Abstract:
A charge pump circuit with a small loop filter capacitor is disclosed in the invention. This is accomplished by providing multiple currents which add to the desired current, I. A switching circuit switches one of the currents through the capacitor, while directing a combination of the multiple currents through the resistors. In this way, a smaller current is provided through the capacitor, allowing the capacitor to be much smaller in size.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to charge pump circuits used in a Phase Locked Loop (PLL), and in particular to techniques for reducing the capacitor size of the loop filter capacitor. 
     A conventional differential charge pump circuit with a loop filter included is shown in FIG.  1 . Typically, the input to the charge pump circuit is the phase difference between a received signal and a signal attempting to lock-on to the receive signal. The output of the charge pump would typically be provided to a voltage controlled oscillator (VCO) to adjust the oscillator frequency accordingly to match the phase of the input signal. The low pass loop filter is coupled to the output of the charge pump to provide an analog output control voltage to the VCO. 
     As shown in FIG. 1A, a pump up signal from the phase detector is provided on an input  10 , and a pump down signal on an input  12 . These low-active signals switch on switching transistors M 1  and M 3 , respectively. The current to M 1  and M 3  is provided by current source transistors  14  and  16 , which each provides a current I. 
     When the pump down signal is applied on line  12 , current I flows through turned on transistor M 3 , through resistor R N , capacitor C loop  resistor R P , and through transistor M 5 . The positive version of a pump down signal (pdn) will turn on an MOS transistor M 5 , providing a sink path for the current. At the same time, the pump up signal will be off, turning off transistor M 1  and corresponding transistor M 7 . Transistors  13  and  15  are current sink transistors. 
     When the pump down signal is inactive, and the pump up signal is active, and transistors M 1  and M 7  will be on, providing current I in the opposite direction through the capacitor. Transistors  17 ,  18 ,  20  and  22  are provided to give a path to steer the current away from the loop filter when their corresponding switching transistors M 3 , M 1 , M 5  and M 7  are turned off. Amplifier  24  is a Common Mode Feed Back (CMFB) amplifier. 
     Typically, the capacitor is a discrete capacitor which is off-chip. This is because the capacitor needs to be large enough to satisfy design requirements. For example, for SONET, there is a 0.1 dB peaking limit, which requires a large capacitor. This cannot be achieved by simply using a smaller current and smaller capacitor, since that would adversely impact other design requirements. 
     FIG. 1B illustrates, in a simplified manner, the loop filter composed of resistors R N , R P  and capacitor C loop  in FIG.  1 A. The shunt capacitors to ground C S , for the lead-lag filter are also included. 
     One approach to provide an on-chip capacitor which is smaller, and thus doesn&#39;t require a large silicon area, is illustrated in U.S. Pat. No. 6,344,772, inventor Patrik Larsson. This patent shows an operational amplifier based capacitor multiplication circuit. In essence, through the use of an operational amplifier the effective capacitance value can be multiplied. Unfortunately, this introduces problems of its own, such as problems with Power Supply Rejection Ratio (PSRR), offset, bandwidth and noise. However, it does avoid the problems associated with off-chip capacitors, such as leakage and parasitic capacitance. 
     FIG. 2 illustrates in simplified format the operational amplified-based circuit, using an operational amplifier  30  with resistors R 1  and R 2 , and capacitor C. The right side of FIG. 2 shows the equivalent circuit with the effective resistance, R eff  and the effective capacitance, C eff . The below analysis demonstrates how this capacitor multiplication effect is achieved. 
     Since                  V   x     =       1     1   +     sCR   1                         V   o         ,           (   a   )                 I   1     =             V   o     -     V   x         R   1                     and                   I   2       =           V   o     -     V   x         R   2                     and               (   b   )                   V   o     -     V   x       =         sCR   1       1   +     sCR   1                         V   o               (   c   )                                
     Now                    Z   =                  V   o         I   1     +     I   2                       =                    R   1                     R   2                     V   o           (       R   1     +     R   2       )                     (       V   o     -     V   x       )           ,               =                      R   1                     R   2           R   1     +     R   2                         (     1   +     1     sCR   1         )                   =                      R   1                     R   2           R   1     +     R   2         +     1     sC                   (     1   +       R   1       R   2         )                         ≈                      R   1                     R   2           R   1     +     R   2         +     1     sC                     R   1       R   2               ,         if                     R   1       R   2         =   K     &gt;&gt;   1                 =                  R   1     //       R   2     +     1     sC   eff                         from                   Eqs   .                (   a   )            –          (   c   )                                  
     where 
     
       
         
           C 
           eff 
           ≈KC 
         
       
     
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a charge pump circuit with a small loop filter capacitor. This is accomplished by providing multiple currents which add to the desired current, I. A switching circuit switches one of the currents through the capacitor, while directing a combination of the multiple currents through the resistors. In this way, a smaller current is provided through the capacitor, allowing the capacitor to be much smaller in size. 
