, and a switching network. The switching network configured to provide the following modes of circuit operation: 1) a first mode where the positive electrode of the flying capacitor is connected to an input voltage and the negative electrode of the flying capacitor is connected to ground; 2) a second mode where the negative electrode of the flying capacitor is connected to the input voltage and the positive electrode of the flying capacitor is connected to the first output node; and 3) a third mode where the positive electrode of the flying capacitor is connected to ground and the negative electrode of the flying capacitor is connected to the second output node. 
     U.S. Patent Application Publication (US 2003/0179593 to Burt et al.) proposes a charge pump circuit configured for charging of parasitic capacitances associated with charge pump capacitors in a manner that minimizes voltage ripple. The charge pump circuit is suitably configured with an independent charging circuit configured for supplying the current needed to charge the parasitic capacitances, rather than utilizing the reservoir capacitor to supply the needed current. The independent charging circuit can be implemented with various configurations of charge pump circuits, such as single phase or dual phase charge pumps, and/or doubler, tripler or inverter configurations. The independent charging circuit comprises a parasitic charging capacitor or other voltage source configured with one or more switch devices configured to facilitate charging of the parasitics during any phases of operation of the charge pump circuit. In addition, the independent charging circuit comprises an independent cell, and is local to charge pump circuit for supplying the current for charging the parasitic capacitances, instead of having the current supplied through external bussing or wire bonds. 
     Furthermore U.S. Patent (U.S. Pat. No. 7,626,445 to Lesso et al.) discloses a dual mode charge-pump circuit and associated method and apparatuses for providing a plurality of output voltages, using a single flying capacitor, the circuit including a network of switches that is operable in a number of different states and a controller for operating said switches in a sequence of the states so as to generate positive and negative output voltages together spanning a voltage approximately equal to the input voltage and centered on the voltage at the common terminal, in a first mode and positive and negative output voltages each up to substantially the input voltage in a second mode. and 
     U. S. Patent (U.S. Pat. No. 7,622,984 to Lesso et al.) discloses a charge pump circuit and associated method and apparatuses for providing a plurality of output voltages using a single flying capacitor. The circuit includes a network of switches that are operable in a number of different states and a controller for operating the switches in a sequence of states so as to generate positive and negative output voltages together spanning a voltage approximately equal to the input voltage and centered on the voltage at the common terminal. 
     SUMMARY OF THE INVENTION 
     A principal object of the present invention is to achieve a charge pump having doubler, invert and step-down functionality. 
     A further object of the invention is to achieve a triple mode charge pump with one flying capacitor only. 
     A further object of the invention is to achieve a charge pump allowing the doubling of input voltages without creating a noisy negative rail. 
     A further object of the invention is to achieve a charge pump improving the performance for AC coupling due to higher level signals while maintaining a clean negative supply. 
     A further object of the invention is to achieve an internal or an external charge pump, allowing a reduced number of external components and reduced pin count compared to prior art. 
     In accordance with the objects of this invention a method to generate from a single supply voltage Vdd output voltages in three modes by a charge pump has been achieved. The method invented comprises firstly the steps of: (1) providing an input voltage Vdd and a charge pump circuit, having a positive and a negative output port, comprising a digital controller, a set of eight switches, one flying capacitor, and two reservoir capacitors, (2) setting output voltage modes desired on the digital controller, wherein the possible output modes comprise doubler, inverter, or half the input supply voltage, and (3) setting switches in order to put voltages on the flying capacitor and on the output ports according to a switching sequence and phases required for the output mode selected. Furthermore the method disclosed comprises the steps of: (4) check if the charge pump is still on, if it so, the process flow goes to step (5), else the process flow goes to step 7, (5) check if the process will be continued with the actual output voltage mode, if it so, the process flow will go back to step (3), else the process flow goes to step (6), (6) setting output voltage mode as required and go to step (3), and (7) end of operation. 
     In accordance with the objects of this invention a charge pump generating from a single supply voltage Vdd output voltages in three operation modes has been achieved. The charge pump invented firstly comprises: a digital controller, controlling the operation of the charge pump by setting the operation modes and accordingly a sequence of operation of switches, a first input port connected to Vdd voltage, and a second input port connected to ground. Furthermore the charge pump comprises a positive output node, a negative output node, and two reservoir capacitors, wherein a first reservoir capacitor is connected between the positive output node of the charge pump and ground and a second reservoir capacitor is connected between the negative output node of the charge pump and ground. Moreover the charge pump comprises one flying capacitor, and a set of eight switches charging/discharging the flying capacitor and connecting a first or second plate of the flying capacitor to the positive and negative output nodes according to the operation mode selected, wherein the set of switches and the related charging of the flying capacitor is controlled by the digital controller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings forming a material part of this description, there is shown: 
         FIG. 1  shows a preferred embodiment of the charge pump  1  of the present invention with all NMOS switches. 
         FIG. 2  illustrates a flowchart of a method invented to generate from a single supply voltage Vdd output voltages by a charge pump in three modes. 
         FIG. 3  shows a block diagram of the architecture of the present invention. 
