Patent Application
20090051340
The present invention relates to the domain of integrated circuits, and more precisely to transconductors which can be used in some integrated circuits.
As it is known by one skilled in the art transconductors are electronic devices which are frequently used into integrated circuits and for instance into integrators, themselves being liable to be used into one-cycle controllers, for instance.
A one-cycle controller is an integrated device implementing a recently proposed nonlinear control technique, and which may be used into a DC-DC switching converter, for instance, to take advantage of the pulsed and nonlinear nature of its switching converters. It aims at achieving an instantaneous dynamic control of the average value of a switched variable, such as a voltage. One-cycle control offers many advantages over existing voltage or current controls because only one switching cycle is needed for the average value of the switched variable to reach a new steady-state after a transient. Therefore there is no steady-state error or dynamic error between the control reference and the averaged value of the switched variable. Moreover this nonlinear control technique provides a fast dynamic response, an excellent power source perturbation rejection, a robust performance and an automatic switching error correction, and is intended for general switching applications.
Most of the proposed one-cycle controllers have been realized with discrete components in laboratories. For a proper operation such controllers require not only positive and negative supplies but also a negative reference voltage, as this is notably described in the documents i) of K. M. Smedley and S. Cuk, “One-cycle control of switching converters”, IEEE Trans. Power Electronics, vol. 10, No. 6, Nov. 1995, pp. 625-633, ii) E. Sandi and S. Cuk, “Modelling of one-cycle controlled switching converters”, Proceedings of 14th International Telecommunication Energy Conference, 4-8 Oct. 1992, pp. 131-138, and iii) Y. Wang and S. Shen, “Research on one-cycle control for switching converters”, Proceedings of the 5th World Congress on Intelligent Control and Automation, Jun. 15-19, 2004.
When one tries to implement the one-cycle control scheme disclosed in these documents on a standard CMOS chip, several problems arise. Firstly, because of the discrete components, bipolar power supplies (Vdd and -Vss) are required. This results from the fact that a conventional one-cycle controller comprises an integrator (which is in an inverting configuration using an operational amplifier, a resistor and a capacitor, and which for a positive input signal presents an output which starts from zero and then goes negative) and a comparator (which is fed with a signal delivered by the integrator and with a negative reference voltage -Vref), which both must have a negative supply. Secondly, because of the use of bipolar power supplies, it is difficult to fabricate the integrator in a standard digital CMOS process.
An integrated one-cycle controller has also been proposed in a document of D. Ma, W.-H. Ki and C.-Y. Tsui, “An integrated one-cycle control buck converter with adaptive output and dual loops for output error correction”, IEEE J. Solid-State Circuits, vol. 39, No. 1, Jan. 2004, pp. 140-149. This integrated one-cycle controller is interesting because it does not need for the negative supply and negative reference voltage.
Unfortunately, such an integrated one-cycle controller implements a DC level shifting technique to allow the use of a single positive power supply and a positive reference voltage, which induces new problems. Indeed this DC level shifting technique requires an integrator for integrating a voltage (Vx), every cycle, and a comparator arranged to compare the signal outputted by the integrator's output with a reference voltage (Vref). The integrator comprises three operational amplifiers and six additional resistors to accomplish what one operational amplifier, one resistor and one capacitor can do otherwise when an additional negative supply is used.
Such an integrator has a rather complex design, is more expensive and cumbersome, and consumes more power. Moreover, it introduces additional delay and degrades reliability.
More, the output of a real operational amplifier is unable to reach the potential of its power supply while still providing an adequate gain. It is recall that when the output of the first operational amplifier is set to be half of the supply voltage, the second operational amplifier should theoretically amplify this voltage by 2 so that its output voltage reaches its supply voltage. However, in practice when the output voltage of an operational amplifier tries to approach its power supply, the pMOS output transistor of this operational amplifier may be strongly driven into the triode region. As a result, the gain of the output operational amplifier drops dramatically, leading to errors in the voltage conversion and/or to instability.
So, the object of this invention is to offer a new linear transconductor which can be notably used in a new integrator capable, notably, to overcome at least some of the drawbacks of the integrated one-cycle controllers, for instance used in DC-DC switching converters.