     In one embodiment, two switching transistors are provided in parallel for each of the pump up and pump down inputs, with the size of the transistor providing the appropriate fractional current. One switching transistor provides the current I through the resistor, but a separate switching transistor diverts a fractional part of it so that it does not flow through the capacitor. A third switching transistor provides current through a second transistor in a differential design, bypassing the capacitor, such that the combined current from the current portion passing through the capacitor and the current portion separately provided to the additional resistor combine to make the total current I. 
     For a further understanding of the nature and advantages of the invention, reference should be made to the following description in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a circuit diagram of a conventional differential charge pump with a discrete capacitor loop filter of the prior art. 
     FIG. 1B is a schematic diagram of the second-order loop filter of FIG. 1A, of the prior art. 
     FIG. 2 is a diagram of operational amplifier-based capacitor multiplication circuit of the prior art and an equivalent circuit representation. 
     FIGS. 3A-3D are diagrams illustrating the current flow for different pump up and pump down conditions for a loop filter according to the prior art. 
     FIGS. 4A-4D are diagrams illustrating the current flow for a differential loop filter according to an embodiment of the present invention. 
     FIG. 5 is a diagram of a differential charge pump with an integrated loop filter according to an embodiment of the present invention. 
     FIG. 6 is a diagram of a single ended charge pump with an integrated loop filter according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     To understand the present invention, a description of the current flow in the prior art is appropriate. FIGS. 3A-3D illustrate different switch configurations and current flows for a simplified version of FIG.  1 A. Shown are switches corresponding to transistors M 1 , M 3 , M 5  and M 7  of FIG. 1A, as well as current sources corresponding to current sources  14 ,  16 ,  13  and  15 , also referred to as current sources I PP , I NP , I PN , and I NN . As can be seen, switches M 1  and M 7  are closed, corresponding to their associated current sources being “ON”. Switches M 3  and M 5  are open, effectively turning off their corresponding current sources  16  and  13 . 
     Consider a conventional differential loop filter (for simplicity in discussion, we neglect the shunt capacitors from V P  and V N  to ground) shown in FIG. 3A with pump-up condition applied. All current sources are of magnitude I. Current sources I PP , I NN  are “ON” and I PN , I NP  are “OFF”. All components in the filter see a current of magnitude I. And therefore, the impedance between V P  and V N  is              Z   =           V   p     -     V   n       I     =     R   +     1     SC   loop                   (   1   )                                
     In FIG. 3B, a pump down condition is applied. Current sources I PN  and I NP  are “ON” and I PP  and I NN  are “OFF”. All components in the filter again see a current of magnitude I. And therefore, the impedance between V P  and V N  is identical to the expression in Eq. (1). 
     In FIG. 3C, both pump up and pump down conditions are applied at the same time, all current sources are “ON” and current sources I PP  and I NP  are drained by I PN  and I NN  respectively. Therefore, no current flows in the filter. 
     Finally, in FIG. 3D, both pump up and pump down conditions are “OFF”. All current sources are “OFF”. Again no current flows in the filter. These complete all combinations of pump up/pump down conditions of the conventional differential loop filter. 
     FIGS. 4A-4D correspond to FIGS. 3A-3D, with the additional circuitry of the present invention. In particular, referring to FIG. 4A, there is the addition of transistor switches M 2 , M 4 , M 6  and M 8  with their corresponding current sources. The current provided through transistors M 4  and M 6  I npi  or I pni  is a fractional current of I pp  or I nn . 
     Consider the filter in accordance with an embodiment of the present invention shown in FIG. 4A with pump-up condition applied. Current sources I PP , I NN  of current I and I PNI , I NPI  of current            N   -   1     N                   I                          
     are “ON”. It can be seen that current I flows through the resistors while current            I   N     =     I   -         N   -   1     N                   I         ,                          
     goes through the capacitor C′ loop . Therefore, the voltage between V p  and V n  is            V   p     -     V   n       =       IR   +       I   N                     1     SC   loop   ′           =     IR   +     I     S                   (     NC   loop   ′     )                                    
     Hence,              Z   =           V   p     -     V   n       I     =     R   +     I     S                   (     NC   loop   ′     )                     (   2   )                                
     Therefore, the contribution to the total impedance from capacitor C′ loop  in this case, is seen as though it is coming from an effective capacitance C eff =NC′ loop  if the prior arts conventional filter configuration in FIG. 3 had been used. If the impedance in Eq. (2) is to be equal to that of Eq. (1), in other words, C eff =C loop , then,            C   loop   ′     =       C   loop     N       ;                          
     A N-time smaller capacitor can be used to yield the same result. 
