Abstract:
Some embodiments of the present invention provide a system that implements a resonant oscillator circuit. This system includes a first inductor with a constant potential terminal coupled to an input voltage, and a time-varying potential terminal coupled to a first phase output. The system also includes a second inductor with a constant potential terminal coupled to the input voltage, and a time-varying potential terminal coupled to a second phase output. The system additionally includes a first n-type transistor with a source terminal coupled to a base voltage, a drain terminal coupled to the first phase output, and a gate terminal coupled to the second phase output. The system also includes a second n-type transistor with a source terminal coupled to the base voltage, a drain terminal coupled to the second phase output, and a gate terminal coupled to the first phase output. Finally, the system includes a startup circuit configured to commence oscillations in the resonant oscillator circuit by energizing the first inductor before energizing the second inductor.

Description:
RELATED APPLICATION 
       [0001]    This application is a continuation-in-part of, and hereby claims priority under 35 U.S.C. §120 to, pending U.S. patent application Ser. No. 12/535,974, entitled “High-Efficiency Switched-Capacitor Power Conversion,” filed on 5 Aug. 2009 by inventors William C. Athas and P. Jeffrey Ungar (Attorney Docket No. APL-P7723US1). 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The disclosed embodiments generally relate to oscillator circuits. More specifically, the disclosed embodiments relate to a method and an apparatus for starting up a resonant oscillator circuit in a manner which provides inrush protection and mitigates metastability problems. 
         [0004]    2. Related Art 
         [0005]    Oscillator circuits are commonly used to generate pulses in electrical systems. However, commonly used oscillator circuits can consume a significant about of power, which is a disadvantage for systems that need to conserve power, such as portable computing devices. To solve this problem, “resonant oscillator circuits,” which transfer energy back and forth between inductive and capacitive circuit elements, can be used to generate clock pulses without dissipating a significant amount of power. (For example, see U.S. Pat. No. 5,559,478, entitled “Highly Efficient, Complementary, Resonant Pulse Generation,” by inventor William C. Athas, filed 17 Jul. 1995.) 
         [0006]    Unfortunately, existing designs for resonant oscillator circuits are not well suited for certain applications. For example, in one application a resonant oscillator circuit is used to clock the two phases of a switched capacitor block (SCB) in a voltage conversion system. (See patent application Ser. No. 12/535,974, entitled “High-Efficiency Switched-Capacitor Power Conversion,” filed on 5 Jul. 2009 by inventors William C. Athas and P. Jeffrey Ungar (Attorney Docket No. APL-P7723US1) which is hereby incorporated by reference). In this application, if the clock signals are first turned on such that power field-effect transistors (FETs) in the SCBs are at or near their minimum on-resistance and the capacitors of the SCB are well below their normal operating voltage levels, the resulting inrush current can damage the FETs. Hence, it is desirable to first clock the SCBs with a low voltage swing, such that the power FETs in the SCB have a high on-resistance, and then gradually decrease the on-resistance by increasing the amplitude of the clock signals. 
         [0007]    Another problem is the uncontrolled and unpredictable way in which oscillations commence in a resonant oscillator during power up. As power is applied to the basic two-FET and two-inductor implementation disclosed in U.S. patent application Ser. No. 12/535,974 (cited above), current more or less equally divides between the two inductor branches. Asymmetries in the component values and the layout will eventually cause the two branch currents to become unequal which allows the oscillations to commence. However, the circuit may exist in a balanced, metastable state for an indeterminate period of time. Meanwhile, current builds up in each inductor in proportion to the duration of the metastable state. Consequently, when the circuit eventually exits the metastable state, the initial output pulses can be almost unbounded in magnitude, and may damage downstream components, such as the SCBs, or even the components of the resonant oscillator circuit itself. 
         [0008]    Hence, what is needed is a resonant oscillator circuit, which does not suffer from the above-listed problems. 
