Abstract:
A DC/DC switching regulator has a semiconductor switch coupled to an inductor, a first capacitor and a rectifier. A circuit to improve the switching efficiency of the semiconductor switch has a transmission gate coupled between the gate of the semiconductor switch and a second capacitor. The transmission gate is turned ON only when the gate of the semiconductor switch is about to make a positive or negative transition and isolated from the first and second voltage sources. A portion of the charge stored in the parasitic capacitance of the gate of the semiconductor switch can be stored in the second capacitor and reused to partially drive the semiconductor switch from the second to the first ON/OFF state. A further embodiment employs this technique with a synchronous rectifier in the regulator circuit.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a DC/DC switching regulator and more specifically to a DC/DC switching regulator for low power applications, such as a cellular telephone. 
     DC/DC switching regulators are part of many power management systems because of the improved power conversion efficiency provided by these regulators over that of linear regulators or low-dropout linear regulators (LDO). Switching regulator designs can achieve efficiency as high as 80-90% compared with efficiencies 25-50% for a LDO. In modem power management systems, switching regulators are often utilized to perform the task of pre-regulating the power supply used by the LDO to produce an overall system efficiency of about 75%. 
     FIG. 1A shows a typical prior art buck (voltage reducing) DC/DC switching regulator. FIG. 1B shows a typical prior art voltage boost switching regulator. Both circuits have the same essential components including a feedback and control circuit  104 ,  154 ; a driver  106 ,  156 ; a switching transistor  108 ,  158 ; an inductor  112 ,  162 ; an output capacitor  116 ,  166 ; a rectifier  114 ,  164 , such as a Schottky diode; and a feedback loop  118 ,  168 . Circuits that perform both boost and buck are also known in the art (not shown). The operation of these circuits is well known in the art and need not be described in detail here. One of the components of the total power loss of the switching regulator is the power switch gate drive loss. Each of the switching transistors  108 ,  158  have an associated parasitic capacitance  110 ,  160  which is charged up from the power supply or boosted supply and discharged to ground. All of the power used to charge the gate of the MOSFET is lost when the parasitic capacitance is discharged to ground. The power loss can be expressed as Psw=Cg*Vin*fs where Cg is the parasitic gate capacitance, Vin is the input voltage swing, of the switching transistor and fs is the switching frequency. The power switch, which can be internal or external to the integrated circuit, is typically very large having a width/length exceeding 50k μm having an associated parasitic gate capacitance in excess of 100 pF. Thus, the switching losses can be a significant portion of the overall losses in the switching regulator, especially at light loads. 
     In DRAM circuits it is common to drive the gate of a pass transistor to substantially above the array voltage supply level (Vdd) in order that the storage elements of the memory be charged to the full array supply voltage. U.S. Pat. Nos. 5,185,721 and 5,216,290 show circuits in which the capacitor utilized to generate the boost voltage is utilized to store the charge on the parasitic capacitor of the gate of the pass transistor when the gate is to be discharged to ground. See also U.S. Pat. Nos. 3,691,537; 4,030,083; 4,070,653; 4,292,677; and 4,430,730. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to improve the switching efficiency of a DC/DC switching regulator. 
     This and other objects are achieved, in accordance with one aspect of the invention by a voltage converter comprising a semiconductor switch coupled to an inductor, a first capacitor and a rectifier, the semiconductor switch having a gate and being driven between ON and OFF states from first and second voltage sources by a control circuit. A circuit for improving switching efficiency of the semiconductor switch includes a second capacitor and a first transmission gate coupled between the second capacitor and the gate of the semiconductor switch. The control circuit is coupled to the first transmission gate and generates a control signal to turn ON the first transmission gate when the first control circuit isolates the semiconductor switch from the first and second voltage sources, turns OFF the first transmission gate before the control circuit drives the semiconductor switch to a second one of the ON and OFF states, turns on the first transmission gate after the first control circuit isolates the semiconductor switch from the first and second voltage sources and turns OFF the first transmission gate before driving the semiconductor switch from the second of the ON and OFF states, to the first of the ON and OFF states. Thus, a portion of charge stored on a parasitic capacitance of the gate of the semiconductor switch is stored in the second capacitor and reused to partially drive the semiconductor switch from the first of the ON and OFF states to the second one of the ON and OFF states. 