     In FIG. 4B, a pump down condition is applied. Current sources I PN , I NP  of current I and I PPI , I NNI  of current            N   -   1     N                   I                          
     are “ON”, I pp , I nn  of current I and l pni , I npi  of current            N   -   1     N                   I                          
     are “OFF”. Again, it can be seen that current I flows through the resistors while current            I   N     =     I   -         N   -   1     N                   I         ,                          
     goes through the capacitor C′loop and the impedance between V p  and V n  is identical to that of Eq. (2). FIGS. 4C and 4D are for both pump up, pump down are “ON” and “OFF” conditions respectively. Similar to the conventional cases, no current flows in the filter. These complete all combinations of pump up/pump down conditions for the modified filter topology. 
     FIG. 5 shows an embodiment of the present invention as an integrated loop filter in a charge pump circuit with the additional transistor switches of FIGS. 4A-4D. As can be seen, compared to FIG. 1A, transistor switches M 1 , M 3 , M 5  and M 7  have been added, along with different connections to the loop filter. In place of the current steering transistor  17  of FIG. 1A, two differently sized current steering transistors  40  and  42  have been added. The same applies to current steering transistors  44  and  46 ,  48  and  50 , and  52  and  54 . 
     Again, in FIG. 5, consider first the pump up condition where ‘pup’ (the pump up signal) and ‘pdn 13 b’ (the complement of pump down signal) are “HIGH”, and ‘pup 13 b’ (the complement of pump up signal) and ‘pdn’ (the pump down signal) are “LOW”. Transistors M 1  (with W/L=N), M 2  (with W/L=N−1), M 7  (with W/L=N) and M 8  (with W/L=N−1) are “ON”; M 3  (with W/L=N), M 4  (with W/L=N−1), M 5  (with W/L=N) and M 6  (with W/L=N−1) are “OFF”. The flow of currents in this situation is exactly identical to what has been discussed previously in FIG. 4A, i.e., current I from transistor M 1  flows into V p , then through resistor R p . A fraction of the current              N   -   1     N        I     ,                          
     branches out to transistor M 8 . The remaining portion,        I   N                          
     flows through the capacitor, C′loop and recombines with current              N   -   1     N        I     ,                          
     from transistor M 2  to recover back to its original value I and then flows through resistor R n  and is subsequently drained by transistor M 7  to ground. This completes the charging cycle of the circuit. Next, for a pump-down condition (i.e., pdn and pup 13 b are “HIGH” and pdn 13  b and pup are “LOW”), the reverse is true; i.e., current I from transistor M 3  flows into V n , then through resistor R n . A fraction of the current              N   -   1     N        I     ,                          
     branches out to transistor M 6 . The remaining portion        I   N                          
     flows through the capacitor C′ loop  and recombines with current              N   -   1     N        I     ,                          
     from transistor M 4  to recover back to its original value I and then flows through resistor R p  and is subsequently drained by transistor M 5  to ground. For the condition of pump-up and pump-down both turned on at the same time (i.e., pup and pdn are “HIGH” and pup_b and pdn_b are “LOW”), currents from M 1 , M 2 , and M 3 , M 4  flow directly to M 5 , M 6  and M 7  and M 8  respectively without going through the filter. Finally for no pump-up and no pump-down condition (i.e., pup_b and pdn_b are “HIGH” and pup and pdn are “LOW”), all currents turn off and again no current goes through the filter. Therefore, the charge pump circuit of the preferred embodiment handles all combinations of pump-up and pump-down conditions. Furthermore, for either a pump up or a pump down condition, resistors R p  and R n  see a current of magnitude I and the capacitor C′ loop  sees a magnitude of          I   N     .                          
     From Eq. (2), the capacitor C′ loop  behaves effectively as if it is N times as big and hence an integrated on-chip capacitor can be utilized. 
     Finally, it is worthwhile to mention in passing that the above description of the present invention applies when shunt capacitors from V p  and V n  to ground are included. There might be a slight deviation from the designed value. 
     With the present invention one can avoid problems associated with an operational amplifier based capacitor multiplication circuits and the problems associated with the off-chip capacitor. 
     FIG. 6 illustrates a single-ended version of the present invention, as contrasted with the differential designs of FIG.  4 . This design uses current sources  60  and  62  providing a current I, and current sources  64  and  66  providing a current.            N   -   1     N          I   .                            
     The current sources are switched on by switches  68 ,  70 ,  72  and  74 . Alternately, the switches could be sized to provide the desired current, similar to as described in FIG. 5, with the unneeded current being shunted away from a current source providing a standard current I. 
     As can be seen, by appropriate closing and opening of the switches, a full current I can be provided through resistor  76 , while shunting off similar current so that only a fractional current flows through capacitor  78 . 
     As will be understood by those with skill in the art, the present invention may be embodied in other specific forms without departing from the essential characteristics thereof. For example, instead of using transistor sizing to provide the different currents, separate current sources could be used. In the embodiment of FIG. 5, the current steering transistors could be combined, such as combining transistors  40  and  42  into a single appropriately sized transistor. Accordingly, the foregoing description is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.