       SUMMARY 
       [0009]    Some embodiments of the present invention provide a system that implements a resonant oscillator circuit. This system includes a first inductor with a constant potential terminal coupled to an input voltage, and a time-varying potential terminal coupled to a first phase output. The system also includes a second inductor with a constant potential terminal coupled to the input voltage, and a time-varying potential terminal coupled to a second phase output. The system additionally includes a first n-type transistor with a source terminal coupled to a base voltage, a drain terminal coupled to the first phase output, and a gate terminal coupled to the second phase output. The system also includes a second n-type transistor with a source terminal coupled to the base voltage, a drain terminal coupled to the second phase output, and a gate terminal coupled to the first phase output. Finally, the system includes a startup circuit configured to commence oscillations in the resonant oscillator circuit by energizing the first inductor before energizing the second inductor. 
         [0010]    In some embodiments, the resonant oscillator circuit additionally comprises a first capacitance and a second capacitance, which operate in concert with the first inductor and the second inductor to produce resonant oscillations. The first capacitance includes capacitive components from a load on the first phase output and the gate terminal of the second n-type transistor. Similarly, the second capacitance includes capacitive components from a load on the second phase output and the gate terminal of the first n-type transistor. 
         [0011]    In some embodiments, the startup circuit comprises a startup resistor coupled between the input voltage and the constant potential terminal of the second inductor, and a startup capacitor coupled between the constant potential terminal of the second inductor and the base voltage. During operation, the startup resistor and the startup capacitor introduce a phase shift between when the first inductor starts energizing and when the second inductor starts energizing. 
         [0012]    In some embodiments, the system further comprises an envelope-generation circuit coupled between the input voltage and the resonant oscillator circuit, wherein the envelope-generation circuit ramps up the input voltage across multiple oscillation periods when the resonant oscillator circuit starts up. 
         [0013]    In a variation on these embodiments, the envelope-generation circuit includes an envelope-generation resistor coupled between the input voltage and the power source, and an envelope-generation capacitor coupled between the input voltage and a base voltage, wherein a time constant associated with the envelope-generation resistor and the envelope-generation capacitor exceeds multiple oscillation periods of the resonant oscillator circuit. 
         [0014]    In a further variation, the envelope-generation circuit additionally comprises a bypass circuit, which bypasses the envelope-generation resistor after the input voltage finishes ramping up. 
         [0015]    In some embodiments, the startup circuit includes a third n-type transistor with a source terminal coupled to a base voltage, a drain terminal coupled to the first phase output, and a gate terminal coupled to the second phase output. It also includes a fourth n-type transistor with a source terminal coupled to the base voltage, a drain terminal coupled to the second phase output, and a gate terminal coupled to the first phase output. The third and fourth n-type transistors have lower threshold voltages than the first and second n-type transistors, which allows oscillations to commence at lower input voltages. 
         [0016]    In some embodiments, the startup circuit includes a first p-type transistor with a source terminal coupled to the input voltage, a drain terminal coupled to the constant potential terminal of the first inductor, and a gate terminal coupled to a first control signal. The startup circuit also includes a third n-type transistor with a source terminal coupled to the base voltage, a drain terminal coupled to the time-varying potential terminal of the first inductor, and a gate terminal coupled to a second control signal. The startup circuit additionally includes a second p-type transistor with a source terminal coupled to the input voltage, a drain terminal coupled to the constant potential terminal of the second inductor, and a gate terminal coupled to a third control signal. Finally, the startup circuit includes a fourth n-type transistor with a source terminal coupled to the base voltage, a drain terminal coupled to the time-varying potential terminal of the second inductor, and a gate terminal coupled to the second control signal. During startup of the resonant oscillator circuit, the first control signal, the second control signal and the third control signal are sequenced so that the first inductor starts energizing before the second inductor starts energizing. 
         [0017]    In a variation on these embodiments, the second control signal is coupled to the first control signal through a first delay element, and the third control signal is coupled to the second control signal through a second delay element. 
