Abstract:
A system switches between application of a first supply voltage and a second supply voltage to a load. The second supply voltage is a regulated voltage that is generated from the first supply voltage, or is alternatively generated from a reference voltage, such as bandgap. When the load is supplied from the first supply voltage, the regulated voltage is also generated from the first supply voltage. At or after switching the load to the second supply voltage, the regulated voltage is generated instead from the reference voltage. The load is a clock circuit, such as an oscillator. The controlled switching of the supply voltage for the load in the manner described addresses concerns over introducing errors in the output clock signal when the clock circuit&#39;s supply voltage is changed.

Description:
TECHNICAL FIELD 
       [0001]    The present invention broadly relates to a system and method for switching between a first supply voltage and a second supply voltage of a load; to a load, and to a system for generating a frequency. 
       BACKGROUND 
       [0002]    In electronic circuits, where two or more supplies are needed to operate the circuit and it is required to switch between the multiple supplies, it is typically necessary to assure a switch-over which does not lead to glitches. This requirement becomes more important in case of jitter-free, highly stable circuits. For example, in a frequency generating system, with high stability (minimum variation in frequency), where a regulator is being used to supply a voltage to an oscillator circuit, a smooth switch over is crucial. 
         [0003]    However, in such circuits, variation in a core supply (VddCore) of a system-on-a-chip (SoC) typically introduces variation in frequency, and noise due to the high SoC current often results in jitter in the RC clock. Usually, to achieve a 1 microsecond (μs) start-up time, the oscillator can be started with ‘VddCore’ first. Subsequently, the supply voltage to the oscillator must be switched from ‘VddCore’ to a regulator output (VREG), which can provide an accurate frequency as desired. The switch-over from ‘VddCore’ to VREG should therefore be smooth (e.g. no overshoot/undershoot) and clean, to prevent any variation/jitter in frequency due to this switch-over. 
         [0004]    Also, with low consumption and high power supply rejection ratio (PSRR) (e.g. −35 dB) criteria, the bandwidth of a conventional bandgap driven regulator system is typically restricted, thus making it a slow responding system. This results in a large settling time to attain the final voltage value when the system is switched from the core supply voltage to the regulated voltage. The large settling time can cause cycle-to-cycle jitter in the on-chip oscillator. 
         [0005]    For example, VddCore can vary from 1.08 Volts (V) to 1.32V, while VREG remains stable at 1.2V. Thus, switching the supply or the load from 1.08V (or from 1.32V) to 1.2V may force the load to be suddenly provided with 1.2V instead of 1.08V (or 1.32V). Thus, this transition may become jittery and un-controlled. 
         [0006]    A need therefore exists to provide a system and method for switching from a SoC core supply to a regulated supply that seek to address at least one of the above problems. 
       SUMMARY 
       [0007]    In accordance with a first aspect of an example embodiment, there is provided a system for switching between a first supply voltage and a second supply voltage of a load, the system comprising: a supply configured to apply, at a first time, the first supply voltage to the load; a regulator circuit configured to apply, at a later second time, the second supply voltage to the load, the second supply voltage being a regulated output voltage of the regulator circuit; wherein the first supply voltage and the second supply voltage provide power to the load at the respective times, and wherein the supply is further configured, prior to the second time, to apply the first supply voltage to an input node of the regulator circuit. 
         [0008]    The supply may be configured to be disconnected from the load prior or at the second time. 
         [0009]    The system may further comprise a voltage reference circuit configured to apply, at a third time equal to or later than the second time, a reference voltage to the input node of the regulator circuit, wherein the supply may be configured to be disconnected from the input node of the regulator circuit prior or at the third time. 
         [0010]    The regulator circuit may be configured, prior to the second time, to draw a sink current from the regulated output of the regulator circuit. 
         [0011]    The sink current may be substantially equal to a load current drawn by the load. 
         [0012]    The input node of the regulator circuit may comprise a node configured to receive a constant voltage. 
         [0013]    The input node of the regulator circuit may be configured to receive the reference voltage via a low pass filter circuit. 