For this purpose, it provides a linear transconductor, for an integrated circuit, comprising:
For instance, the voltage divider means comprises i) a first resistor comprising a first terminal defining the voltage divider means first terminal, and a second terminal defining the voltage divider means second terminal, and ii) a second resistor comprising a first terminal connected to ground and a second terminal connected to the second terminal of said first resistor.
The first and second matched transistors of this linear transconductor may be of the pMOS type.
Moreover, the respective gates of the first and second matched transistors may be parts of a single common gate.
More, the operational amplifier may comprise an input stage comprising a pair of differential pMOS transistors.
The invention also provides a non-inverting integrator comprising:
The invention further provides a one-cycle controller comprising:
The invention still further provides a DC-DC (switching) converter comprising:
For instance, the power switch comprises i) a driver means having one input fed with the second control signal and first and second outputs for delivering this second control signal, and ii) first and second switches respectively connected to the first and second outputs of the driver means to be driven by the second control signal.
The invention still further provides an electronic equipment comprising a battery arranged to deliver a DC voltage and a DC-DC converter such as the one above introduced and arranged to convert a first voltage defined from the DC voltage into an output DC voltage.
Such an electronic equipment may be a battery-powered or portable electronic device such as a mobile (or cellular) phone, a cordless phone, a digital still camera, a MP3 player, or a personal digital assistant (PDA), for instance.
Other features and advantages of the invention will become apparent on examining the detailed specifications hereafter and the appended drawings, wherein:
The appended drawings may not only serve to complete the invention, but also to contribute to its definition, if need be.
As mentioned before, the invention firstly provides a new linear transconductor which is intended to be part of an integrated circuit.
In the following description it will be considered that the linear transconductor according to the invention is part of an integrated non-inverting integrator, which itself is part of an integrated one-cycle controller of an integrated DC-DC (switching) converter of an electronic equipment (or device). For instance, such a DC-DC (switching) converter may be part of a battery-powered or portable electronic device such as a mobile (or cellular) phone, a cordless phone, a digital still camera, a MP3 player, or a personal digital assistant (PDA).
But, the invention is not limited to these applications. Indeed, the linear transconductor may be used in any integrated circuit where a linear, stable and accurate transconductance (Gm) over an input voltage range from 0 to X volts is mandatory, and notably each time an input voltage larger than the so-called rail-to-rail needs to be processed (for instance integrated). Moreover, the one-cycle controller according to the invention may be used in any integrated circuit where an instantaneous dynamic control of the average value of a switched variable, such as a voltage, is required, for instance.
As it is schematically illustrated in
The power switch SD comprises at least a first input intended to be connected to a DC voltage VBAT (variable or not), for instance provided by a power supply such as an external battery BAT, a second input connected to the output of the one-cycle controller OC to be driven by a (second) control signal it outputs, and an output for outputting a first voltage Vx to be integrated by the one-cycle controller OC.
This first voltage Vx is defined from the DC voltage VBAT, by means of a driver DR and first T3 and second T4 switches which may be respectively made of transistors. More precisely, the first transistor (or switch) T3 comprises a source connected to the DC voltage VBAT, a drain connected to the drain of the second transistor (or switch) T4 and a gate controlled by a first output of the driver DR. The second transistor (or switch) T4 also comprises a source connected to ground and a gate controlled by a second output of the driver DR. The driver DR also comprises an input fed with the (second) control signal outputted by the one-cycle controller OC.
With this configuration the voltage at the output node of the power switch SD, defined by the connexion between the first transistor drain and the second transistor drain, is the first voltage Vx. The value of this first voltage Vx depends on the duty cycle of the second control signal which is provided by the driver DR onto the first and second transistor gates.
The LC circuit CC comprises an inductance L, comprising a first terminal connected to the output node (Vx) of the power switch SD and a second terminal connected to an output node of the DC-DC converter CV, and a capacitor C′ comprising a first terminal connected to ground and a second terminal connected to the output node of the DC-DC converter CV. With this configuration the DC voltage VBAT is converted to an output voltage Vo accessible onto the output node of the DC-DC converter CV.
