Abstract:
The present invention is directed to a system and method which utilizes a fixed base current to control the output voltage instead of a variable base current. In one embodiment, instead of modulating the base current, the converter uses output current to determine the voltage produced at the output. By sinking current out of the DC-to-DC converter, a high output impedance is achieved which, in turn, allows a fairly low modulating current to offer a large change in output voltage. This circuit eliminates at least one of the feedback loops found in existing designs, further increasing stability. As a result of the circuit design, there is achieved a DC-to-DC converter which allows the user to easily define the frequency at which the circuit operates and which is tolerant of component variations.

Description:
BACKGROUND OF THE INVENTION 
     It is well known in the art that DC-to-DC converters are utilized to increase and decrease voltage levels within a system. One way to accomplish such conversion is to use a voltage controlled oscillator as a control element within this converter. This VCO feedback circuit typically has been arranged with other feedback loops forming the DC-to-DC converter. 
     A varying amount of current provided to the base of a bipolar transistor determines the output voltage generated. Problems with this approach exist with regard to the stability of such a circuit. For example, in some situations, the DC-to-DC circuit is required to run at a low frequency, which interferes with the frequency of the VCO causing the VCO to output spurious signals. Additional problems exist in the fact that DC-to-DC converters are part of an operational amplifier driving a loop filter for a VCO resulting in a closed-loop filter and a closed-loop operational amplifier fixed together. Variations of component tolerances with this arrangement produce stability problems. 
     For instance, a transistor with a large beta or gain variation modifies the bandwidth of the operational amplifier loop, thereby creating instability. Also, as the number of feedback loops increases instability due to component variations becomes more and more problematic. Accordingly, problems with oscillation frequency level and instability make the existing DC-to-DC converters unsuitable for many applications. Furthermore, the complexity of existing DC-to-DC converter circuits requires a user to compute various component values needed for specific bandwidths, input voltages and output voltages, all of which, once selected, are difficult to change for changing situations. 
     There exists a desire for a low cost DC-to-DC converter which produces few spurious emissions, provides simple computation of required components and input values, operates at easily controllable frequencies and is operable to produce output to input ratio of voltages, in the order of 6 to 1 or more. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to a system and method which utilizes a fixed base current to control the output voltage instead of a variable base current. In one embodiment, instead of modulating the base current, the converter uses output current to determine the voltage produced at the output. The high output impedance allows a fairly low modulating current to offer a large change in output voltage. This circuit eliminates at least one of the feedback loops found in existing designs, further increasing stability. As a result of the circuit design, there is achieved a DC-to-DC converter which allows the user to easily define the frequency at which the circuit operates and which is tolerant of component variations. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
     FIG. 1 depicts the usage of a DC-to-DC converter in a typical prior art feedback loop to control output voltage; 
     FIG. 2 depicts a typical prior art DC-to-DC converter as used in FIG. 1 in greater detail; 
     FIG. 3 shows the details of a prior art operational amplifier as used in the system of FIG. 2; 
     FIG. 4 details an operational amplifier, showing the effective mathematical equivalent circuitry of an exemplary embodiment of the invention; and 
     FIG. 5 shows the details of the operational amplifier of the exemplary embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is ablock diagram of system  100  which employs circuit DC-to-DC converter  200  to provide an output voltage which, in turn, controls the frequency of voltage controlled oscillator (VCO)  101 . System  100  operates to control the output DC level at node  121  as follows: Assume a voltage between, for example ½ volt and  33  volts applied to node  121 , (as will be discussed below, this voltage is the output from the converter where no control voltage is applied) such a voltage would be applied to voltage controlled oscillator  101 , which in turn would force an output oscillation from VCO  101  in direct relationship to the applied voltage in the well known fashion. This oscillation, or frequency, would then be divided by N, if desired, as shown by division circuit  102 . The output of division circuit  102  is then provided to one input of summer  103 . The other input of summer  103  is a frequency reference, which in our example is 4 megahertz. If the 4 megahertz reference frequency matches the frequency input from division circuit  102 , then the output voltage of summer  103  would essentially be zero. This output is applied to the input of CP  104 , which would then convert the error voltage from summer  103  into an error current which then would be applied to node  120 . 
     In the example the input from division circuit  102  matched the input frequency reference at summer  103  and thus there is no voltage applied to node  120  and the circuit is said to be locked or stable. Let&#39;s now assume that the voltage at terminal  121  were to be higher (or lower) than necessary, then that voltage would affect the frequency of the output of VCO  101 , which in turn would be divided by  102  and applied to summer  103 . Since the frequency reference stays constant at 4 megahertz in our example, then there would be generated an error voltage at the output of summer  103 , which in turn would cause an error current to be provided to input node  120  this error current in the manner to be discussed hereinafter, causes the voltage to change at node  121  in a manner to adjust the frequency output of VCO to achieve balance, i.e., a locked condition. 
     The problems with a system such as this in the past have been to be able to stabilize converter circuit  200  so that the circuit locks and stays locked and is independent of manufacturing tolerances on components, and also which can be partially built on silicon chips for ease of manufacturing critical components. A problem also exists in that the typical voltage used in a silicon chip of this nature is approximately 5 volts, and the high end output necessary at node  121  ranges between ½ volt, which is lower than 5 volts, up to 30 to 33 volts, which is significantly higher than the 5 volts the silicon is designed to withstand. Accordingly, unless one is willing to use silicon which could tolerate 30-35 volts, building the converter on silicon is problematic. 