         [0014]    A time constant of the low pass filter may be greater than an inverse of a bandwidth of the regulator circuit. 
         [0015]    The regulator circuit may comprise a unity gain buffer circuit. 
         [0016]    The voltage reference circuit may comprise a bandgap reference circuit. 
         [0017]    In accordance with a second aspect of an example embodiment, there is provided a method for switching between a first supply voltage and a second supply voltage of a load, the method comprising: at a first time, applying the first supply voltage to the load; at a later second time, applying the second supply voltage to the load, the second supply voltage being a regulated output voltage of a regulator circuit; wherein the first supply voltage and the second supply voltage provide power to the load at the respective times, and wherein, prior to the second time, the first supply voltage is also applied to an input node of the regulator circuit. 
         [0018]    The method may further comprise, prior to or at the second time, disconnecting the first supply voltage from the load. 
         [0019]    The method may further comprise, prior to or at a third time equal to or later than the second time, disconnecting the first supply voltage from the input node of the regulator circuit, and at the third time, applying a reference voltage to the input node of the regulator circuit. 
         [0020]    Prior to the second time, a sink current may be drawn from the regulated output of the regulator circuit. 
         [0021]    The sink current may be substantially equal to a load current drawn by the load. 
         [0022]    The input node of the regulator circuit may comprise a node at which a constant voltage is provided. 
         [0023]    The reference voltage may be applied to the input node of the regulator circuit via a low pass filter circuit. 
         [0024]    A time constant of the low pass filter may be greater than an inverse of a bandwidth of the regulator circuit. 
         [0025]    The regulator circuit may comprise a unity gain buffer circuit. 
         [0026]    The reference voltage may be generated from a voltage reference circuit, the voltage reference circuit comprising a band-gap reference circuit. 
         [0027]    In accordance with a third aspect of an example embodiment, there is provided a load configured to receive, at a first time, a first supply voltage, and at a later second time, a second supply voltage, the second supply voltage being a regulated output voltage from a regulator circuit; wherein, prior to the second time, the first supply voltage is also applied to an input node of the regulator circuit. 
         [0028]    The load may comprise an oscillator. 
         [0029]    The load may comprise a phase-locked loop. 
         [0030]    In accordance with a fourth aspect of an example embodiment, there is provided a system for generating a frequency, the system comprising: an oscillator configured to generate said frequency; a supply configured to provide a first supply voltage to the oscillator; a regulator circuit configured to provide a second supply voltage to the oscillator; and a system for switching between the first supply voltage and the second supply voltage as defined in the first aspect. 
         [0031]    The system may further comprise a reference circuit configured to provide a reference voltage to the regulator circuit at a third time equal to or later than the second time, wherein the supply may be configured to be disconnected from the regulator circuit prior or at the third time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    Embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which: 
           [0033]      FIG. 1  shows a schematic circuit diagram illustrating a system for switching between a first supply voltage and a second supply voltage of a load according to an example embodiment at a time T 1 ; 
           [0034]      FIG. 2  shows a schematic circuit diagram illustrating the system of  FIG. 1  at a time T 2 ; 
           [0035]      FIG. 3  show a flow chart illustrating a method for switching from a core supply voltage to a regulated voltage according to an example embodiment; 
           [0036]      FIG. 4  shows a flow chart illustrating a method for switching between a first supply voltage and a second supply voltage of a load according to an example embodiment; and 
           [0037]      FIG. 5  shows a block diagram illustrating a system for generating a frequency according to an example embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]      FIG. 1  shows a schematic circuit diagram illustrating a system  200  for switching between a first supply voltage and a second supply voltage of a load  220  according to an example embodiment at a time T 1 .  FIG. 2  shows a schematic circuit diagram illustrating the system  200  of  FIG. 1  at a time T 2 , later than time T 1 . 
