Patent Publication Number: US-6710643-B1

Title: Circuit technique to eliminate large on-chip decoupling capacitors

Description:
TECHNICAL FIELD 
     The field of the invention is that of integrated circuits having on-chip power supplies. 
     BACKGROUND OF THE INVENTION 
     On-chip voltage regulators and dc to dc converters are increasingly used in integrated semiconductor chips. Charge pumps are typically used to convert supply voltages to higher voltages or to lower voltage. In voltage converters, the standard supply voltage is used to drive an oscillator. The oscillator signal is used in turn to charge the output up or down to the required value. Charge pumps usually use voltage regulation to compensate for process and supply voltage variations and to maintain the output at the required voltage level. They also use large decoupling capacitors to reduce ripple voltage when load current is drawn from the regulated output. 
     When the magnitude of the converted voltage output goes below the required levels, one or more pump cycles are needed to restore the output back to the required voltage. 
     DECOUPLING CAPACITOR 
     The oscillator frequencies used in these charge pumps are typically very small compared to the frequency of the active cycle. For example, a memory chip with-access time and cycle time less than 10 ns may use oscillator frequencies 1 MHz to 20 MHz. Even during an active cycle, load current is drawn from the regulated output only during a fraction of the active cycle. For example, for a system with active clock period of 10 ns, load current may be active only for 2 ns. The lower oscillator frequencies are used in the charge pump to minimize inefficiencies of the charge pump, as well as to reduce power consumption. As a result, several active chip cycles may take place during one pump cycle. A large decoupling capacitor is necessary during this time, to provide charge for the load current with low ripple in the output voltage of the charge pump. Decoupling capacitors occupy considerable chip area. Planar gate area capacitors, or trench capacitors, may be used for decoupling. Trench decoupling capacitors would use less area compared to planar capacitors, but trench capacitors would add to processing cost. Trench capacitors also have higher ohmic and parasitic losses associated with it. 
     SUMMARY OF THE INVENTION 
     The invention relates to a circuit technique that not only eliminates the need for large decoupling capacitors, but restores the output voltage to the required level at a faster rate. 
     A feature of the invention is an on-chip power supply that employs at least two decoupling capacitors connected in parallel. 
     A feature of the invention is the use of a smaller decoupling capacitor together with a supplementary capacitor for supplying reserve charge. 
     Another feature of the invention is a controllable connection for connecting the two capacitors in parallel when the output node declines in voltage by a threshold amount. 
     Another feature of the invention is the maintenance of the supplementary capacitor at a higher voltage than the decoupling capacitor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic drawing of the invention. 
     FIG. 2 shows the time dependence of voltage shift using the invention. 
     FIG. 3 shows a schematic of a prior art circuit. 
     FIG. 4 shows time dependence corresponding to FIG.  3 . 
    
    
     DETAILED DESCRIPTION 
     The typical output stage of a charge pump is schematically shown in FIG.  3 . Charge is pumped and stored in the decoupling capacitor  130 ′, and an output voltage V out  is maintained. Note that V out  can be positive or negative. When V out  reaches the required magnitude, the pump is disabled. When the magnitude of V out  goes below the required level due to leakage, or load current, the pump becomes active again. Ripple Voltage, 
     
       
           dV=I   L   t   a /( C   1 )  (1) 
       
     
     where I L  is the load current and t a  is the active time period. 
     A large decoupling capacitor is required to bring the ripple voltage dV to acceptable levels. FIG. 4 schematically shows how the load current affects the V out  voltage in curve  30 ′. The load current brings V out  down. Voltage V out  stays low until the next pump cycle starts. One or more pump cycle may become necessary to restore V out . In this example here, we are assuming V out  to be positive. FIG. 1 shows a circuit according to the invention attached to the pump output to eliminate large decoupling capacitors. Capacitor  130  and capacitor  135  are two decoupling capacitors, T 1  ( 110 ) and T 2  ( 120 ) are transistors. T 1  controls the charging of capacitor  130  and T 2  controls the charging of capacitor  135 . Normally transistor T 3  ( 220 ) is turned off, capacitor  130  is charged to V out  and capacitor  135  is charged to a storage voltage having a magnitude V out +ndV, where n, the storage voltage factor, is any number and dV is the magnitude of the Ripple Voltage. The pump first charges capacitor  130  to V out , and when capacitor  130  is not drawing current, capacitor  135  is charged to V out ,+n dV by the same pump  100 . 
     The differential amplifier  210  that controls transistor T 3  is fast, and its response time is very short compared to the active time period t a . When the load current, I L , is drawn from V out , the magnitude of the voltage V out  drops. During the active time period, if the magnitude of V out  decreases, the differential amplifier turns the transistor T 3  on, and V out  is quickly restored. Charge is drawn from capacitor  135  until the capacitor  130  is charged to V out . When V out  is fully restored, T 3  is turned off. Now the Ripple Voltage, 
     
       
           dV=I   L   t   a /( C   1 + nC   2 )  (2) 
       
     
     For simplicity, assume C 1 =C 2 =C. Now the Ripple Voltage becomes, 
     
       
           dV=I   L   t   a /( n+l ) C   (3) 
       
     
     Thus, the decoupling capacitance can be effectively reduced by a factor of n. The additional elements used here are, control  1 , control  2 , transistors T 1 , T 2 , and T 3 , and the differential amplifier. The combined area for these is only a small fraction of typical decoupling capacitors. Note that V out  can be positive or negative. What is important for a small ripple in the output voltage is that the magnitude of the sum of capacitance (C 1 +nC 2 ). Preferably, the designer will choose capacitance C 2  to be (1/n)C 1 . 
     FIG. 2 schematically shows how the load current (curve  20 ) affects the V out  voltage with the new circuit. The load current brings V out  down. The differential amplifier detects this voltage drop, and turns T 3  on. Charge is now transferred from capacitor  135  to capacitor  130  through transistor T 3 . The voltage V out  is quickly restored. There is an overshoot in V out  (shown in curve  30 ) and the fast comparator/differential amplifier turns T 3  off. V out  is now restored within the active period itself. 
     Control  1  and control  2  are conventional circuits, well known to those skilled in the art. Control  1  is similar to the prior art circuit controlling pump  100 . It senses the voltage on output node  150  and, when it is less than its nominal value by a threshold amount, turns on pump  100 . Control  2  is similar, except that it contains logic preventing it from turning transistor T 2  on when transistor T 1  is on. Optionally, control circuit  112  could have logic overriding the normal sequence in order to maintain a minimum amount of charge on capacitor  135 . A circuit designer will make a design decision on the relative priority to award to the two charge systems. 
     While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced in various versions within the spirit and scope of the following claims.