         FIG. 4  illustrates an integrated charge pump providing the necessary gate-source voltage for a transistor switch. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Circuits and methods for generating output voltages from a single supply voltage (Vdd) by using charge pump technique are disclosed; wherein the charge pump invented can act as doubler, divider by two, or inverter of the input supply voltage. 
       FIG. 1  shows a preferred embodiment of the charge pump  1  of the present invention with all NMOS switches. The charge pump is made up of a digital controller  30  (shown in  FIG. 3 ), a set of eight switches S 1 -S 8 , a single flying capacitor  2  and two external reservoir capacitors  3  and  4 . The charge pump is part of the headphone amplifier  5  with a loudspeaker  6  providing just the amount of power that according to gain settings is required. In normal operation the charge pump is driven according to the audio volume.  FIG. 1  provides a schematic overview of the switches involved to realize the various operating modes. In total 8 switches are employed. Some of the switches have an integrated bulk switch. 
     An example implementation with all NMOS switches is shown in  FIG. 1 . The switches with associated capacitor symbol have an integrated charge pump which provides the necessary gate-source voltage for NMOS transistors, which are used as high-side switches. Switches S 1 -S 3  and S 5  are NMOS switches with internal integrated charge pump, signified in  FIG. 1  by a capacitor sign, where an internal capacitor in each switch is charged to Vdd voltage and then connected between source and gate of a power NMOS switch to switch ON, in order to switch OFF, the gate is shorted to source. These switches are designed to operate only with terminal voltages between 0 to Vdd. 
       FIG. 4  illustrates a preferred embodiment of such an integrated charge pump  40  providing the necessary gate-source voltage for transistor switch  41 , representing switches S 1 -S 3  and S 5  shown in  FIG. 1 . The charge pump  40  comprises said internal capacitor  45  and three switches  42 - 44 . Other arrangements of switches and one or more capacitors are also possible to implement a charge pump for such a purpose. 
     The advantage of this system is that it can act as doubler, divided by two or inverter of the input supply voltage. This gives three possible modes of operation that can be controlled by digital controller. Doubling mode can be used when driving an AC coupling load. This has the advantage that the available output signaled is double while maintaining a “noise free” negative supply (CSN). CSN is shorted to ground during doubling through switch “S 8 ”. 
     Example of two-phase implementation of doubler is in a first phase to close switches S 1 , S 6  and S 8 , followed by a second phase to close S 2 , S 3  and S 8 . Thus the voltage level on CSP becomes 2*VDD and GND is applied to CSN. 
     In order to invert the input voltage the switching sequence is in a first phase to close switches S 1 , and S 5  with all other switches open, followed by a second phase to close switches S 4 , and S 7  with all other switches open. Thus GND is applied to CSP and the voltage level on CSN becomes −VDD. 
     In order to achieve the input supply voltage divided by two, the top plate of the flying capacitor has first to be connected to VDD voltage and the bottom plate of the flying capacitor to the positive output voltage. Then the top plate of the flying capacitor has to be connected to ground with the bottom plate of the flying capacitor connected to the negative output voltage. 
     The phases are normally of fixed duration using system clock though this is not a requirement. All phases are required to be non-overlap i.e. switches in one phase must turn “off” before any of preceding phases turn “on”. This is usually referred to as “break-before-make”. The frequency of operation can be adjusted based on the load. 
       FIG. 3  shows a block diagram of the architecture of the present invention. It shows a controller  30 , controlling the setting of the switches of the charge pump  1  according to the mode of operation selected, and the amplifier  5 , which is supplied by the output voltage of the charge pump  1 . The controller  3  controls the frequency of switch controls in a way that the voltages generated on the output nodes are just enough for an audio signal to be correctly generated at the output of an audio amplifier supplied by the charge pump. 
     Furthermore it should be noted that all components except the capacitors are integrated in an integrated circuit. 
       FIG. 2  illustrates a flowchart of a method invented to generate from a single supply voltage Vdd output voltages by a charge pump in three modes. A first step  20  describes the provision of an input voltage Vdd and a charge pump circuit, having a positive and a negative output port, comprising a digital controller, a set of eight switches, one flying capacitor, and two reservoir capacitors. The next step  21  depicts the setting of output voltage mode desired on the digital controller, wherein the possible output modes comprise doubler, inverter, or half the input supply voltage. The digital controller controls the charge pump in a way that just the amount of power, required by a stage supplied by the charge pump, is provided by the charge pump. In a preferred embodiment of the invention where the charge pump is supplying a class-G audio amplifier the amount of power is according to gain setting, i.e. the audio volume. The following step  22  illustrates setting switches in order to put voltages on the flying capacitor and on the output ports according to a switching sequence and phases required for the output mode selected. Step  23  is a check if the charge pump is still on, if it so, the process flow goes to step  24 , else the process flow goes to step  26 , which describes the end of the method. Step  24  is a check if the process will be continued with the actual output voltage mode, if it so, the process flow will go back to step  22 , else the process flow goes to step  25 . In step  25  the output voltage mode will be set by the controller as required and the process flow goes to step  22 . 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.