Charge Pump CMOS Circuit

Abstract

A charge pump CMOS circuit comprises a differential input stage with two parallel circuit branches. Each of the parallel circuit branches has a diode-connected MOS transistor connected in series with a complementary input MOS transistor. There is a common tail current source for both circuit branches. The diode-connected MOS transistors each have their gate/drain node connected to corresponding current sources. The charge pump CMOS circuit is suitable for use in an oscillator.

Description

TECHNICAL FIELD OF THE INVENTION

The technical field of this invention is generally CMOS charge pump circuits. More particularly, but not exclusively, the present invention relates to a pre-bias mechanism for charge pumps in clock control applications.

BACKGROUND OF THE INVENTION

Clock control applications generally require a charge pump controlled by a digital clock signal, for example in regulation of the duty cycle of a crystal oscillator in an ultra-low power microcontroller circuit. When a charge pump is controlled by a clock signal, the charge pump switches between a positive and a negative current controlled by the clock signal. Switching of the full output current causes a larger than required voltage change in diode connected MOS transistors used in current mirror operational amplifiers in the charge pump circuit. This increases the delay of the charge pump.

SUMMARY OF THE INVENTION

The present invention is a charge pump CMOS circuit, including a differential input stage with two parallel circuit branches. Each of the parallel circuit branches has a diode connected MOS transistor connected in series with a complementary input MOS transistor. The parallel circuit branches have a common tail current source. The diode-connected MOS transistors each have their gate/drain node connected to a current source. This provides a pre-bias scheme, which avoids complete discharge of the diode-connected MOS transistors during switching, therefore reducing the delay in charging up the voltage nodes in the driver.

Preferably, each of the parallel circuit branches has an associated current mirror stage. One of the current mirror stages can be a single-ended output stage.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of this invention are illustrated in the drawings, in which:

FIG. 1 illustrates a charge pump CMOS circuit according to the invention; and

FIG. 2 illustrates the clock signals applied to the inputs of the charge pump CMOS circuit according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a charge pump CMOS circuit, which is basically a current mirror OTA. The circuit includes an N-channel MOS transistor MN0 having a source terminal connected to a source terminal of another N-channel MOS transistor MN1. Gate terminals of transistors MN0 and MN1 receive respective differential input signals so that the transistors MN0 and MN1 are differential input stages. The drain terminal of the transistor MN0 is connected to the drain terminal of a P-channel MOS transistor MP3. The drain terminal of the transistor MN1 is connected to the drain terminal of a P-channel MOS transistor MP4. The transistor pairs MN0 and MP3, and MN1 and MP4 form parallel circuit branches. Transistors MP3 and MP4 are diode connected and have interconnected source terminals. The connection of the gate and drain terminals of the transistor MP3 forms a voltage node Vb. The connection of the gate and drain terminals of the transistor MP4 forms a voltage node Va.

The source terminal of the transistor MP3 is also connected to the source terminal of another P-channel MOS transistor MP2. The source terminal of the transistor MP4 is connected to the source terminal of a P-channel MOS transistor MP5. Thus the source terminals of all the transistors MP2-MP5 are interconnected. The transistor pairs MP2 and MP6, and MP5 and MP7 form current mirror stages associated with each of the two parallel branches formed by the transistors MN0 and MN3; and MP4 and MN1, respectively. Each current mirror stage is amplifies the signal output from each of the branches by a factor depending on the actual physical size of the transistors MP2 and MP6, and MP5 and MP7.

The drain terminal of the transistor MP2 is interconnected with the drain terminal of an N-channel MOS transistor MN6. The drain terminal of the transistor MP5 is connected to the drain terminal of another N-channel MOS transistor MN7. The transistor pairs MP2 and MP3, MP4 and MP5, and MN6 and MN7, respectively have interconnected gate terminals. There is also an interconnection between the gate terminal and the drain terminal of the transistor MN6. The gate terminals of the transistors MN0 and MN1 receive respective input voltage signals Inm and Inp.

