Abstract:
A method and circuit which employs negative feedback to generate a ramped voltage having well controlled maximum amplitude. In preferred embodiments, the invention is an integrated circuit (or portion of an integrated circuit) which generates a ramped voltage with controlled maximum amplitude in a manner independent of process and temperature variations in implementing and operating the circuit. Preferably, the circuit includes an amplifier having an input coupled to receive a reference signal, an output coupled to ramped voltage generator, and another input coupled to the output of the ramped voltage generator, thus implementing a negative feedback loop in which the amplifier asserts feedback to control the maximum amplitude of the ramped voltage generated by the ramped voltage generator. Preferably, the ramped voltage generator periodically charges and discharges a capacitor to generate the ramped voltage, the amplifier is a transconductance amplifier which is periodically enabled (for a portion of each charging cycle of the capacitor) in response to a clock signal, a one-shot circuit produces a delayed voltage pulse train in response to the clock signal, and the ramped voltage generator receives the delayed voltage pulse train and discharges the capacitor in response to each pulse of the delayed voltage pulse train.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation-in-part of application Ser. No. 09/231,046, filed Jan. 14, 1999 now U.S. Pat. No. 6,111,440. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to circuitry for generating ramped voltages having controlled maximum amplitude, and to ramped voltage generation methods performed by such circuitry. Preferred embodiments of the invention are circuits for generating ramped voltage signals (e.g., for use in DC to DC converters) having controlled maximum amplitude. 
     2. Description of the Related Art 
     In power supply circuitry, it is often desired to produce ramped voltages. For example, in some DC-DC converters (sometimes referred to as interleaved PWM DC/DC converters, where “PWM” denotes “pulse width modulated”), multiple channels of ramped voltages are provided to comparator circuitry for use in controlling the duty cycle of the DC—DC converter and thus the amplitude of the DC output voltage. The waveforms of the ramped voltages are identical (to the extent possible and practical) except that each has a different phase than the others. An advantage of providing multiple channels of ramped voltages of this type (rather than a single ramped voltage) is that use of multiple channels of PATENT ramped voltages allows the DC/DC converter with parallel power-processing channels to be implemented with smaller power stage inductors, smaller input filter inductors, and smaller output capacitors, thus providing an overall improved step-load transient response and reduced physical size. 
     More generally, circuitry providing ramped voltage signals (with controlled maximum amplitude) is useful for a wide variety of applications, including but not limited to interleaved PWM DC/DC converter applications. However, when implementing circuitry providing ramped voltage signals with controlled maximum amplitude (especially when implementing it as an integrated circuit or part of an integrated circuit), process and temperature variations typically cause variation from circuit to circuit (or variation over time in a single circuit) in the characteristics (e.g., maximum amplitude) of the ramped voltages. 
     SUMMARY OF THE INVENTION 
     In a class of embodiments, the invention is a circuit which employs negative feedback to generate a ramped voltage signal (preferably of a type useful in an interleaved PWM dc/dc converter) having well controlled maximum amplitude. The invention can be implemented as an integrated circuit (or portion of an integrated circuit) which generates a ramped voltage signal with controlled maximum amplitude in a manner independent of process and temperature variations in implementing and operating such integrated circuit. In preferred embodiments, the inventive circuit includes an amplifier (preferably an operational transconductance amplifier) having one input coupled to receive a reference signal indicative of a preselected maximum ramped voltage amplitude, an output coupled to a ramped voltage generation circuit, and another input coupled to the output of the ramped voltage generation circuit. The circuit implements a negative feedback loop in which the amplifier asserts a feedback signal which controls the maximum amplitude of the ramped voltage signal generated by the ramped voltage generation circuity. Preferably, the ramped voltage generation circuit periodically charges and discharges a capacitor to generate the ramped voltage at its output, the amplifier is a transconductance amplifier which is periodically enabled (for a portion of each charging cycle of the capacitor) in response to a clock signal, the invention includes a one-shot circuit which produces a delayed voltage pulse train in response to the clock signal, and the ramped voltage generation circuit receives the delayed voltage pulse train and discharges the capacitor in response to each pulse of the delayed voltage pulse train. The ramped voltage generation circuit is configured to charge or discharge the capacitor by an amount which depends on the feedback signal (which signal is preferably a correction current), and the ramped voltage generation circuit is typically configured to charge the capacitor at a rate which depends on the feedback signal. 
