Voltage conversion circuit and control circuit therefor

Details Voltage conversion circuit and control circuit therefor Voltage conversion circuit and control circuit therefor

2001-02-01

2002-09-24

Vu, Bao Q. (Department: 2838)

Electric power conversion systems

Current conversion

With voltage multiplication means

C363S062000, C307S109000, C307S110000, C327S536000

Reexamination Certificate

active

06456513

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a voltage conversion circuit. More particularly, it relates to a voltage conversion circuit that generates an internal supply voltage by stepping up or stepping down an external supply voltage.
Particularly, a supply voltage generating circuit (generator), which is provided in a semiconductor memory device such as DRAM, generates an internal supply voltage, such as a stepped-up voltage supplied to word lines and a negative voltage supplied to the substrate, using an external supply voltage. When the external supply voltage becomes lower due to reduction in consumed power, the internal supply voltage also becomes lower. This requires a supply voltage generator with a sufficient current supplying capability and low power consumption even when an external supply voltage is relatively low.
FIG. 1A
is a schematic circuit diagram of a conventional stepped-up voltage generator
100
. A supply voltage Vcc is supplied to the anode of a diode D
1
from an external apparatus. The cathode of the diode D
1
is connected to the anode of a diode D
2
. The cathode of the diode D
2
is connected to the anode of a diode D
3
, and a stepped-up voltage Vpp is output from the cathode of the diode D
3
. A switch circuit SW
1
is connected in parallel to the diode D
3
.
A first input signal IN
1
is supplied to a node N
1
disposed between the diodes D
1
and D
2
via a capacitor C
1
. A second input signal IN
2
is supplied to a node N
2
disposed between the diodes D
2
and D
3
via a capacitor C
2
.
The stepped-up voltage generator
100
can selectively perform a one-stage step-up operation or two-stage step-up operation. In the one-stage step-up operation mode, when the switch circuit SW
1
is conducting, the clock input signal IN
1
having a predetermined frequency and the clock input signal IN
2
having a fixed level are supplied as shown in
FIGS. 1B and 1C
. The pumping operation by the diode D
1
and the capacitor C
1
steps up the voltage at the node N
1
to higher than the level of the supply voltage Vcc, so that the stepped-up voltage Vpp is supplied to a load circuit via the diode D
2
and the switch circuit SW
1
. In the one-stage step-up operation, the stepped-up voltage Vpp is ideally twice the supply voltage Vcc.
In the two-stage step-up operation mode, as shown in
FIG. 2A
, the clock input signals IN
1
and IN
2
have different phases and predetermined frequencies as shown in
FIGS. 2B and 2C
, and are supplied when the switch circuit SW
1
is nonconducting. The pumping operation by the diode D
1
and the capacitor C
1
and the pumping operation by the diode D
2
and the capacitor C
2
are alternately performed to step up the voltage at the node N
2
higher than the level of the supply voltage Vcc, so that the stepped-up voltage Vpp is supplied to the load circuit via the diode D
3
. In the two-stage step-up operation, the stepped-up voltage Vpp is ideally three times the supply voltage Vcc.
FIG. 3
is a graph showing the relationship between the output voltage and the maximum supply current in the stepped-up voltage generator. The horizontal axis shows the stepped up voltage Vpp in terms of a magnification with respect to the supply voltage Vcc. The vertical axis represents the allowable supply current.
With the same output voltage Vpp, the allowable supply current I
2
in the two-stage step-up operation mode is larger than the allowable supply current I
1
in the one-stage step-up operation mode. This is because the capacitor C
1
alone contributes to the pumping operation in the one-stage step-up operation mode whereas the capacitors C
1
and C
2
contribute to the pumping operation in the two-stage step-up operation mode. However, the two-stage step-up operation has a lower power efficiency than the one-stage step-up operation and thus suffers greater power consumption. As shown in
FIG. 3
, Ip indicates the consumed current of the load circuit to which the stepped-up voltage Vpp is supplied. The consumed current Ip increases in proportion to the voltage of the stepped-up voltage Vpp.
To reduce the power consumption of the stepped-up voltage generator while keeping a sufficient supply current to the load circuit, it is desirable that the one-stage step-up operation and the two-stage step-up operation should be switched at a voltage Va (set switch voltage) at which the consumed current Ip intersects the allowable supply current I
1
of. the one-stage step-up operation mode. That is, the one-stage step-up operation is performed when Vpp<Va, and the two-stage step-up operation is performed when Va<Vpp.
In a memory device, such as DRAM, the supply voltage Vpp is supplied to a selected word line and is higher than the supply voltage Vcc by the threshold value of cell transistors or larger. The difference between the supply voltage Vpp and the supply voltage Vcc therefore becomes substantially constant regardless of the level of the supply voltage Vcc. The higher the supply voltage Vcc becomes, the smaller the ratio of the supply voltage Vpp to the supply voltage Vcc becomes.
The consumed current Ip is substantially proportional to the supply voltage Vpp, and the absolute amount of the allowable supply currents I
1
and I
2
increase as the supply voltage Vcc rises. Therefore, as the supply voltage Vcc becomes higher, the consumed current Ip is relatively shifted to the lower portion of the graph of FIG.
3
.
When the supply voltage Vcc is relatively high, therefore, the set switch voltage Va moves to a high-voltage side. This widens the range of the supply voltage Vpp that can supply the allowable supply current I
1
greater than the consumed current Ip in the one-stage step-up operation, thus improving the power efficiency of the stepped-up voltage generator
100
.
When the supply voltage Vcc is relatively low, the set switch voltage moves to a low-voltage side. This narrows the range of the supply voltage Vpp that can supply the allowable supply current I
1
greater than the consumed current Ip in the one-stage step-up operation, thus lowering the power efficiency of the stepped-up voltage generator
100
.
The set switch voltage Va is set based on the supply voltage Vcc. It is however difficult to accurately detect the set switch voltage Va based on the supply voltage Vcc. If the one-stage step-up operation is changed to the two-stage step-up operation when the supply voltage Vpp higher than the set switch voltage Va is output, the allowable supply current I
1
falls to or below the consumed current Ip. This causes the supply voltage Vpp to fall.
One way to prevent the allowable supply current I
1
from becoming lower than the consumed current Ip is to change the one-stage step-up operation to the two-stage step-up operation when the supply voltage Vpp sufficiently lower than the set switch voltage Va is output. In this case, however, the two-stage step-up operation is performed in the voltage range that is sufficient for the one-stage step-up operation. This lowers the power efficiency of the stepped-up voltage generator and thus increases the consumed power of the entire device.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a voltage conversion circuit which has an improved power efficiency and lower power consumption.
In a first aspect of the present invention, a voltage conversion circuit is provided. The voltage conversion circuit includes a plurality of voltage conversion cells each including a capacitor element. A switch circuit is connected to the plurality of voltage conversion cells to selectively switch between parallel connections of a plurality of voltage conversion cells and serial connections of a plurality of voltage conversion cells. A control circuit is connected to the switch circuit to control the switch circuit to selectively perform first voltage conversion of an input voltage by the plurality of parallel-connected voltage conversion cells and second voltage conversion of the input voltage by the plurality of series-connected voltage conversion ce

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