     Another aspect of the invention includes a DC to DC converter for generating a voltage at an output which is lower than a voltage supplied at an input having a PMOS transistor coupled between the voltage supply and the series connection of an inductor and a capacitor, the voltage across the capacitor being the output voltage. A rectifier is connected in parallel to the series connected inductor and capacitor and a control circuit for the PMOS transistor is coupled to a gate thereof and provides a drive signal between substantially the supply voltage and substantially a reference voltage. A transmission gate is coupled between the gate, a second capacitor and the reference voltage and is responsive to a control voltage generated by the control circuit for driving the transmission gate ON after the gate of the PMOS transistor is isolated from the supply voltage and the reference voltage, driving the transmission gate OFF after a predetermined time interval and before the control circuit drives the gate of the PMOS transistor to substantially the supply voltage, the control circuit isolating the gate of the PMOS transistor before driving the transmission gate ON for a predetermined time interval before the drive signal at substantially the reference voltage is applied to the gate of the PMOS transistor. 
     A further aspect of the invention comprises a method for operating a voltage converter having a semiconductor switch coupled to an inductor, a first capacitor, and a rectifier, the semiconductor switch being driven between ON and OFF states from first and second voltage sources and having parasitic capacitance at a gate thereof. The semiconductor switch is isolated from the first and second voltage sources. Charge is transferred from the parasitic capacitor to a second capacitor coupled thereto. The charge transfer is terminated and the second capacitor isolated from a remainder of the voltage converter. A ON/OFF state of the semiconductor switch is changed by driving the gate to the other of the first and second voltage sources. The semiconductor switch is isolated from the first and second voltage sources . The second capacitor is coupled to the gate to charge the parasitic capacitor and decoupled prior to driving the semiconductor switch to the other of the ON/OFF states. 
     A still further aspect of the invention includes a cellular telephone having a voltage converter for powering a telephone circuit, the voltage converter comprising a semiconductor switch coupled to an inductor, a first capacitor and a rectifier, the semiconductor switch having a gate and being driven between ON and OFF states from first and second voltage sources by a control circuit. A second capacitor, a first transmission gate coupled between the second capacitor and the gate of the semiconductor switch, wherein the control circuit is coupled to the first transmission gate and generates a control signal to turn ON the first transmission gate when the control circuit isolates the semiconductor switch from the first and second voltage sources, turns OFF the first transmission gate before the control circuit drives the semiconductor switch to a second one of the ON and OFF states, turns on the first transmission gate after the first control circuit isolates the semiconductor switch from the first and second voltage sources and turns OFF the first transmission gate before driving the semiconductor switch from the second of the ON and OFF states to the first of the ON and OFF states. Thus, a portion of the charge stored on a parasitic capacitor of the gate of the semiconductor switch is stored in the second capacitor and reused to partially drive the semiconductor switch from the first of the ON and OFF states to the second one of the ON and OFF states. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a schematic drawing of a prior art buck DC/DC switching regulator, 
     FIG. 1B is a schematic diagram of a prior art boost DC/DC switching regulator; 
     FIG. 2 is a schematic diagram of a buck DC/DC switching regulator according to the present invention; 
     FIG. 3 is a timing diagram for the switching regulator of FIG. 2; 
     FIG. 4 is a schematic diagram of a buck DC/DC switching regulator having a synchronous rectifier; and 
     FIG. 5 is a simulation of the capacitor voltage CR and the signal PDRIVE_OUT of FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 2, a schematic diagram of a buck DC/DC switching regulator (voltage converter) according to the present invention is generally shown as  2 . This circuit is similar to the circuit  1  shown in FIG.  1 A. In FIG. 2, elements also shown in FIG. 1 have similar reference numerals. In FIG. 2, the feedback loop  118  and the feedback and control circuitry  104  have been omitted for clarity. The signal PDRIVE_IN is the signal that would appear on signal  105  of the prior art circuit shown in FIG.  1 A. This signal is generated in response to the feedback on line  118  and may be modified by other control features in the feedback control circuit  104 . The driver circuit  206  corresponds to the buffer/driver  106  shown in FIG.  1 A. The driver circuit  206  consists of PMOS transistor  238  coupled in series with an NMOS transistor  240  between the power supply Vin and ground. The signal PDRIVE_OUT is applied to the gate of switching transistor  208 . When transistor  208  is to be driven OFF, transistor  238  is turned ON to drive to signal PDRIVE_OUT to the power supply voltage supply Vin less the voltage drop across transistor  238 . When the gate of transistor  208  is driven high, the parasitic capacitance  210  will be charged toward the power supply voltage Vin. If transistor  238  is ON for a sufficient period of time, the parasitic capacitance  210  will be charged to the full supply voltage. NMOS transistor  240  is used to drive the gate of transistor  208  to ground to turn transistor  208  ON. When transistor  240  is made conductive, it discharges capacitor  210  to ground, and the charge stored thereon is lost. 
