Pulse or digital communications – Synchronizers – Phase displacement – slip or jitter correction
Reexamination Certificate
1998-09-29
2002-07-02
Tse, Young T. (Department: 2634)
Pulse or digital communications
Synchronizers
Phase displacement, slip or jitter correction
C375S376000, C327S148000, C327S157000
Reexamination Certificate
active
06415007
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a charge pump, and more particularly to a charge pump for use in a PLL (Phase Locked Loop).
The PLL has recently been mounted not only in microprocessors but also in high-speed memories, and is considered more and more important. For particulars of the PLL, see, for example, Deog-Kyoon Jeong et al., “Design of PLL-Based Clock Generation Circuit” IEEE Journal of Solid-State Circuits, Vol. SC-22, No. 2, pp. 255-261, April 1987.
The PLL generally has a charge pump, which will be described.
FIG. 1
is a circuit diagram, showing an example of a charge pump. This charge pump operates as below, supposing that a low pass filter (LPF) is connected to the charging terminal of the charge pump.
A p-channel MOS transistor (hereinafter referred to simply as a “pMOS transistor”) P
100
is driven by a charge signal as shown in FIG.
2
. Then, the charge pump charges the LPF with electricity corresponding to the pulse width of the charge signal. Further, an n-channel MOS transistor (hereinafter referred to simply as an “nMOS transistor”) N
100
is driven by a discharge signal as shown in FIG.
2
. Then, the charge pump discharges, from the LPF, electricity corresponding to the pulse width of the discharge signal.
The relationship between the total charge and the pulse width is controlled depending upon the sizes of current-limiting transistors P
102
and N
102
which are located at the source-sides of the pMOS transistor P
100
and the nMOS transistor N
100
, or upon the input gate voltage (Vref).
Moreover, an nMOS transistor N
104
and a pMOS transistor P
104
are controlled by an enable signal and an enable bar signal which is the inverted signal of the enable signal, respectively. The nMOS transistor N
104
and the pMOS transistor P
104
serve as filters for interrupting a pass current when the charge pump is in a standby state, and removing switching noise while the charge pump operates.
As is shown in
FIG. 1
, there exist PN junctions at the drain-sides of the limiter transistors P
102
and N
102
of the charge pump, and junction capacitors C
102
and C
104
exist at the PN junctions. Accordingly, at the start of the charge pump, the current-limiting transistor P
102
or N
102
operates for the first time after the junction capacitor C
102
or C
104
is charged or discharged. This being so, electricity discharged from the junction capacitor C
102
or that to be accumulated into the junction capacitor C
104
(the electricity will be referred to as a “to-be-offset (or offset) charge”) is added to electricity to be accumulated into or discharged from the low pass filter.
The offset charges will be described in detail. A case where charging is performed at the pMOS transistor side of the charge pump will be examined. If no junction capacitor C
102
exists, the potential of the junction between the limiter transistor P
102
and pMOS transistor P
100
becomes less than a power voltage VDD and reaches the operation voltage of the limiter transistor P
102
at the moment the charge signal level has changed to “L (Low)”. The current which flows when the operation voltage has been reached is an ideal average current.
On the other hand, where there exists the junction capacitor C
102
at the junction between the limiter transistor P
102
and the pMOS transistor P
100
as shown in
FIG. 1
, the potential of the junction has a value between the power voltage VDD and an operation voltage V
1
until electricity flows from the junction capacitor C
102
to the LPF via the pMOS transistor P
100
, even if the charge signal has shifted to “L”, as is shown in FIG.
3
. At this time, even if the pMOS transistor P
100
operates in the pentode area, a current which includes an overshooting portion as shown in FIG.
3
and hence is greater than the ideal average current flows into the LPF. This extra current is the cause of the offset, and the hatched overshooting portion in
FIG. 3
corresponds to the offset current.
When the pulse width of each of the discharge and charge signals is sufficiently large, the effective average current (=(total charge)/(pulse width)) is kept substantially constant even if a slight current is discharged from the junction capacitor C
102
or charged into the junction capacitor C
104
. However, where the pulse width is small, only a slight offset current will increase the effective average current. An increase in the effective average current requires an increase in the capacity of the LPF in order to keep the entire PLL system stable, which will inevitably increase the lockup time or layout area.
BRIEF SUMMARY OF THE INVENTION
It is the object of the invention to provide a charge pump which is substantially free from the influence, upon a total charge, of a junction capacitor existing at a PN junction, even if the pulse width of a driving signal used in the pump is small, and hence in which pump the effective average current shows only a little dependency upon the pulse width during the charging or discharging operation of the pump.
According to a first aspect of the invention, there is provided a charge pump comprising: a first sub charge pump having a plurality of transistors for performing charging and discharging operations; and a second sub charge pump having a charging/discharging terminal common to that of the first sub charge pump, the second sub charge pump discharging electricity which is charged via the charging/discharging terminal when the first sub charge pump has performed a charging operation using electricity accumulated in a junction capacitor at a PN junction which exists between the transistors of the first sub charge pump, the second sub charge pump charging electricity via the charging/discharging terminal when the first sub charge pump has performed a discharging operation to charge the junction capacitor.
The charge pump constructed as above can substantially cancel electricity which is discharged from the junction capacitor existing at the PN junction, and is used as a charging current at the time of charging, and also can substantially cancel electricity which is accumulated in the junction capacitor and used as a discharging current at the time of discharging. As a result, even if the pulse width of a driving signal is small, the influence of the junction capacitor upon the total charge can be minimized, thereby suppressing an increase in average current when the charge pump performs charging and discharging operations, and reducing the dependency of the effective average current upon the pulse width.
According to a second aspect of the invention, there is provided a charge pump comprising: a first sub charge pump having a plurality of transistors for performing charging and discharging operations; and a second sub charge pump having a charging/discharging terminal common to that of the first sub charge pump, the second sub charge pump performing a charging operation for a time period shorter by a predetermined time period than a charging time period for which the first sub charge pump performs a charging operation, the second sub charge pump performing a discharging operation for a time period shorter by a predetermined time period than a discharging time period for which the first sub charge pump performs a discharging operation.
In the charge pump constructed as above, the first sub charge pump performs a charging operation for the charging time period, and the second sub charge pump performs a charging operation for a time period shorter by a predetermined time period than the charging time period of the first charge pump. On the other hand, the first sub charge pump performs a discharging operation for the discharging time period, and the second sub charge pump performs a discharging operation for a time period shorter by a predetermined time period than the discharging time period of the first charge pump. As a result, variations in average current during charging and discharging of the charge pump can be minimized, thereby reducing the dependency of the effective average current upo
Kabushiki Kaisha Toshiba
Tse Young T.
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