         [0018]    In some embodiments, the startup circuit includes a first p-type transistor with a source terminal coupled to the input voltage, a drain terminal coupled to the constant potential terminal of the first inductor, and a gate terminal coupled to a first control signal. The startup circuit also includes a third n-type transistor with a source terminal coupled to the base voltage, a drain terminal coupled to the time-varying potential terminal of the first inductor, and a gate terminal coupled to the first control signal. The startup circuit additionally includes a second p-type transistor with a source terminal coupled to the input voltage, a drain terminal coupled to the constant potential terminal of the second inductor, and a gate terminal coupled to a second control signal. Finally, the startup circuit includes a fourth n-type transistor with a source terminal coupled to the base voltage, a drain terminal coupled to the time-varying potential terminal of the second inductor, and a gate terminal coupled to the first control signal. In these embodiments, the second control signal is a delayed version of the first control signal through a delay element. 
         [0019]    Some embodiments of the present invention provide a method for starting up a resonant oscillator circuit. The method operates by initially energizing a first inductor and a second inductor in the resonant oscillator circuit, wherein the first and second inductors are initially energized so that the first inductor finishes energizing before the second inductor finishes energizing. At the same time, the method ramps up an input voltage for the resonant oscillator circuit across multiple oscillation periods to provide SCB inrush protection during the startup process. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0020]      FIG. 1A  illustrates a resonant oscillator circuit in accordance with an embodiment of the present invention. 
           [0021]      FIG. 1B  presents a timing diagram for the control signals in  FIG. 1A  in accordance with an embodiment of the present invention. 
           [0022]      FIG. 2  illustrates a circuit for generating the control signals in  FIG. 1A  in accordance with an embodiment of the present invention. 
           [0023]      FIG. 3  illustrates an alternative design for a resonant oscillator circuit in accordance with an embodiment of the present invention. 
           [0024]      FIG. 4  illustrates another design for a resonant oscillator circuit in accordance with an embodiment of the present invention. 
           [0025]      FIG. 5A  illustrates yet another design for a resonant oscillator circuit in accordance with an embodiment of the present invention. 
           [0026]      FIG. 5B  illustrates currents and voltages for the resonant oscillator circuit in  FIG. 5A  in accordance with an embodiment of the present invention. 
           [0027]      FIG. 5C  presents a flow chart listing a sequence of operations for some of the inputs of the resonant oscillator circuit in  FIG. 5A  in accordance with an embodiment of the present invention. 
           [0028]      FIG. 6  presents a flow chart listing operations performed by a resonant oscillator circuit in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
         [0030]    The methods and processes described in the detailed description section can be embodied as code and/or data, which can be stored in a computer-readable storage medium as described above. When a computer system reads and executes the code and/or data stored on the computer-readable storage medium, the computer system performs the methods and processes embodied as data structures and code and stored within the computer-readable storage medium. Furthermore, the methods and processes described below can be included in hardware modules. For example, the hardware modules can include, but are not limited to, application-specific integrated circuit (ASIC) chips, field-programmable gate arrays (FPGAs), and other programmable-logic devices now known or later developed. When the hardware modules are activated, the hardware modules perform the methods and processes included within the hardware modules. 
         [0031]    This disclosure describes four implementations which solve the two above-described problems with different trade-offs in complexity, cost, and oscillator efficiency. The first is shown in  FIG. 1A . This implementation includes two p-type transistors, P 1  and P 2 , which are coupled in series to the inductors L 1  and L 2 , respectively. Moreover, there are two pull-down n-type transistors, N 1  and N 2 , which are attached to each clock output: φ 1  and φ 2 . Note that delay circuits T 1  and T 2  provide time delays to produce control signals S 1  and S 2  which starts currents I L1  and I L2  flowing through inductors L 1  and L 2 , respectively, as is illustrated by the timing diagram which appears in  FIG. 1B . Delay circuits T 1  and T 2  may be implemented as simple passive RC sections, or more sophisticated active arrangements with current mirrors, inverters, etc. For example, T 1  and T 2  can be implemented using the circuit illustrated in  FIG. 2 , which produces control signals S 1  and S 2  from STOP signal  104 . 