         [0039]    In an example embodiment, system  200  comprises a voltage regulator circuit  210  configured to receive a reference voltage signal from either a core supply  240 , or a reference voltage source  230 , e.g. a bandgap reference circuit generating a reference voltage, as input voltage. As shown in  FIGS. 1 and 2 , a switch S 11  is coupled between an input node  212  of the voltage regulator circuit  210  and the core supply  240 , and a switch S 21  is coupled between the input node  212  of the voltage regulator circuit  210  and an output of the reference voltage source  230 . The voltage regulator circuit  210  generates a regulated output voltage for outputting at an output node  214  based on the input voltage. In addition, the voltage regulator circuit  210  comprises circuit components (not shown) configured to generate a sink current I sink  flowing from the output node  214  to a ground when a switch SR is closed. 
         [0040]    As also shown in  FIGS. 1 and 2 , the load  220  is configured to receive a voltage signal from either the core supply  240  (i.e. the first supply voltage), or the voltage regulator circuit  210  (i.e. the second supply voltage) as input voltage. Here, a switch S 12  is coupled between node  222  (which is the output node of system  200 ) and the core supply, and a switch S 22  is coupled between the node  222  and the output node  214  of the voltage regulator circuit  210 . During operation of the load  220 , a current I load  is drawn from node  222  through the load  220  and to a ground. The load  220  may comprise, but is not limited to, an RC oscillator, a phase-locked loop (PLL), etc. 
         [0041]    System  200  further comprises circuit components such as resistor R and capacitors C 1 , C 2 , C 3  and C 4 . For example, resistor R is coupled between the switch S 21  and the input node  212 , capacitor C 1  is coupled between an output of the reference voltage source  230  and a ground, capacitor C 2  is coupled between the input node  212  and a ground, capacitor C 3  is coupled between the output node  214  and a ground, and capacitor C 4  is coupled between the load input node  222  and a ground. A low-pass filter circuit comprising resistor R and capacitor C 2  is thus provided between the reference voltage source  230  and the voltage regulator circuit  210 . Examples of relative capacitance values of capacitors C 1 -C 4  are shown in  FIGS. 1 and 2 . In an example embodiment, the value of C is in the range of about 10 picofarads (pF), while the value of C′ is in the range of about 150 pF. However, it should be appreciated that different values and ratios may be used in alternate embodiments, depending on e.g. system requirements. 
         [0042]    As illustrated, the voltage level provided by the core supply  240  is VddCore, the voltage level provided by the reference voltage source  230  is Bgout (a bandgap voltage, for example), the voltage level at the input node  212  is VREF, the voltage level at the output node  214  is Vregin, and the voltage level at the load input node  222  is VREG. For example, in an embodiment, VddCore=1.2V (can vary from about 1.08V to 1.32V), Bgout=1.2V+10 mV, VREG=1.2V+25 mV. 
         [0043]    In an example embodiment, switches S 11 , S 12 , S 21  and S 22  are closed/opened in phases with respect to a start-up time of the reference voltage source  230  and the voltage regulator circuit  210  to effect the switch-over, as described in detail below. A control circuit  215  controls actuation of the switches as needed to operate the circuit. 
         [0044]    Referring to  FIG. 1 , at time T 1 , switches S 11  and S 12  are closed while switches S 21  and S 22  are open. Thus, the voltage level at both the regulator input node  212  (i.e. VREF) and the load input node  222  (i.e. VREG) is VddCore. In an example embodiment, the voltage level Vregin at the regulator output node  214  is also equal to VddCore at this time (e.g. the voltage regulator  210  comprises a unity gain buffer circuit). The sink current I sink  in the voltage regulator circuit  210  is generated from time T 1 . The sink current I sink  is increased incrementally to attain a value which is approximately equal to the load current I load . This allows the voltage at node  214  to attain its final value gradually without any sudden transition. The sink current I sink  is designed with reference to Bgout (a constant voltage) to attain a value that is close to I loath  and with a knowledge of an approximate value of I load . This approximate value is attempted to be available in I sink  at the typical process (P), voltage (V), and temperature (T) conditions. Also, the regulator circuit  210  is set at an operating point which matches with a condition when node  222  is to supply the load current I load . It is expected that variations due to PVT conditions do not significantly alter the outcome of equalizing the operating point before and after T 2  (see below). In exemplary embodiments where the load  220  is an oscillator or a PLL, the start-up of the load current I load  can be independent of T 1 . That is, the load current I Load  can be switched on at T 1  or sometime after T 1 . In some embodiments, the switching on of the load current I load  can be controlled via a switch (not shown). In a preferred embodiment, the load current I load  drawn from the load input node  222  is switched on at time T 1 +ΔT to minimize noise due to simultaneous switching inside the load  220  and at switch S 12 . 