The one-cycle controller OC comprises a first input connected to the output node of the power switch SD where the first voltage Vx is defined, a second input connected to a reference voltage Vref, a third power supply input connected to the DC voltage VBAT, a fourth input connected to a clock means (for instance an integrated oscillator) CLK to be fed with periodical clock signals, and an output connected to the input of the driver DR to drive it with the second control signal.
The value of the reference voltage Vref is equal to the desired output voltage Vo, or can be made to be a portion of Vo (for instance Vo/2).
It is recall that a conventional integrator generally comprises a resistor, a capacitor and an operational amplifier, which, in the present application, must be in an inverting configuration, that would require a bipolar power supply. Using a non-inverting integrator NI according to the invention makes it possible to abandon the previously required negative power supply, although both input and output signals are still referred to ground.
The non-inverting integrator NI comprises a linear transconductor LT and an integrating capacitor C.
The linear transconductor LT comprises a non-inverting input Vin+ defining the first integrator input connected to the first voltage Vx, an inverting input Vin− defining a second integrator input connected to a second voltage (which is the ground in this non limitative example), a power supply input connected to the DC voltage VBAT, and an output delivering an output current Io.
The integrating capacitor C comprises a first terminal connected to ground and a second terminal connected to the output of the linear transconductor LT.
As it is illustrated in
The (first) control signal is outputted by a reset-set flip flop component (or RS-FF) RF of the one-cycle controller OC, which will be described later on. This (first) control signal takes alternatively first and second values respectively adapted to switch on and switch off the shunt switch SW at the chosen (switching) frequency, in order the integrator output delivers an integrated output voltage which is integral of Vx over one cycle of the chosen frequency.
When the shunt switch SW is open (or switch off), the integrating capacitor C is charged by the output current lo delivered by the linear transconductor LT. So it integrates the output current Io which is representative of the first voltage Vx. When the shunt switch SW is closed (or switch on), the integrating capacitor C is short circuited and quickly discharged completely to 0, ready for next cycle.
The output current Io being generated by applying a voltage Vx at the positive (non-inverting) input Vin+ of the linear transconductor LT, the integrator NI is clearly of the non-inverting type and presents a transfer function H(s)=Gm/sc, where Gm is the transconductance of the linear transconductor LT, c is the capacitance of the integrating capacitor C.
As it is known by one skilled in the art, conventional MOS transconductors suffer from poor linearity and instable transconductance as temperature varies. More precisely, practical transconductors can be considered linear only within a very small input range because their MOS transistors are non linear devices. Linearization techniques can be used to extand the transconductor linear range, but it is not possible to extend this input range from 0 to their supply voltage. Moreover, the resultant transconductance becomes strongly temperature dependent due to the mobility. Therefore, it would be of interest to have at one's disposal transconductors with a linear, stable and accurate transconductance Gm over such a large input range.
The invention aims at offering such a transconductor.
As it is schematically illustrated in
The first R1, second R2 and third R3 resistors are preferably of the same type in order to allow to track each other for better performance.
The operational amplifier OA comprises non-inverting (+) and inverting (−) inputs, a power supply input connected to the DC voltage VBAT, and an output OO.
The first resistor R1 comprises a first terminal defining the transconductor non-inverting input Vin+, which is here connected to the first voltage Vx, and a second terminal connected to the inverting input (−) of the operational amplifier OA.
The second resistor R2 comprises a first terminal connected to ground and a second terminal connected to the second terminal of the first resistor R1 (and therefore to the inverting input (−) of the operational amplifier OA).
The third resistor R3 comprises a first terminal defining the transconductor inverting input Vin−, which is here connected to the ground (second voltage), and a second terminal connected to the non-inverting input (+) of the operational amplifier OA.
The first T1 and second T2 transistors are preferably of the pMOS type. They are in a common-source configuration, are matched and have identical size. Moreover, the respective gates of the first T1 and second T2 transistors are connected together and are preferably parts of a single common gate which is connected to the output OO of the operational amplifier OA. More, their respective sources are connected to the power supply input (VBAT) of the operational amplifier OA. The drain of the first transistor T1 is connected to the non-inverting input (+) of the operational amplifier OA (and therefore to the second terminal of the third resistor R3), while the drain of the second transistor T2 defines the transconductor output, which delivers the output current Io (so, the output current lo is the drain current of the second transistor T2).