     The invention can be implemented in a wide variety of applications to generate a ramped voltage signal having well controlled maximum amplitude, for example, in any of a variety of converters having an external clock synchronization input in which it is desirable for an internal voltage ramp (within the converter) to have a maximum amplitude which is independent of frequency. 
     Another aspect of the invention is a method for generating a ramped voltage signal (preferably of a type useful in an interleaved PWM dc/dc converter) having well controlled maximum amplitude. The method includes the steps of periodically charging and discharging a capacitor to generate the ramped voltage; feeding back the ramped voltage to an amplifier (e.g., an operational transconductance amplifier); periodically enabling the amplifier to generate a correction signal indicative of the difference between the ramped voltage and a reference voltage; and employing the correction signal as negative feedback to control the rate at which the capacitor is charged. Preferably, the amplifier is enabled in response to a clock signal, and the method also includes the steps of generating a delayed voltage pulse train in response to the clock signal, and discharging the capacitor in response to each pulse of the delayed voltage pulse train. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a preferred embodiment of the inventive circuit. 
     FIG. 2 is a timing diagram showing waveforms of several signals asserted during operation of the circuit of FIG.  1 . 
     FIG. 3 is a schematic diagram of an implementation of the FIG. 1 embodiment. 
     FIG. 4 is a schematic diagram of an implementation of the transconductance amplifier of FIG.  3 . 
     FIG. 5 is a schematic diagram of an implementation of the one-shot circuit of FIG.  3 . 
     FIG. 6 is a graph of simulated output potential V 3  for the FIG. 3 circuit (in response to a CLK signal having waveform as shown in FIG. 2 and a frequency of 100 KHz, with V 1 =1.8 Volts and V dd =2.8 volts). 
     FIG. 7 is a graph of simulated output potential V 3  for the FIG. 3 circuit (in response to a CLK signal having waveform as shown in FIG. 2 and a frequency of 1 MHz, with V 1 =1.8 Volts and V dd =2.8 volts). 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the inventive circuit will be described with reference to FIG.  1 . The FIG. 1 embodiment includes operational transconductance amplifier  2 , ramped voltage generation circuit  4  (which produces ramped voltage signal V 3  at its output), and one-shot circuit  6 , connected as shown. FIG. 2 is a timing diagram showing waveforms of several signals asserted during operation of the circuit of FIG. 1 (each waveform of FIG. 2 is voltage as a function of time). 
     Amplifier  2  is enabled by rising edges, and disabled by falling edges, of clock signal CLK. Clock signal CLK is a train of voltage pulses (and has a waveform similar or identical to that of the upper waveform of FIG.  2 ), with the pulses typically occurring with frequency 2.67 MHz (or a lower frequency such as 1 MHz or 100 KHz). In a typical implementation, each pulse typically has 125 nsec (nanosecond) duration. In another typical implementation, each pulse has 30 nsec duration. 
     The FIG. 1 circuit also includes one-shot circuit  6  which produces a delayed voltage pulse train (signal V 2 ) in response to clock signal CLK. In a typical implementation of circuit  6 , the rising edge of each pulse of signal V 2  coincides with the falling edge of a pulse of clock signal CLK (as in FIG.  2 ). Typically, each pulse of signal V 2  has the same (or substantially the same) duration as does a corresponding pulse of clock signal CLK (as indicated in FIG.  2 ). For example, the duration of each pulse of CLK and V 2  can be 30 nsec. In some applications, it is desirable to implement one-shot circuit  6  so that each pulse of signal V 2  has shorter duration than a corresponding pulse of clock signal CLK (e.g., each pulse of V 2  can have duration 10 nsec where each pulse of CLK has duration 125 nsec. In general, each pulse of CLK should have duration which is sufficiently long to allow the feedback loop (including amplifier  2  and elements C 1 , MN 1 , MP 1 , and MP 2 ) to correct the charge rate of capacitor C 2  sufficiently to compensate for foreseeable variations in the desired maximum amplitude of signal V 3 , and each pulse of V 2  should have duration which is sufficiently long to allow capacitor C 2  to discharge completely (but not significantly longer than such minimum duration). 