         [0045]    Just before time T 2  when the switching of supply to the load  220  is initiated, the voltage level Vregin at the regulator output node  214  is equal to the voltage level VddCore at the load input node  222  as discussed above, and the sink current I sink  drawn from regulator output node  214  is set at a value close to the load current I load  drawn from the load input node  222 . 
         [0046]    In an example embodiment, first, switch S 12  is opened and after a very small delay switch S 22  can be closed. It will be appreciated that even if this delay is not maintained, there should not be an issue as both voltage levels Vregin and VREG are at substantially the same value at this moment (Vregin may be slightly lower depending upon the offset of the regulator circuit  210 ). During this transition period, capacitors C 3  and C 4  are used to prevent the respective voltage levels from dropping. Along with the closing of switch S 22 , in the example embodiment, switch SR is opened such that the sink current I sink  is replaced by the load current I Load . This helps to make sure that the biasing point for the regulator circuit  210  is substantially not changing much. In the next step, switch S 11  is opened and switch S 21  is closed. The voltage level VREF at node  212  finally reaches Bgout, depending upon the RC 2  time constant. In a preferred embodiment, this time constant is chosen to be high such that there is no sudden change in the voltage level VREF, hence the voltage level VREG. The voltage level VREG is changed from VddCore to Bgout very gradually such that cycle to cycle jitter in clock can be reduced in the example embodiment. 
         [0047]    In the example embodiments, the regulator circuit  210  comprises a voltage regulator circuit which regulates the voltage levels at node  214  and node  222  (when switch S 22  is closed) at varying values of the load current I load  (e.g. from about 0 μA to 300 μA). The regulator circuit  210  uses, for example, the output of the reference voltage source  230  to generate required voltage level at node  214 . Here, the capacitor C 2  is not intended to charge node  212 , but rather, the resistor R and capacitor C 2  are used as a low pass filter (LPF) for the voltage level Bgout (output of the reference voltage source  230 ). When switch S 21  is closed, voltage level Bgout is applied to node  212 , and the LPF is used in the example embodiments to eliminate any high frequency noise during this transition and, make transition from voltage level VddCore to voltage level Bgout. Also, the time constant RC 2  is used in the example embodiments to determine the time in which the value of VREG is changed from value of VddCore to value of VREF. In a preferred embodiment, this time is chosen to be large in order to minimize the cycle to cycle jitter. 
         [0048]    In the example embodiments, by opening switch S 11  and closing switch S 21 , the regulator input node  212  is disconnected from the core supply  240  and coupled to the reference voltage source  230  at time T 2 . As a result, the voltage level VREF at the regulator input node  212  changes from VddCore to Bgout. Further, a time constant RC 2  is chosen such that the voltage level VREF changes substantially slowly during and after this switch from the core supply  240  to the reference voltage source  230 . For example, the time constant RC 2  is chosen to be higher than (1/Bandwidth) of the voltage regulator circuit  210 . This may provide a slow discharge of capacitor C 2 , hence a slow change in the voltage level VREF at the regulator input node  212 . Thus, the voltage level VREG at the load input node  222  (which is equal to the voltage level Vregin at the regulator output node  214 ) slowly reaches a final value derived by the voltage level Bgout (which is now provided to the voltage regulator circuit  210 ). That is, system  200  of an example embodiment may generate substantially no voltage overshoot/undershoot or frequency variation/jitter during and after the switch from a core supply  240  to a reference voltage source  230 . 