         [0032]    Each delay is of duration (½)(π+φ)√{square root over (LC)} where L=L 1 =L 2  and wherein C is the balanced clocked capacitance per output. Initially, when STOP signal  104  is high, transistors N 1  and N 2  actively pull down both clock outputs, φ 1  and φ 2 , and no current flows through the circuit because transistors P 1  and P 2  are turned off. When STOP signal  104  subsequently goes low, P 1  turns on and L 1  starts energizing. It will build up a current starting from zero and ramping linearly up to a normal level+I 0 . At this point, S 1  goes low and N 1  and N 2  turn off. Note that transistor N 1  allows the normal blip pulse to emit on φ 1 , while φ 2  will be held low by transistor M 2  of the blip circuit. After a delay T 2 , transistor P 2  turns on and L 2  begins to energize for a half cycle starting from zero current. L 2  will then emit a normal blip pulse and the circuit will operate normally with P 1  and P 2  on and N 1  and N 2  off. When STOP signal  104  goes high, power is cut from the two inductors L 1  and L 2  and transistors N 1  and N 2  will clamp the two clock outputs φ 1  and φ 2  low. Note that the delays T 1  and T 2  may be implemented by other means such as general-purpose inputs and outputs (GPIOs) from a microcontroller. 
         [0033]    The implementation illustrated in  FIG. 1A  is advantageous in that the two clocks are cleanly started at the correct amplitude. Also note that one delay T 2  may be eliminated at the expense of a higher than normal initial φ 2  blip pulse.  FIG. 3  illustrates this alternative embodiment. In  FIG. 3 , STOP signal  104  is tied directly to the gate inputs of transistors P 1  and N 1  and N 2 . When STOP signal  104  subsequently goes low, current flows through L 1  to the output. The pulse width will be 2π{square root over (LC)} and of lower amplitude than the standard blip waveform, e.g., twice that of V IN    102 . The inductor L 2 , however, now energizes for a much longer period and will produce a much higher pulse level. The effect of this higher pulse level can be mitigated by inserting a voltage regulator V REG    304  between the input voltage and the resonant oscillator circuit as shown in  FIG. 3 . Initially, the output voltage of V REG    304  is set to a minimum level. The voltage is then ramped up to a normal operating voltage. As before, when STOP signal  104  goes high, power is cut to inductors L 1  and L 2 , and the two clock outputs φ 1  and φ 2  are clamped low. 
         [0034]    A drawback of the implementations in  FIG. 1A  and  FIG. 3  is that the p-type transistors, P 1  and P 2 , are in series with the inductors. This arrangement is a source of additional cost and I 2 R on  loss.  FIG. 4  illustrates another embodiment, which includes a “slow ramp on” feature for SCB inrush protection and a different approach to ensuring that the inductors energize sequentially with reasonable initial amplitudes. This embodiment includes a programmable voltage regulator V REG    404  that starts the oscillator at the lowest feasible voltage and increases to a higher voltage for normal operation under external control. Specially selected low-threshold n-type transistors, N 1  and N 2 , help to lower the minimum startup voltage while exhibiting higher dissipation due to their higher on-resistance characteristics. They are placed in parallel with low on-resistance but higher threshold voltage power MOSFETS, M 1  and M 2 . 
         [0035]    Initially, M 3  is off when voltage regulator  404  is powered up. Current first flows to the two inductors L 1  and L 2  and capacitor C 2  through resistor R 2 . The resistor R 2  limits the current flow and there is an exponential rise in voltage at the top side of each inductor. The voltage applied to the second inductor, L 2 , is delayed by an additional R and C element. Note that the two low-threshold n-type transistors N 1  and N 2  will start the oscillation. As the oscillation voltage amplitude rises, the diode D 1  forward biases and begins to charge-up the capacitor C 1  which is tied to the gate of M 3 . The steady-state amplitude of the oscillator output is approximately 3.2 times that of the input voltage. Hence, M 3  will be fully turned on (low on-resistance) during normal operation. Note that the impedance of R 5  is 11MΩ, whereas the impedance of resistor R 6  is 10MΩ. This asymmetry in resistors R 5  and R 6  is provided to further assist the startup process by reducing the impedance of the L 1  branch relative to the L 2  branch. Moreover, the 10MΩ resistor R 6 , which is tied across capacitor C 1 , ensures that M 3  will turn off when V IN    102  powers down. 