A current source Ib is connected between a node interconnecting the source terminals of the transistors MN0 and MN1 and a node interconnecting the source terminals of the transistors MN6 and MN7 so that the two parallel circuit branches have a common tail current source. A current source I1 is connected to the node Vb interconnecting the gate and drain terminals of the transistor MP3, the drain terminal of the transistor MN0 and the gate terminal of the transistor MP2. The current source I1 is also connected to the node interconnecting the source terminals of the transistors MN6 and MN7. A current source 12 is connected to the node Va interconnecting the source terminals of the transistors MN6 and MN7 and a node interconnecting the gate and drain terminals of the transistor MP4, the drain terminal of the transistor MN1 and the gate terminal of the transistor MP5. The current sources I1 and I2 provide respective bias currents to the nodes Vb and Va. The output node Out of the driver is provided at a node interconnecting the drain terminals of the transistors MP5 and MN7. Thus the current mirror stage comprising the transistors MP5 and MN7 is a single-ended output stage. An interconnection of the gate and drain terminals of the transistor MN6 forms a voltage node Vc.

In operation, differential input signals Inp and Inm are applied to the respective gates of the transistors MN0 and MN1. These input signals are illustrated in FIG. 2. The voltage node Vb is biased by the current source I1 and the voltage node Va is biased by the current source 12. The bias currents I1 and I2 cancel each other at the output so that they introduce no error to the output signal. The averaged output current from the driver then depends on the duty cycle of the input signals Inm and Inp.

The voltage nodes Va, Vb and Vc do not discharge fully when the input signals Inm and Inp to the corresponding transistors MN0 and MN1 are switched from high to low due to the current sources I1 and I2. Therefore the charge pump driver has a reduced switching delay. For example, if the input signal Inm applied to the gate of the transistor MN0 is at its maximum value and then switches to zero for the next cycle, this causes a large change in voltage of the voltage node Vb because the transistor MP3 discharges completely. This introduces a large delay because the voltage node Vb must be fully charged again when the input signal Inm switches back to high. However, because the node Vb is permanently charged via current source I1, the time required to charge the node Vb to its maximum voltage is reduced.

Furthermore, the presence of the current sources I1 and I2 means that the current generated by the current source Ib can be lower, while still keeping the switching time of the driver constant.

Although the present invention has been described with reference to a specific embodiment, it is not limited to this embodiment and no doubt further alternatives will occur to the skilled person that lie within the scope of the invention as claimed.

Claims

1. A charge pump CMOS circuit for use in an oscillator, comprising: a first diode-connected MOS transistor (MP3) having a source connected to a first common node and a drain and gate connected together (Vb);a first input MOS transistor (MN0) having a drain connected to the common drain and gate of said first diode-connected MOS transistor (Vb), a source connected to a second common node and a gate receiving a first differential input signal;a second diode-connected MOS transistor (MN1) having a source connected to said first common node and a drain and gate connected together (Va);a second input MOS transistor (MP4) having a drain connected to the common drain and gate of said second diode-connected MOS transistor (Va), a source connected to said second common node and a gate receiving a second differential input signal;a first current source (Ib) connected between said second common node and a third common node;a second current source (I1) connected between said common drain and gate of said first diode-connected MOS transistor and said third common node; anda third current source (I2) connected between said common drain and gate of said second diode-connected MOS transistor and said third common node.

2. The charge pump CMOS circuit of claim 1, wherein: said first and second diode-connected MOS transistors (MP3, MP4) are P-channel MOS transistors.

3. The charge pump CMOS circuit of claim 1, wherein: said first and second input MOS transistors (MN0, MN1) are N-channel MOS transistors.

4. The charge pump CMOS circuit of claim 1, further comprising: a first current mirror circuit including a first current mirror MOS transistor (MP2) having a source connected to said first common node, a gate connected to the common drain and gate (Vb) of said first diode-connected MOS transistor (MP3) and a drain, anda second current mirror MOS transistor (MN6) having a drain connected to said drain of said first current mirror transistor (MP2), a gate connected to said drain and a source connected to said third common node; and a second current mirror circuit includinga third current mirror MOS transistor (MP5) having a source connected to said first common node, a gate connected to the common drain and gate (Va) of said second diode-connected MOS transistor (MP3) and a drain,a fourth current mirror MOS transistor (MN7) having a drain connected to said drain of said third current mirror transistor (MP5), a gate connected to the common drain and gate of said second current mirror MOS transistor (MN6) and a source connected to said third common node, andan output terminal connected to said drain of said third current mirror MOS transistor (MP5) and said drain of said fourth current mirror MOS transistor (MN7).

5. The charge pump CMOS circuit of claim 4, wherein: said first and third current mirror MOS transistors (MP2, MP5) are P-channel MOS transistors.

6. The charge pump CMOS circuit of claim 4, wherein: said second and fourth current mirror MOS transistors (MN6, MN7) are N-channel MOS transistors.