     Circuit  200  is shown in more detail in FIG.  2 . Circuit  300  is used to control voltage gain and ideally would function independently from the overall performance of system  100 . Resistor  214 , capacitor  215  is used to provide output node  121  with the desired output voltage of the circuit. Two nested loops, one consisting of capacitor  211  and the other consisting of resistor  212  and capacitor  213  are shown within system  200  for controlling the operation of circuit  300 . Elements  211 ,  212 ,  213  are the loop filter components of the PLL that control PLL stability. 
     FIG. 3 shows a prior art version of circuit  300  such that the input current to node  120  passes to current control  301  which causes current to be supplied to transistor  332  in voltage control circuit  301 . Note that voltage control circuit  302  is a well known circuit and could be, for example, a Philips TSA5523M, as shown on Philips Integrated Circuits Data Sheet dated Dec. 17, 1966 (ICO2), the disclosure of which is hereby incorporated herein by reference. Circuit  302  is designed to provide an output voltage at node  220 , dependent upon the current coming into the base of transistor  322 . The current to the base of transistor  322  is provided by current (I) generator  333  and is variable dependent upon the desired output voltage and is a function of the closed-loop feedback path formed by the circuit. A higher base current in  322  makes its collector current higher by a factor of β (transistor gain). The higher collector current increases the amplitude of oscillation in the LC oscillator formed by inductor  325  and capacitor  327 . This increased amplitude makes voltage  121  higher. 
     FIG. 4 shows DC-to-DC converter circuit  300  modified to form circuit  500  in accordance with the invention. Various portions of circuit  500  are on-chip and certain portions are off-chip as denoted by the dotted line. The on-chip portion is designed around a  5  volt power source, while the off-chip portion handles voltage higher than 5 volts. On-chip transconductance switch  410  accepts an input current at node  120  and generates an output current which is provided to the off-chip portion of the converter composed of components  401  (R-out),  402  (V-out), and  403  (C-out). The circuitry shown in FIG. 4 is the small signal AC equivalent, which is typically used for stability purposes of the full circuit shown in FIG.  5 . Note that, as will be discussed, input current  433  to the base of transistor  322  (FIG. 5) is now constant, whereas in the prior art it had been variable. By fixing current  433 , the circuit can be reduced for stability calculation purposes. It is this reduction in the equivalent circuit, (by fixing the base current to transistor  322 ) which then allows transconductance amplifier  410  to control the output voltage of the circuit. We have found that variations in tolerances of transistor  322  was a major cause of the instability problems of the prior art. 
     Thus, in operation, the current generated at the output of  410  modulates the final output voltage of the circuit as follows: by fixing input current  433  as discussed above to a fixed value such that if nothing else were to change and if transconductance amplifier  410  had no output then the output at node  220  of FIG. 4 would be the maximum. In our example, node  220  would then have 30 volts on it. At this point any change in this voltage would be due to a change in the output of the current coming from transconductance amplifier  410 . 
     If circuit  500  were then to be reintroduced back into FIG. 1 (as circuit  400  ( 300 ) and we assume that with transconductance  410  not providing an output current, the output voltage at node  220  (shown in FIG. 2) would be 30 volts. This voltage, would then cause a frequency at the output of VCO  101 , which, inturn, would cause an error current at node  120 , which would be fed to the input of transconductance amplifier  410  (FIG. 4) within circuit  500 , which, in turn, would provide a current to elements  401 ,  402 , and  403  to reduce the voltage at node  220 . This reduced voltage would then be fed to VCO  101  which would change the frequency and the process would be repeated until the output voltage on node  121  caused the frequency output from VCO  101  to match (after being manipulated by circuit  102 ) the input reference frequency at which time the voltage would lock in place. 
     Since elements  401 ,  402 , and  403  in FIG. 4 are representations of circuit  502  (FIG.  5 ), the transconductance output of element  410  then would go to the source of mosfet transistor  520  (as opposed to the prior art (FIG. 3) where it went to node  220 ). The fixed current from the on-chip current source would go to the base of the transistor  322  via resistor  321  and  503 . In all other respects, the equivalent circuit  401 ,  402 ,  403  acts as discussed above with respect to circuit  301 , FIG.  3 . 
     In a preferred embodiment, a transconductance with a high value of g m  may be implemented, thereby inducing a large gain amount. As it may be desired to have a high gain bandwidth, an appropriate bandwidth may be calculated using g m  divided by the value of effective C out    402 . This results in a gain bandwidth independent of R out    401 , resulting in a tolerance of different component values. The ability to straightforwardly interchange off-chip components with components that may not be equivalent allows for easy manufacture and maintenance. By operating amplifier  500  in open loop mode, gain is not dependent on specific transistor values and types, allowing interchangeability of transistors, which greatly simplifies design and increases stability. Furthermore, the ability to select a high oscillating frequency in the range of 2.5 MHz (±500 MHz) decreases the total spurious signals of the system. Note that in circuit  500 , elements  321 ,  322 ,  323 ,  324 ,  325 ,  326 , and  327  form an oscillator circuit with its frequency set by the LC combination of elements  325  and  327 . The prior art circuit ran in the range of 350 KHz. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.