WO2003088494A1

WO2003088494A1 - Low leakage charge pump

Info

Publication number: WO2003088494A1
Application number: PCT/SE2003/000574
Authority: WO
Inventors: Fredrik Jonsson
Original assignee: Spirea Ab
Priority date: 2002-04-16
Filing date: 2003-04-09
Publication date: 2003-10-23
Also published as: SE0201158L; SE522959C2; AU2003224527A1; SE0201158D0

Abstract

This invention relates to a circuitry connectable to a charge pump for reducing leakage current associated with said charge pump. The invention is based on the idea that the voltage in the node from which a charge pump load is charged or discharged is measured. The measuring device measures the potential of the node. When the charge pump is disconnected from the load, the measuring device drives its output to the same potential as its input. By employing this technique, a leakage current that in the prior art structure is driven via the node into the first switch because of a voltage drop on the switch input is reduced. The measuring device will force its output to the same potential as its input, and the corresponding leakage current is now produced and driven by the measuring device output instead. Thus, no leakage current is driven via the node.

Description

LOW LEAKAGE CHARGE PUMP

Technical Field of the Invention

This invention relates to a circuitry connectable to a charge pump for reducing leakage current associated with said charge pump.

Background Art

- In a frequency shift keying (FSK) radio communication system, such as the DECT and Bluetooth, the Local Oscillator (LO) can be used both to set the correct frequency channel and to modulate the carrier. To achieve this a VCO having two inputs is used, where one input receives a DC control voltage and the other receives a modulation signal. Thus, the output signal of the oscillator could be regarded as a modulated carrier. Usually, the carrier frequency is determined by means of a phased locked loop (PLL) . The PLL contains a phase detector, one input of which detector is connected to a frequency divider, which in turn receives the output of the VCO. The other input of the detector is connected to a reference frequency generator. The output of the detector is a function of the difference between the phases of the two input signals. The output of the detector is input to a low pass filter via a charge pump for generating the very control voltage, which is supplied to the VCO. Within the VCO, circuitry is arranged so as to generate an output signal which has a frequency that is dependent on both the control voltage and the modulation signal.

When it is desirable to use the VCO to generate a modulation signal only, it is necessary to open the PLL as it otherwise will try to lock to the modulation ■ signal. This is done by disconnecting the charge pump from the low pass filter. It is desirable that the amount of leakage current produced by the charge pump is small, as this current will enter the low pass filter and change the control voltage of the VCO, resulting in the fact that the output frequency of the VCO is altered.

Summary of the Invention

An object of the present invention is therefore to reduce the leakage current associated with the charge pump when the charge pump is disconnected from the filter.

This object is achieved by a circuitry connectable to a charge pump for reducing leakage current associated with the charge pump. The circuitry comprises a measuring device, a first switch and a second switch. The input of the measuring device is connected to the node of the charge pump from which a load is energized, the first switch is connected between the node and the output of the measuring device.- The first switch is also connected to a substrate on which the measuring device is arranged and the second switch is connected between the output of the measuring device and the substrate. Alternatively, the measuring device and the second switch can be merged into one single component, thereby forming a measuring device having a switchable output. The first switch is arranged to be in its non-conductive mode when the charge pump is disconnected from the load, the second switch is arranged to be in its conductive mode when the charge pump is disconnected from the load and the measuring device is arranged to drive the voltage drop across a main current path of the first switch to zero and/or to drive the voltage drop across a current collecting electrode of the first switch and the substrate to zero when the first switch is in its non-conductive mode. The leakage current associated with the charge pump is thereby reduced. Preferred embodiments are defined by the dependent claims. The invention is based on the idea that the voltage in the node from which a charge pump load is charged or discharged is measured. Since the node is connected to the first switch and to the input of a measuring ,device,- and since the first switch as well as the measuring device, via the second switch, are connected to the substrate, they are connected in parallel. The measuring device measures the potential of the node. When the charge pump is disconnected from the load, the measuring device drives its output to the same potential as its input- By employing this technique, a leakage current that in the prior art structure is driven via. the node into the first switch because of a voltage drop on the switch input is reduced. The measuring device will force its output to the same potential as its input, and the corresponding leakage current is now produced and driven by the measuring device output instead. Thus, no leakage current is driven via ^• the node.