         [0049]      FIG. 3  shows a flow chart  300  illustrating a method for switching from a core supply voltage to a regulated voltage according to an example embodiment. At step  302 , voltage level VddCore from the core supply  240  ( FIG. 1 ) is applied to input node  212  of the regulator circuit  210  ( FIG. 1 ), and to node  222  of the load  220  ( FIG. 1 ). At step  304 , the reference voltage source  230  ( FIG. 1 ) and the regulator circuit  210  are started for generating voltage levels Bgout and Vregin respectively. At step  306 , the load  220  ( FIG. 1 ) is started. At step  308 , the sink current I sink  is started in increasing steps such that it reaches a final value approximately equal to the load current I load . 
         [0050]    After a stipulated time, in a preferred embodiment, first, Vregin is applied to node  222  (step  310   a ) and at the same time the sink current I sink  is stopped (step  310   b ). Then, in the next step (step  312 ), voltage level Bgout is applied to input node  212  of the regulator circuit  210 . The stipulated time can be, for example, the time for the voltage level Bgout to attain a stable value in all process, voltage and temperature conditions. In an alternate embodiment, voltage level Bgout is applied to input node  212  of the regulator circuit  210  simultaneously with the voltage level Vregin being applied to node  222  of the load  220  and the stopping of the sink current I sink . At step  314 , the setting attained at this point is maintained until a reset is provided to the system. 
         [0051]    In the example embodiment, control signals, for example one to draw the sink current I sink  in increasing steps, another to apply the voltage level Bgout to the regulator input node  212 , and another to apply the voltage level Vregin to node  222 , are generated with reference to a clock signal. For example, the clock signal is an output of the load  220 , which is an oscillator in the example embodiment. These control signals can be generated by the control circuit  215  which receives a clock input. 
         [0052]    In an alternate embodiment, the application of voltage level Bgout to node  212  and the application of voltage level Vregin to node  222  can be done at different times. In yet another embodiment, the load  220  can be configured to start only after the voltage levels Bgout and Vregin have attained respective stable values. 
         [0053]      FIG. 4  shows a flow chart  400  illustrating a method for switching between a first supply voltage and a second supply voltage of a load according to an example embodiment. At step  402 , at a first time, the first supply voltage is applied to the load. At step  404 , at a later second time, the second supply voltage is applied to the load, the second supply voltage being a regulated output voltage of a regulator circuit; wherein the first supply voltage and the second supply voltage provide power to the load at the respective times, and wherein, prior to the second time, the first supply voltage is also applied to an input node of the regulator circuit. 
         [0054]      FIG. 5  shows a block diagram illustrating a system  500  for generating a frequency according to an example embodiment. System  500  comprises an oscillator  520 , a supply  540  coupled to the oscillator  520  for providing a first supply voltage to the oscillator  520 , a regulator circuit  510  coupled to the oscillator  520  for providing a second supply voltage to the oscillator  520 , and a reference circuit  530  coupled to the regulator circuit  510  for providing a reference voltage to the regulator circuit  510 . The oscillator  520 , e.g. an RC oscillator, is capable of generating a selected frequency, e.g. from a range of possible frequencies. In an example embodiment, system  500  employs the mechanism for switching between the first supply voltage and the second supply voltage, as described above with respect to  FIGS. 2 and 3 . For example, in  FIG. 5 , the solid lines show connections after the switch to the second supply voltage is completed (the dotted lines show alternative connections for the first supply voltage). 
         [0055]    While this detailed description has set forth some embodiments of the present invention, the appended claims cover other embodiments of the present invention which differ from the described embodiments according to various modifications and improvements. For example, other types of reference voltage source may be used in place of the bandgap reference circuit. Also, the values of R, C 1 -C 4 , and thus the time constant RC 2 , may be adjusted accordingly based on e.g. operation requirements. 
         [0056]    Within the appended claims, unless the specific term “means for” or “step for” is used within a given claim, it is not intended that the claim be interpreted under 35 U.S.C. 112, paragraph 6.