Static information storage and retrieval – Read/write circuit – Including level shift or pull-up circuit
Reexamination Certificate
2001-08-22
2002-09-03
Le, Vu A. (Department: 2824)
Static information storage and retrieval
Read/write circuit
Including level shift or pull-up circuit
Reexamination Certificate
active
06445623
ABSTRACT:
FIELD OF INVENTION
The present invention relates to charge pumps for use in integrated circuits. More particularly, the present invention relates to a charge pump circuit having current sources configured for supplying current to the charge pump capacitors to regulate the charging and discharging current of the charge pump.
BACKGROUND OF THE INVENTION
The demand for less expensive, and yet more reliable integrated circuit components for use in communication, imaging and high-quality video applications continues to increase rapidly. As a result, integrated circuit manufacturers are requiring improved performance in the voltage supplies and references for such components and devices to meet the design requirements of such emerging applications.
One device utilized for providing a regulated voltage supply is a charge pump circuit. Charge pumps are DC/DC converters that utilize a capacitor instead of an inductor or transformer for energy storage, and are configured for generating positive or negative voltages from the input voltage. One more common type of charge pump utilized in circuits comprises one configured for doubling the input voltage, i.e., a charge pump voltage doubler, while other frequently utilized charge pumps comprises tripler and inverter configurations. These charge pumps can operate to multiply the input voltage by some factor, such as by one-half, two, or three times or any other suitable factor of the input voltage, to generate the desired output voltage.
Charge pumps typically utilize transistors and/or diodes as ideal switching devices to provide current paths for charge transfer. On occasions when the sum of any residual voltages of the charging capacitors in a charging/discharging circuit loop is smaller than the voltage of the power supply, a significantly large amount of transient current tends to flow within the loop. Further, an uncontrolled peak current occurring during charging or discharging of the capacitors is only limited by the “on-resistance” of the switching devices and the equivalent series resistance (ESR) of the capacitors. Moreover, the uncontrolled alternating peak current generally flows out of the positive power supply and into the negative power supply, e.g., into ground, thus causing significantly large electromagnetic interference (EMI) to associated electronic circuits. Still further, the large uncontrolled current also tends to charge the output reservoir capacitor which results in large voltage ripples at the output of the charge pump.
To limit the voltage ripple to a tolerable level, the reservoir capacitor can be configured with a larger capacitance value. However, such an arrangement is not desirable in that such a larger value capacitor results in a larger total printed circuit board area and higher manufacturing costs.
Another problem related to charge pump voltage doublers, triplers and inverters is the requirement for higher voltage processes than the nominal supply voltage. Since the output voltage, or the voltage at the internal nodes compared to the lowest potential in the charge pump circuit, can reach a level twice or more of the supply voltage, the breakdown voltage requirement for the processes used to manufacture these charge pumps is not limited by the supply voltage or the output voltage. Moreover, the voltage across some of the switching devices within the charge pumps may exceed the maximum allowable voltage for a given process. Accordingly, processes with higher breakdown voltages are essential requirements for charge pump regulators implemented within integrated circuit applications, thus resulting in increased costs and circuit size compared to circuits implemented in low voltage processes.
With reference to
FIG. 1
, a conventional charge pump
100
configured as a voltage doubler is illustrated. Charge pump doubler
100
comprises four switches S
1
, S
2
, S
3
and S
4
, a pump capacitor C
PUMP
, and a reservoir capacitor C
RES
. The charging and discharging current of capacitor C
PUMP
is determined by the difference between an supply voltage V
IN
and an output voltage V
OUT
, the on-resistance for the switches S
1
, S
2
, S
3
and S
4
, and the ESR of capacitors C
PUMP
and C
RES
.
Charge pump doubler
100
is typically controlled by a clock having a 50% duty cycle, i.e., a clock having a clock phase-one and phase-two. During clock phase-one, switches S
2
and S
3
are turned “on” to charge capacitor C
PUMP
to approximately the supply voltage V
IN
, while switches S
1
and S
4
remain in an “off” condition. During clock phase-two, switches S
1
and S
4
are turned “on”, while switches S
2
and S
3
are turned “off”, to charge capacitor C
RES
to a higher voltage potential. If the output voltage V
OUT
is not otherwise regulated, output voltage V
OUT
will reach a value of approximately twice the supply voltage V
IN
.
The selection of the on-resistance for switches S
1
, S
2
, S
3
and S
4
is difficult, in that while switches S
1
, S
2
, S
3
and S
4
generally need to be configured to provide a high enough on-resistance to limit any inrush current, switches S
1
, S
2
, S
3
and S
4
must also be configured to provide a low enough on-resistance to output sufficient current at a low level of supply voltage V
IN
. Due to the concurrent need to provide such a low enough on-resistance, a large inrush of current can occur during the closing of switches S
2
and S
3
, or S
1
and S
4
, thus resulting in conductive noise within the charge pump doubler
100
. In addition, the on-resistances of switches S
1
, S
2
, S
3
and S
4
are generally configured on the same order of magnitude in an attempt to optimize performance and minimize the die size, and thus the voltage drops on all switches S
1
, S
2
, S
3
and S
4
are also on the same order of magnitude, regardless of the location and purpose of switches S
1
, S
2
, S
3
and S
4
, e.g., whether for charging or discharging functions.
As discussed, higher voltage processes are generally required for prior art charge pumps. For example, with reference to
FIG. 1
, let's assess a case where V
IN
and V
OUT
have the same voltage level and S
1
and S
4
have the same on-resistance, RON, i.e., the sizes of S
1
and S
4
are similar. In steady state, pump capacitor C
PUMP
may be charged to a voltage close to V
IN
in the charging phase, and S
2
and S
3
are “closed,” and S
1
and S
4
are “open.” At the moment S
2
and S
3
are opened and S
1
and S
4
are closed, due to the fact that S
1
and S
4
have the same on-resistance R
ON
and the voltage across C
PUMP
, i.e., V
PUMP
equals V
IN
and V
OUT
, the inrush current, I
INRUSH
becomes:
(V
IN
+V
PUMP
−V
OUT
)/(2*R
ON
) or V
OUT
/(2*R
ON
).
Therefore, the voltage at node B, V
B
, becomes much higher than V
IN
or V
OUT
:
V
B
=V
OUT
+I
INRUSH
*R
ON
, or
V
B
=1.5*
V
OUT
.
This result means that the breakdown voltage of the process needs to be at least 1.5 times the rated input or output voltage of the regulator.
Accordingly, this scenario prevents processes with voltage ratings as low as the supply voltage V
IN
and the output voltage V
OUT
from being utilized for charge pump doublers. Moreover, as the supply voltage V
IN
increases from a low level to a high level, the on-resistance of switches S
1
, S
2
, S
3
and S
4
tends to decrease exponentially to a very low level due to a higher gate driving voltage. Accordingly, the current flowing through, i.e., the current flowing to ground and to capacitor C
RES
, becomes excessively large, thus resulting in uncontrolled output voltage ripple and conducted EMI.
A large number of currently available charge pumps, including charge pump regulators known as switched-capacitor regulators, utilize low on-resistance switches in order to boost current output capability at low supply voltages. The use of low on-resistance switches causes the injection of large uncontrolled pulsed current into the output or reservoir capacitor and to ground, thus generating larger EMI and output voltage ripple.
One approach for resolving the problems discusse
Waterfall Gregory S.
Zhang Haoran
Brady W. James
Le Vu A.
Swayze, Jr. W. Daniel
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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