     In the embodiment shown in FIG. 2, the driver  206  contains additional circuitry for recovering a portion of the charge stored on parasitic capacitance  210 . A transmission gate  250  is coupled between the gate of transistor  208  and a capacitor  254 . The other electrode of capacitor of  254  is coupled to ground. When transmission gate  250  is ON, capacitors  210  and  254  are coupled in parallel. Thus, a portion of the charge stored on capacitor  210  will be transferred to capacitor  254  until the voltage across the parallel coupled capacitors  210  and  254  reach a value consistent with the law of conservation of charge, given sufficient time. When transmission gate  250  is turned ON at the time when capacitor  210  has been charged to the input voltage Vin, but before transistor  240  has been turned on to remove the charge away from capacitor  210 , a portion of the charge on capacitor  210  can be saved on capacitor  254  for later reuse. After the charge has been redistributed between capacitors  210  and  254 , the transmission gate  250  is turned OFF, causing the electrode  252  to store a portion of the charge on capacitor  210 . Then the transistor  240  is turned ON in order to pull the gate of transistor  208  to ground potential less the voltage drop across transistor  240 . When transistor  240  is turned ON, the remainder of the charge stored in capacitor  210  that has not been transferred to capacitor  254  is lost. However, a portion of the charge has still been saved on capacitor  254 . 
     Circuit  202  is added to circuitry shown in FIG. 1A to provide the switching waveforms for the transistors  238 ,  240  and the transmission gate  250 . The generation of these waveforms and the operation of the circuit will now be described in connection with FIG.  3 . In FIG. 3, the events are defined by the time line at the bottom of the figure which are designated by appropriate letters. During period A, the input signal to circuit  202  PDRIVE_IN is low and the signal PDRIVE_OUT is also low. At this time, signals PBUF and NBUF are high coupling the gate of transistor  208  to ground through NMOS transistor  240  which renders transistor  208  conducting. The signal REFRESHN is low and the corresponding signal REFRESHP is high rendering transmission gate  250  OFF so that the storage capacitor  254  is separated from the signal PDRIVE_OUT. The charge on the capacitor  254 , less any leakage current, is held and the voltage across it remains constant, since the ungrounded end of the capacitor at electrode  252  is essentially floating. 
     Time period B starts when the signal PDRIVE_IN rises. At the beginning of this time period, signal NBUF is pulled low, which turns OFF the NMOS pulldown transistor  240 . At this time, in a conventional circuit as shown in FIG. 1A, the signal PDRIVE_OUT would be pulled up to the power supply voltage Vin through the PMOS pullup transistor  238 . Instead, in the present invention, the signal REFRESHN is driven high and the signal REFRESHP is driven low to turn ON the transmission gate  250 . This allows charge stored on the capacitor  254  (see below) to be shared between the capacitor  254  and the parasitic capacitance  210 . As shown in FIG. 3, the signal CR drops from a level at  302  to a level at  306  while the signal PDRIVE_OUT rises from a point  304  to the same point  306 . This occurs, given enough time in period B, for the two voltages to equal (neglecting the voltage drop across the transmission gate  250 ) at a magnitude determined by the capacitance of the two capacitors and the voltage across the storage capacitor  254 . 
     At the end of period B, the signal REFRESHN is pulled low and the signal REFRESHP is pulled high which once again turns OFF the transmission gate  250  and separates the capacitor  254  from the signal PDRIVE_OUT. After the transmission gate  250  has been turned OFF, PBUF is pulled low, which turns on the PMOS pullup transistor  238  and connects PDRIVE_OUT to Vin. The time period C is the time during which the signal PDRIVE_OUT is pulled the rest of the way to Vin. In FIG. 3, the signal PDRIVE_OUT at  306  is equal to the voltage on the capacitor  254  starts at  306  and rises to level  308  (Vin). Transistor  208  is now OFF. 