     Circuit  4  of FIG. 1 generates ramped voltage V 3  (having waveform similar or identical to that of the lowest waveform of FIG. 2) by cyclically charging and discharging capacitor C 2 . Capacitor C 2  discharges while voltage V 2  is high (i.e., when NMOS transistor MN 2  is on in response to a high level of voltage V 2  at its gate). When voltage V 2  is low (so that MN 2  is off), capacitor C 2  charges in response to the flow of current i 2  (current i 2  flows from the top rail through the channel of PMOS transistor MP 2  to the top plate of capacitor C 2 ). 
     The FIG. 1 circuit is designed to provide ramped voltage V 3  with a maximum amplitude determined (for a given frequency of the pulses of signal V 2 ) by the magnitude of current i 2  (in turn determined by the characteristics of capacitor C 1 , NMOS transistor MN 1 , and the current mirror comprising transistors MP 1  and MP 2 ) and the capacitance C 2 . However, due to process and temperature variations in implementing and operating FIG. 1 as an integrated circuit (or portion of an integrated circuit), the values of capacitor shift and variation of frequency of clock signals CLK and V 2  with process and temperature will directly affect the maximum amplitude of ramped voltage V 3 . The variation in maximum amplitude (from circuit to circuit, and over time for each circuit implementation of the invention) of the ramped voltage V 3  (unless corrected in accordance with the invention) has a number of undesirable consequences. 
     To alleviate the problems that would otherwise be caused by variation in the maximum amplitude of the ramped voltage signal it produces, the FIG. 1 circuit includes current mirror CM 1 , operational transconductance amplifier  2 , NMOS transistor MN 1 , and capacitor C 1  (connected as shown) in accordance with the invention. A reference signal V 1 , which is indicative of a desired (pre-selected) maximum amplitude for ramped voltage signal V 3 , is provided to the noninverting input of amplifier  2 . The ramped voltage signal V 3  is fed back to the inverting input of amplifier  2 . In response, current I o  (whose magnitude depends on the difference between reference signal V 1  and the instantaneous level of ramped voltage signal V 3 ) flows to (or from) the output of amplifier  2  from (or to) the top plate of capacitor C 1 . The output current I o  of amplifier  2  is negative feedback which is used in a negative feedback loop which tends to move the level of the ramped voltage signal toward the desired maximum amplitude. 
     Current mirror CM 1  comprises PMOS transistors MP 1  and MP 2  connected as shown (with their sources at the top rail potential V dd , and the gates of MP 1  and MP 2  and the drain of MP 1  connected to the drain of NMOS transistor MN 1 ). 
     Next, the operation of the maximum amplitude correction circuitry of the invention is explained in more detail with reference to FIG.  1 . When CLK goes high, amplifier  2  is enabled and a feedback loop comprising amplifier  2 , capacitor C 1 , transistor MN 1 , the current mirror comprising MP 1  and MP 2 , and output node A (whose potential determines voltage signal V 3 ) is established. Capacitor C 1  charges or discharges at a rate which depends on the value of current I o . 
     If V 1 &gt;V 3  while amplifier  2  is enabled, then current I o  flows from amplifier  2  to the top plate of capacitor C 1 , thus raising the gate potential of transistor MN 1 . In response, the current through the channel of transistor MP 2  increases, and thus the voltage across capacitor C 2  (and thus the level of V 3 ) increases at greater rate than it would if I o  were zero (I o =0 when amplifier  2  is disabled). In other words, while amplifier  2  is enabled (during a short interval of time immediately before capacitor C 2  discharges), if V 1  is greater than V 3 , then the value of current I o  is negative feedback which causes the current mirror to raise the level of ramped voltage signal V 3  more rapidly (toward the desired maximum amplitude) than if I o  were zero. 
     If V 1 &lt;V 3  while amplifier  2  is enabled, then amplifier  2  sinks a positive current I o  from the top plate of capacitor C 1 , thus lowering the gate potential of transistor MN 1 . In response, the current through the channel of transistor MP 2  decreases, and thus the voltage across capacitor C 2  (and thus the level of V 3 ) increases at lower rate than it would if I o  were zero (as when amplifier  2  is disabled). In other words, while amplifier  2  is enabled (during a short interval of time immediately before capacitor C 2  discharges), if V 1  is less than V 3 , then the value of current I o  is negative feedback which causes the current mirror (MP 1  and MP 2 ) to raise the level of ramped voltage signal V 3  less rapidly (toward the desired maximum amplitude) than if I o  were zero. 
