Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Synchronizing
Reexamination Certificate
2003-03-13
2004-09-21
Nguyen, Linh M. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Synchronizing
C327S148000
Reexamination Certificate
active
06794911
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a charge-pump circuit for the PLL (Phase Lock Loop), and more particularly to a feedback charge-share suppressing charge-pump circuit.
2. Description of the Related Art
When generating required system clocks, it is necessary to input referenced clocks in phase lock loops (PLL) or clock synthesizers.
FIG. 1
is a block diagram illustrating a conventional phase lock loop system. The phase lock loop system
100
comprises a phase frequency detector
102
, a charge-pump circuit
104
, a voltage-controlled oscillator
106
, a divider
108
and a loop filter
110
. The phase lock loop system
100
receives a referenced clock F
in
. The charge-pump circuit
104
drives the voltage-controlled oscillator
106
to generate a required clock F
out
at a signal generated by the phase frequency detector
102
. The required clock F
out
is fed back to the phase frequency detector
102
through the divider
108
. The loop filter
110
is coupled to an output terminal of the charge-pump circuit
104
. A detailed diagram of the conventional charge-pump circuit
104
and loop filter
110
is shown in
FIG. 2
a.
FIG. 2
a
is a circuit diagram illustrating a conventional charge-pump circuit and loop filter as shown in FIG.
1
. As shown in
FIG. 2
a
, the charge-pump circuit
104
comprises current sources
202
and
204
, switches
206
and
208
, and a capacitor
210
. The loop filter
110
is composed of a capacitor
212
. The loop filter
110
is coupled to an output terminal of the charge-pump circuit
104
. When the switch
206
is in “On” state, the loop filter
110
is charged by the current sources
202
. When the switch
208
is in “On” state, the loop filter
110
supplies the stored power to the switch
208
and the current source
204
. A charge-share problem occurs in the charge-pump circuit
104
.
FIG. 2
b
is a diagram illustrating the charge-share problem in the charge-pump circuit shown in
FIG. 2
a
. The X axis is time, in units of seconds (s). The Y axis is the output voltage Vc, in units of volts (V). Line
22
shows a normal voltage curve. Dotted line
24
shows a curve of the voltage affected by the charge-share problem when the switch
206
changes from “Off” state to “On” state. Because of the charge-share problem, the signal driving the controlled oscillator
106
is incorrect. Thus, the phase lock loop system cannot generate the required clock.
To preventing the charge-share problem, Ian A. Young, Jeffrey K. Greason, and Keng L. Wang provide a charge-pump circuit for charge-share suppression with an operational amplifier (referring “A PLL Clock Generator with 5 to 110 MHz of Lock Range for Microprocessors,” IEEE J. Solid State Circuits, vol. 27, pp. 1599-1607 November 1992).
FIG. 3
is a circuit diagram illustrating the conventional charge-pump circuit for charge-share suppression with the operational amplifier OP
1
. As shown, when the switches S
1
and S
4
are in “Off” state and the switches S
2
and S
3
are in “On” state, through the operational amplifier OP
1
, the voltage on the node N
1
is equal to V
c
. When the switches S
2
and S
3
is in “Off” state and the switches S
1
and S
4
is in “On” state, through the operational amplifier OP
1
, the voltage on the node N
2
is equal to V
c
. Thus, the charge-share problem does not occur. The disadvantage of this circuit is that the operational amplifier must work in the wide range of the input frequency and respond quickly to all input frequencies. The result of the charge-share suppression in this circuit completely depends upon the operational capacity of the operational amplifier. A fine design of the operational amplifier is preferred to the result of the charge-share suppression. It is difficult to design such an operational amplifier. Thus, the design of the charge-pump circuit becomes more complex.
To overcome the above problem, Hee-Tae Ahn and David J. Allstet provide a charge-pump circuit for charge-share suppression with transistors (referring to Hee-Tae Ahn and David J. Allstet “A Low-Jitter 1.9 V CMOS PLL for UltraSPARC Microprocessor Applications, ” IEEE J. Solid-State Circuits, vol. 35, pp. 450-454 March 2000).
FIG. 4
is a circuit diagram illustrating the conventional charge-pump circuit for charge-share suppression with the transistors Q
1
and Q
2
. As shown in
FIG. 4
, when the switch S
1
is in “Off” state and the switch S
2
is in “On” state, a difference in voltage between a source and a gate of the transistor Q
1
is V
Q1
. Thus, the voltage on the node N
1
is equal to V
c
+V
Q1
. If the voltage depleted in an impedance of the switch S
1
in “On” state is V
Q1
, the charge-share problem does not occur. When the switch S
2
is in “Off” state and the switch S
1
is in “On” state, a voltage between a source and a gate of the transistor Q
2
is V
Q2
. Thus, the voltage on the node N
2
is equal to V
c
+V
Q2
. If the voltage depleted in an impedance of the switch S
2
in “On” state is V
Q2
, the charge-share problem does not occur. Therefore, his circuit has many problems. When the switch is in “On” state, there are two current paths, and it is difficult to detect the current through the switch. Thus, the impedance of the switch in “On” state is difficult to determine, to resolve the charge-share problem. Also, when the current through the switch is small, the impedance of the switch in “On” state must be large. However, in the design of the switch, the impedance of the switch in “On” state must be small. The large impedance of the switch in “On” state cannot be implemented in practical circuit. Finally, the large impedance of the switch in “On” state can be influenced by different procedures and environments. Thus, the design the charge-pump circuit becomes more complex.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a charge-pump circuit for charge-share suppression without adding operational amplifiers to decrease the difficulty of design for the charge-pump circuit.
Another object of the present invention is to provide a charge-pump circuit for charge-share suppression with a feedback path to resolve the problems in the conventional charge-pump circuit shown in FIG.
4
.
Accordingly, the present invention provides a charge-pump circuit for charge-share suppression comprising a first current source, a first switching element, a first load, a second switching element, a second current source, a second load, a first feedback circuit and a second feedback circuit. The first current source receives a voltage from a voltage generator and provides a current output. The first switching element is coupled between a first connecting node and an output terminal. The first switching element is controlled by an input signal. The first load is coupled between the first switching element and the output terminal. The first load receives the current and outputs an output voltage at the output terminal when the first switching element is in “On” state. The second switching element is controlled by the input signal and opposite to the first switching element. The second current source is coupled between the second switching element and ground and is coupled to the second switching element through a second connecting node. The second load is coupled between the second switching element and the output terminal. The second load receives the output voltage when the second switching element is in “On” state. The first feedback circuit maintains a constant relation between the output voltage and a voltage of the first node, and is not influenced by the status of the first and second switching elements. The second feedback circuit maintains a constant relation between the output voltage and a voltage of the second node, and is not influenced by the status of the first and second switching elements.
REFERENCES:
patent: 5801578 (1998-09-01), Bereza
patent: 6255872 (2001-07-01), Harada et al.
patent: 6586976 (2003-07-01), Yang
Industrial Technology Research Institute
Nguyen Linh M.
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