According to an embodiment of the invention, the measuring device that is used comprises an operational amplifier arranged in a voltage follower configuration. In this configuration, the output of the amplifier will track the input. In the circuitry according to the invention, both PMOS and NMOS transistors are used. In the case of the first transistor, its drain is connected to the node from which the charge pump load is charged or discharged. By controlling the conduction of current through this transistor, it is possible to control the charging and discharging of the charge pump load. In case it is an NMOS transistor, the drain is attached to an n- doped connector. Accordingly, ^' in case it is a PMOS transistor, the drain is connected to a p-doped connector. In a conventional charge pump, a voltage drop will occur across this connector and the p-doped substrate (for an NMOS transistor) of the first transistor, resulting in a leakage current into the junction between the n-doped and p-doped material. When the charge pump is disconnected from the load, this first transistor is in a non-conductive mode. The input of the amplifier is connected to the charge/discharge node and the drain of the second transistor is connected to the output of the amplifier. This second transistor is in conductive mode when the charge pump is disconnected from the load. When the second transistor conducts, the amplifier will drive the substrate to the same potential as the n-doped material, causing the previously mentioned voltage drop to occur at the output of the amplifier, and the output of "the amplifier will drive the leakage current instead.^' Therefore, no leakage current will be drawn from the filter and driven into the pn junction at the drain of the first transistor, which current otherwise would cause the control voltage of the VCO to change. The change in the control signal applied would cause the output frequency of the VCO to be altered.

Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description.

Brief Description of the Drawings

Embodiments of the present invention will be described in greater detail with reference to the accompanying drawings, in which:

Fig. 1 shows a typical PLL comprising a charge pump in which the present invention advantageously can be employed;

Fig. 2 shows the basic structure of an NMOS transistor;

Fig. 3 shows a charge pump according to an embodiment of the invention and a load in the form of a loop filter connected to the charge pump; and ^■ Fig. 4 shows the circuitry according to the invention and the structure of a transistor according to the invention. Detailed Description of the Present Invention

A typical use of a VCO 1 is in a PLL. An example thereof is shown in Fig. 1. In the PLL, the output of the VCO 1 is connected to the input of a programmable divider 2. The output of the programmable divider is connected to a first input of a phase/frequency detector 3. A second input of the detector 3 receives a reference frequency to which the PLL should lock. The phase/frequency detector 3 detects the phase/frequency difference between the VCO signal and the reference signal. A phase/frequency detector acts'-as a.phase detector during lock and a frequency detector when the loop is out of lock. The outputs of the detector 3 are connected to a charge pump 4 and applies charge-up or charge-down signals to the charge pump 4 depending on the difference between the VCO signal and the reference signal. The charge pump 4 comprises a charge switch 6 controlled by the charge-up signal from the detector 3, a charge current source 8 connected to the charge switch 6, a discharge switch 7 controlled by the charge.-down signal from the detector 3, a discharge current source 9 connected to the discharge switch 7 etc. If the frequency of the VCO signal is higher than the frequency of the reference signal, a charge-down signal will be applied and if the frequency of the VCO signal is lower than the frequency of the reference signal, a charge-up signal will be applied. The charge pump converts the charge-up and charge-down signals to a control voltage for the VCO by means of a loop filter 5. A charge-up signal will close the charge switch 6 and thus a current will charge the filter. A charge-down signal will close the discharge switch 7 and consequently the filter will discharge to ground. The output of the loop filter 5 is connected to a second input of the VCO 1, thereby providing the VCO 1 with a control voltage signal setting the carrier frequency of the output signal Vout of the VCO 1. A first input of the VCO 1 receives a modulating voltage signal. The output of β the PLL, i.e. the output Vout of the VCO 1, is connected to a power amplifier, a mixer or the like of the radio communication device in which the PLL is mounted.

As mentioned hereinabove, when it is desirable to use the VCO 1 to generate a modulation signal only, it is necessary to open the PLL as it otherwise will try to lock to the modulation signal. This is done by disconnecting the charge pump 4 from the loop filter 5. It is desirable that the amount of leakage current produced by the charge pump 4 is small, as this current will" enter the' loop filter 5 and produce a voltage to the VCO resulting in the fact that the frequency of the output Vout of the VCO 1 is altered.

Turning to Fig. 2, the structure of an NMOS transistor is depicted. When denoting relative doping concentrations in materials, it is common to use - and + symbols. Thus, a lightly ^• doped p-material would be referred to as p^~, as can be seen in Fig. 2. A heavily doped p-material would consequently be referred to as p⁺. The transistor T indicated by the dotted box in Fig. 2 is comprised in the discharge switch 7 in Fig. 1. The charge switch β is implemented by means of PMOS transistors. Because of the drain potential of the transistor, a difference in potential between the n-type material and p-type material will occur and this voltage drop V across the pn junction between the drain and the substrate will result in a leakage current into the junction. This might not be a problem when the transistor is in its conducting mode, as the leakage current normally is relatively small compared to the current flowing through the transistor. However, when the transistor is in its non-conducting mode, this leakage current is comparatively significant, as the current flow through the transistor is virtually zero¹. The leakage current will slightly charge or discharge the loop filter, resulting in a small change in the control voltage fed to the VCO and thus the output frequency of the VCO will be affected. To overcome this weakness in prior art charge pumps, the circuitry according to an embodiment of the invention is introduced in fig. 3.