     With reference to FIGS.  3 - 5 , we next describe a typical integrated circuit implementation of the FIG. 1 embodiment. FIG. 3 is a schematic diagram of the circuit. FIG. 4 is a schematic diagram of an implementation of transconductance amplifier  2  of FIG.  3 . FIG. 5 is a schematic diagram of an implementation of one-shot circuit  6  of FIG. 3 (which produces delayed voltage pulse train V 2  (whose pulses have width 30 nsec) in response to clock signal CLK (whose pulses also have width 30 nsec). 
     The characteristics of the components and reference signals of the circuit of FIGS.  3 - 5  are indicated (for example, capacitor C 1  has a capacitance of 5 picoFarads, capacitor C 2  has a capacitance of 15 picoFarads, capacitor C PD  of FIG. 5 has a capacitance of 250 femtoFarads, reference potential V 1  is 1.8 volts above ground, and each of the transistors shown in FIGS.  3 - 5  is a MOSFET transistor having channel width to length ratio as indicated, where the indicated widths and lengths are in microns). Those of ordinary skill in the art will appreciate what are appropriate levels for bias potentials pbias and nbias, and regulated top rail potential V dd , in view of the present disclosure. The top rail potential V dd  is typically in the range from 1.5 to 10 volts above ground (V dd  is 2.8 volts above ground in the example shown in the Figures), and the pulses (i.e., the rising edges) of signal CLK typically occur with frequency 3 MHz or less. 
     Transconductance amplifier  2  of FIG. 4 is implemented so that it is capable of responding rapidly to an enabling signal (a rising edge of the “CLK” signal). In response to a rising edge of the CLK signal, transistors MN 88  and MP 89  rapidly turn on, and diode-connected NMOS transistor MN 86  (whose gate and drain are coupled to the gate of NMOS transistor MN 87 ) and diode-connected PMOS transistor MP 1   5  (whose gate and drain are coupled to the gate of PMOS transistor MP 85 ) set the gate potentials of transistors MN 87  and MP 85  (during the charging portion of the operating cycle) to levels which result in the appropriate (positive or negative) output current I o  flow from the output node (the common drains of MP 85  and MP 85 ). Since the gate potentials of MP 85  and MN 87  are set during the charging portion of the operating cycle, they need not be set rapidly and the transconductance amplifier thus does not need to be extremely fast. Since transistors MN 88  and MP 89  rapidly turn on (in response to a rising edge of CLK), current can travel in transistors MN 87  and MP 85  for only a brief portion of each cycle of operation. 
     One-shot circuit  6  of FIG. 5 has a conventional design, in which the output potential V 2  is the output of a NOR gate, one of the inputs of the NOR gate is the output of an inverter (whose input is the common drain of PMOS transistor MP 26  and NMOS transistor MN 24 ), and the other of the inputs of the NOR gate is coupled to receive the CLK signal. Capacitor C PD  is coupled between the gate and source of transistor MN 24 . Transistor MP 0  is a current source transistor (the current through its channel is set by the potential “pbias”). 
     Simulations (using SPICE software) have confirmed that the invention, implemented as in FIGS.  3 - 5  (with V 1 =1.8 Volts, V dd =2.8 volts, and the rising edges of signal CLK occurring with frequency of either 100 KHz or 1 MHz), can produce output signal V 3  with a ramped (sawtooth) waveform with substantially linear ramp slope, and with well controlled maximum amplitude equal to about 1.8 Volts. Results of the simulations are shown in FIG. 6 (for CLK frequency=100 KHz) and in FIG. 7 (for CLK frequency=1 MHz). 
     Although only a preferred embodiment has been described in detail herein, those having ordinary skill in the art will certainly understand that many modifications are possible without departing from the teachings hereof. For example, one or both of capacitors c 1  and C 2  can be replaced by a set of two or more capacitors connected in parallel. All such modifications are intended to be encompassed within the following claims.