         [0036]      FIG. 5A  provides a schematic illustrating another implementation in accordance with an embodiment of the present invention.  FIG. 5B  shows the relationships between two inductor currents, I L1  and I L2 , and the two clock outputs, φ 1  and φ 2  in  FIG. 5A . At the halfway point of a clock phase, the inductor currents cross at zero while one clock phase is held low and the other is at its maximum amplitude. We can use this relationship between the two currents and voltage outputs to cleanly initiate oscillation in the blip circuit without first having to energize the inductors. 
         [0037]    The overall idea is to simultaneously apply power to the two inductors while setting one clock output to its peak voltage amplitude, and then release the clock output and allow the oscillator to continue as normal. A narrow pulse is applied to circuit node  506  simultaneously with turning on M 1  and M 2  with the GO signal  504 . Circuit node  508  is tied to either a Low Drop-Out voltage regulator (LDO)  522 , or alternatively, a Zener-diode circuit that provides the initial voltage amplitude for the clock output (as is illustrated in the box with dashed lines). Because of the inherent body diode, two FETs, M 2  and M 3 , are required to isolate the full clock swing from the voltage source. 
         [0038]    The ratio between the oscillator voltage V OSC    526  and the initial voltage at circuit node  502  is important for correct startup. In one embodiment, the oscillator is set to 0.5V and the output of the voltage source is set to 1.5V (LDO) or 1.8V (Zener). The microcontroller then ramps up the oscillator voltage under programmed control. 
         [0039]    The timing between energizing circuit node  511  and circuit node  502  is also important. The microcontroller sequences the FETs that turn power on and off to the oscillator and set the clock output to its initial value. Note that the microcontroller may output only 2.5V, which may result in a slow rise time on the clock output. A bootstrap circuit enhances the clock-pulse voltage by driving circuit node  506  to a voltage substantially higher than the high output voltage of the microcontroller. Initially, the signal K O    530  is high and circuit node  506  is tied low. Next, referring to the flow chart illustrated in  FIG. 5C , the microcontroller negates signal K O    530  (step  530 ) and asserts signal K S    529  (step  532 ) which charges up node  506  through an isolation n-type transistor M 6 . Node  506  then charges up to V IN    102  minus one threshold drop. At the same time, capacitor C 7  is also charged up. The microcontroller then asserts signal K B    528  and simultaneously asserts GO signal  504  (step  534 ) which raises node  506  to a higher voltage. Next, the microcontroller reverses the sequence to end the pulse applied to circuit node  506 . This involves negating signal K S    529  (step  536 ) and asserting signal K O    530  (step  536 ). 
         [0040]    Note that  FIG. 5A  uses the same rectification circuit as the circuit illustrated in  FIG. 4 . Additionally, GO signal  504  and the bootstrapped voltage from circuit node  506  are ORed in via Schottky diodes. Additionally, a low-threshold FET, M 5 , is tied directly to GO signal  504  to time applied power to the oscillator coincident with setting the clock output to its initial value. 
       Energizing Process 
       [0041]      FIG. 6  presents a flow chart listing operations performed while starting up the resonant oscillator circuit illustrated in  FIG. 5A  in accordance with an embodiment of the present invention. First, V IN    102  and V LDO  from node  508  are programmed to provide a minimum peak pulse amplitude (step  602 ). Next, the system charges an output capacitor for the resonant oscillator to the peak amplitude, which energizes L 1  and L 2 , wherein L 1  and L 2  are initially energized so that L 1  finishes energizing before L 2  finishes energizing (step  604 ). (This output capacitor is described in more detail with reference to the SCBs in U.S. patent application Ser. No. 12/535,974 which is discussed above.) The system also gradually ramps up V IN    102  under program control across multiple oscillation periods to provide SCB inrush protection during the startup process (step  606 ). 
         [0042]    The foregoing descriptions of embodiments have been presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the present description to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present description. The scope of the present description is defined by the appended claims.