Fig. 3 shows a charge pump according to an embodiment of the invention and a loop filter connected to the charge pump. One charge-up signal Pup and one charge-down signal Pdn is connected from the phase/frequency detector (PFD) according to Fig. 1 to the charge pump. When the PFD requires the VCO to increase the frequency, switches S5 and S4 will be closed and S6 will' be open,' 'while S^'2 and SI will be open and S3 will be closed. The control signal Inv(Pup) of S6 is the inverse of the control signal Pup of S5 and S4. This operation causes the filter to be charged by the current generator 8, thereby generating a control voltage Vctr to the VCO. If the PFD requires the VCO to increase the frequency, switches S2 and Si will be closed and S3 will be open, while S5 and S4 will be open and S6 will be closed. This operation causes the filter to be discharged through the S2 and SI switches. Opening switches S5, S4, S2 and SI and closing switches Sβ and S3 will disconnect the charge pump from the filter. The switches S5 and SI in Fig. 3 corresponds to the switches 7 and 6 in Fig. 1.

Fig. 4 will show another view of the circuitry according to an embodiment of the invention, explaining why the leakage current in the pn junction between the drain and the substrate is avoided when the charge pump is disconnected from the filter. As in Fig. 2, the switches depicted are implemented in the form of NMOS transistors. When SI and S2^"are open and S3 is closed, the charge pump is disconnected from the filter. An operational amplifier A with a gain of 1 is measuring the drain (i.e. the current collecting electrode) potential of the transistor and drives the source (i.e. the current emitting electrode) to the same potential, resulting in a drain-source (i.e. a main current path) voltage drop of zero. This will minimize the channel current of the transistor. The amplifier also drives the bulk, i.e.- the p-substrate, to the same potential. Since the n-material and the p-material between the drain and the substrate now has the same potential, there will be now voltage drop across this junction. However, a voltage drop V will instead occur over the junction between the n^~-material and the p^"-material. A leakage current will flow into the junction, but this time the current is driven by the output of the operational amplifier. Consequently, and as opposed to Fig. 2, no voltage drop will occur across the pn junction at the drain of transistor S2 and thus the loop filter will not be energized. It should be noted that in the circuitry according to the invention, both PMOS transistors and NMOS transistors are used. In fig. 3, transistors S4, S5 and Sβ are PMOS and transistors SI, S2 and S3 are NMOS.

It should also be noted that instead of using an amplifier' with a switch connected to the output, it is possible to use an operational amplifier in which the driving of the output can be disabled, e.g. a so called switched operational amplifier. The key feature is that the output of the amplifier can be disconnected, if desired.

Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. The described embodiments are therefore not intended to limit the scope of the invention, as defined by the appended claims.

Claims

1. Circuitry connectable to a charge pump for reducing leakage current associated with said charge pump, which circuitry comprises a measuring device with a switchable output and a first switch, the input of the measuring device being connected to a node of the charge pump from which a load is energized, the first switch being connected between said node and the output of the measuring device, the first switch also being connected to a substrate on which said measuring device is arranged, wherein the first switch is arranged to be in its non-conductive mode when the charge pump is disconnected from the load, the measuring device is arranged to be in its conductive mode when the charge pump is disconnected from the load and the measuring device is arranged to drive the voltage drop across a main current path of the -first switch to zero and/or to drive the voltage drop across a current collecting electrode of said first switch and said substrate to zero when said first switch is in its non-conductive mode, thereby reducing the leakage current associated with the charge pump.

2. The circuitry according to claim 1, wherein said measuring device with a switchable output is an amplifier having a second switch connected to its output, said first switch is a first transistor and said second switch is a second transistor.

3. The circuitry according to claim 2, wherein the current collecting electrode of the first transistor is connected to the node of the charge pump from which a load is energized, the first transistor being in its non- conductive mode when the charge pump is disconnected from the load and wherein the input of the amplifier is connected to said node and the output of the amplifier is connected to the current collecting electrode of the second transistor and a current emitting electrode of the second transistor is connected to a current emitting electrode of the first transistor, the second transistor being in its conductive -mode when the charge pump is disconnected from the load and which second transistor connects said output of- the amplifier to the substrate of the first transistor.

4. The circuitry according to claim 1, wherein said measuring device with a switchable output is a switched operational amplifier^' and said first switch is a first transistor.

5. The circuitry according to claim 4, wherein the current collecting electrode of the first transistor is connected to the node of the charge pump from which a load is energized, the first transistor being in its non- conductive mode when the charge pump is disconnected from the load and wherein the input of the amplifier is connected to said node and^' the output of the amplifier is connected to the current emitting electrode of the first transistor.

6. The circuitry according to any one of claims 2-5, wherein the transistors used are NMOS transistors.

7. The circuitry according to any one of claims 2-5, wherein the transistors used are PMOS transistors.

8. The circuitry according to any one of claims 3-7,. wherein the main current path consists of the drain- source current path, the current collecting electrode is a drain electrode and the current emitting electrode is a source electrode.

9. The circuitry according to any one of claims 2-8, wherein the amplifier is connected in a voltage follower configuration, thereby causing the output of the amplifier to track the input.