With such a configuration, the transconductance Gm of the transconductor LT is given by Gm=ξ/r3, where ξ=r2/(r1+r2), and r1, r2 and r3 are the respective resistance values of the first R1, second R2 and third R3 resistors. Therefore, at least the linearity, stability, accuracy and temperature dependency of the transconductor LT are all as good as the ones offered by the passive resistors used in the state-of-the-art integrators. Moreover, the linear transconductor LT, according to the invention, has a much simpler scheme, which leads to lower cost, less design effort, lower risk, and enables a quick development.
The presence of the first R1 and second R2 resistors relaxes the requirement on the common-mode input voltage range of the operational amplifier OA. Without theses two resistors R1 and R2 mounted in series (i.e. without R1), the required input common-mode range of the operational amplifier OA would be from 0 to VBAT. Now, with the first R1 and second R2 resistors this common-mode voltage range is reduced to ξVBAT. Because ξ<1, the required common-mode voltage range at the higher end is reduced.
As the input voltage Vx goes down to as low as 0V, it is preferable to use a pMOS input stage for the operational amplifier OA. For instance, this input stage comprises a pair of differential pMOS transistors.
Although the first T1 and second T2 transistors are pMOS transistors in the preferred linear transconductor embodiment, it is also possible to replace them by nMOS transistors if the circuit is modified accordingly, which is obvious to the man skilled in the art.
Moreover, if the circuit CC to be powered by the DC-DC converter CV draws large current and represents an equivalent load resistance of the order of few ohms to the DC-DC converter CV, the resistance values r1 and r2 of the first R1 and second R2 resistors can be chosen of the order of few tens of kilo ohms. With such resistance values the power drained by R1 and R2 is completely negligible.
The one-cycle controller OC also comprises a comparator CO comprising a first (−) and second (+) inputs and an output.
The first input (−) is connected to the node to which are connected the second terminal of the shunt switch SW and the second terminal of the integrating capacitor C (and therefore the output of the linear transconductor LT), in order to be fed with an integrated output voltage (representative of the integral of the input voltage Vx over one cycle). The second input (+) is connected to the reference voltage Vref. Each time before the integration starts the integrating capacitor C is authorized by the shunt switch SW to discharge completely at the end of each cycle, the comparator CO compares the integrated output voltage with the reference voltage Vref, and delivers a two-level signal, one indicating an integrated output voltage larger than the reference voltage Vref and the other indicating an integrated output voltage lower than the reference voltage Vref.
Finally the one-cycle controller OC further comprises the above mentioned reset-set flipflop component (or RS-FF) RF.
This component RF comprises a first input (for instance the reset one) R connected to the output of the comparator CO, a second input (for instance the set one) S connected to the clock means CLK (for instance an oscillator) to be fed with a clock signal, a first output (for instance
As it is well known by the man skilled in the art, the first Q* and second Q outputs of a RS-FF component RF deliver complementary first and second control signals intended respectively to the command input of the shunt switch SW and the driver DR. For instance, the shunt switch SW turns on when
The periodicity of the clock signals CLK defines the switching frequency.
With the transconductance Gm=ξ/r3 of the linear transconductor LT, if the period Ts of the switching frequency (of SW) is set equal to r3c (Ts=r3c), then the DC-DC converter CV can convert the DC voltage VBAT down to the output voltage Vo by replacing Vref by ξVref at the second input (+) of the comparator CO.
It is important to notice that the DC-DC converter CV above described with reference to
The invention is not limited to the embodiments of linear transconductor (LT), non-inverting integrator (NI), one-cycle controller (OC), DC-DC (switching) converter (CV) and electronic equipment described above, only as examples, but it encompasses all alternative embodiments which may be considered by one skilled in the art within the scope of the claims hereafter.
| Number | Date | Country | Kind |
|---|---|---|---|
| 05300691.2 | Aug 2005 | EP | regional |
| PCT/IB2006/052693 | Aug 2006 | IB | international |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2006/052693 | 8/4/2006 | WO | 00 | 10/30/2008 |