     During period D, transistor  208  remains in the OFF state. It remains in the OFF state until the signal PDRIVE_IN falls, thus starting the time period E. As soon as PDRIVE_IN falls, PBUF is driven high, turning OFF the PMOS pullup transistor  238  and separating PDRIVE_OUT from the supply voltage Vin. After the signal PBUF rises, the signal REFRESHN rises and the signal REFRESHP falls again, turning ON the transmission gate  250  and coupling the capacitor storage node  252  to PDRIVE_OUT. The voltage on PDRIVE_OUT is now higher than that on the parasitic capacitor  254  and the parasitic capacitor  210  discharges into capacitor  254 . In FIG. 3, the signal PDRIVE_OUT drops at point  312  to point  314  while the voltage CR of the capacitor  210  rises from point  318  to point  314 . If the time period E is long enough, the two voltages will equal out at a magnitude determined by the law of conservation of charge. The parasitic capacitance  254  is now charged and ready for the time period B again. As in the beginning of time period C, at the end of a delay, the signal REFRESHN is pulled low and REFRESHP is pulled high, again separating the capacitor node  252  from the signal PDRIVE_OUT. Once the transmission gate  250  has been turned OFF, the signal NBUF is pulled high turning ON the NMOS pulldown transistor  240  and connecting PDRIVE_OUT to ground. The time period F is the time during which the signal PDRIVE_OUT is pulled the remainder of the way to ground. In FIG. 3 this is the transition of PDRIVE_OUT from  314  to ground potential at  320 . It should be noted that the capacitor  254  stays at a value  316 , which is essentially the same as the value  302  from which the process started. Time period A follows again during which the signal PDRIVE_OUT is low and before the signal PDRIVE_IN rises. 
     It should be noted that in the prior art techniques, there is no time period B. Time periods C and D follow immediately after time period A. In addition, there is no time period E. Time periods F and A follow immediately after time period D. 
     It is well know in the art that the efficiency of a DC/DC switching regulator can be improved by replacing the rectifier diode  114 ,  164  or  214  with a synchronous rectifier. The voltage drop across the synchronous rectifier can be made much smaller than the voltage drop across the rectifier diode. FIG. 4 shows alternate embodimentof the circuit of FIG. 2 in which the rectifier diode has been replaced by a synchronous rectifier. The circuit, generally shown as  4 , is identical to the circuit shown in FIG. 2 except for the synchronous rectifier transistor  468  and its associated drive circuitry. In FIG. 4, like components to FIG. 2 have similar reference numerals. The synchronous rectifier transistor  468  is chosen as NMOS transistor because it is being switched to ground. However, it should be noted that, depending upon the circuit in which it is used, a PMOS device may be more appropriate. The gate of transistor  468  has an associated parasitic capacitance shown by capacitor  466 . The gate is coupled to a line NDRIVE_OUT which is at the juncture of PMOS transistor  462  and NMOS transistor  464  which are coupled between the input supply Vin and ground. Also coupled to this node is a transmission gate  470  and a storage capacitor  476 . One electrode  472  of the storage capacitor is coupled to the transmission gate  470  and the other electrode is coupled to ground. The gate of PMOS transistor  462  is coupled to the signal PBUF and the gate of NMOS transistor  464  is coupled to the signal NBUF. The transmission gate is coupled to the signal REFRESHP and REFRESHN. These signals are generated by a signal generator circuit  402  which is identical to the circuit  202  in FIG.  2 . 
     As well known to those skilled in the art, when transistor  408  is shut OFF, the inductor  412  will try to maintain the current flow and the refore will generate a voltage which is the inverse of the voltage generated when the transistor  408  is on. It is common to use a diode such as diode  114  and  214  to allow this energy to charge the capacitor  116 ,  216  or  416  as the magnetic field of the inductor collapses. A synchronous rectifier transistor, such as transistor  468 , allows the current to flow with a lower voltage drop than can be produced by the diode  114 ,  214  which improves the efficiency of the converter. Because transistor  468  has been chosen to be an NMOS transistor and transistor  408  has been chosen to be a PMOS transistor, then the control voltage waveforms can look the same. This allows the circuit  460  to be driven by the circuit  402  which also drives circuit  406 , thus saving the need for additional drive circuitry. The signal NDRIVE_OUT will be the same as PDRIVE_OUT but will have the opposite effect. Thus, in time period A of FIG. 3, the gate voltage on both transistor  408  and transistor  468  is at a low voltage shown at  304 , when transistor  408  is conducting and transitor  468  is nonconducting. As the voltage of the gate of transistor  408  rises from  304  to  306  to  308 , thus turning transistor  408  OFF, the voltage on the control gate of transistor  468  will rise from  304  to  306  to  308 , thus turning transistor  468  ON. During the time interval B, the transmission gate  470  is turned ON by driving the signals REFRESHN high and the signal REFRESHP low. This couples the capacitor  476  to the capacitor  466  so that, given adequate time, the voltage across both capacitors will equalize to a level determined by the law of conservation of charge. Just as the transmission gate  450  is turned OFF so that capacitor  454  is isolated from signal PDRIVE_OUT, the transmission gate  470  is turned OFF so that capacitor node  472  is isolated from signal NDRIVE_OUT. Transistor  462  is then driven ON by the signal PBUF going low just as the transmission gate  470  is turned OFF to drive the gate of the transistor  468  to the supply voltage Vin. This takes place in time interval C. During time interval D, the voltage  308  on the signal PDRIVE_IN maintains transistor  468  in the ON state and transistor  408  in the OFF state. During time interval E, the transistor  408  turns ON and the transistor  468  turns OFF. Again, the signal REFRESHN is driven high and the signal REFRESHP is driven low to turn ON the transfer gates  470  and  450 . At this time, parasitic capacitor  466  has a higher voltage on it than capacitor  476 , so that charge flows from the parasitic capacitor to charge storage capacitor  476  until the voltages on both capacitors is equal. During interval F, the signal REFRESHN then goes low and the signal REFRESHP then goes high to isolate the capacitor node  472  from the signal NDRIVE_OUT so that the electrode  472  of capacitor  476  is floating. The signal NBUF then goes high, turning ON transistor  464  and pulling NDRIVE_OUT the remainder of the way to ground, thus turning transistor  468  OFF. At the same time, as described above, the signal PDRIVE_OUT drops from  312  to  314  during interval E and then to  320 , thus turning ON transistor  408 . 
     FIG. 5 is a computer simulation of the transient response of the signals CR and PDRIVE_OUT shown in FIG.  3 . Waveform  502  applies shows the signal PDRIVE_OUT and wave form  504  show the signal CR. This figure allows one to calculate that the use of the present invention (Vin=3.2V and Cp=Cr=150pf allows the storage capacitor  254 ,  454  and  476  to store about 30% of the charge on power switching transistor gate capacitance. About 70% of the charge has to be delivered by Vin. Thus, this produces a savings of about 30% of the gate drive power. In a buck DC/DC switching regulator switching at 1MHz with a 150pF gate capacitance and a 200mA load, the gate drive losses will account for approximately 2% of the total power loss. The present invention will save about 30% of that 2%. However, under low load conditions, where the gate drive loss is one of the major power loss contributors, the savings will be higher. These low load conditions occur frequently in battery-powered equipment, such as cellular telephones, when the devices go into a low power rode often referred to as a “sleep” mode. In the “sleep” mode, the device is not OFF but is switched to a low power operation by powering down circuits that are not needed. 
     In regulators utilizing a synchronous rectifier, the losses of the DC/DC switching regulator can be made significantly smaller because of the much smaller voltage drop across the synchronous rectifier than the voltage drop across the diode. The diode voltage drop is the largest contributor to the losses in a non-synchronous regulator. The gate drive loss on each of the switching transistors, including the power switch and the synchronous rectifier, now accounts for approximately 8% of the total loss because the total loss is less than the loss described above using a diode rectifier. Thus the combined loss of these two devices accounts for approximately 16% of this reduced loss. The charge saving technique of the present invention can reduce this by about 30% depending on the voltage and the capacitance Cp and CR to produce a noticeable 4.8% improvement in efficiency. Under light loading conditions discussed above where the power losses from the parasitic resistances in the circuit are small, the gate drive loss can account for as much as half of the total loses, making the efficiency savings as high as 50% or possibly more. 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof, it is well understood by those skilled in the art that various changes and modifications can be made in the invention without departing from the spirit and the scope of the invention as defined by the appended claims. For example, the present invention has been illustrated in connection with a buck switching regulator. Those skilled in the art will recognize that the same technique can be applied to a boost switching regulator such as shown in FIG. 1B, or to a buck/boost regulator. Furthermore, while the semiconductor switch is illustrated as a PMOS transistor and the synchronous rectifier is illustrated as a NMOS transistor, those skilled in the art will recognize that transistors of the opposite type can be utilized with the present invention.