An embodiment of the invention relates to electronic circuits in general, and more specifically to a DC-to-DC voltage converter.
In certain electronic circuits, a converted voltage may be used. For example, in a modern PC (personal computer) system, a real time clock (RTC) produces a frequency output that is then used to provide a time base for the system, which thus requires constant power. For this purpose, an RTC Crystal Oscillator (RTCCO) resides on an I/O (input/output) controller hub chip, which is sometimes referred to as the “south-bridge”. An RTC circuit provides an accurate oscillator output (commonly a frequency of 32.768 kHz) that is used as the main clock to maintain system time. The output of the RTC circuit is divided to obtain time in units of seconds, minutes, and hours. The time is stored by the system and used as the time basis for the system, which is maintained when the system power is either on or off.
When PC system is powered down, the RTC circuit derives power from another power source, such as a self-contained source in the PC. A 3.0-volt coin cell lithium battery is generally used because such batteries are widely available and very inexpensive. In certain systems, another power source, such as a charged capacitor, may provide the power for the RTC circuit when the system is powered down. A PC system may be turned off for long periods of time, possibly for years, depending upon usage and the length of time a system may stay in storage. Therefore, an RTC circuit may potentially need to derive power from a coin cell battery or other such power source for a period of years to maintain system time.
As computer processes move towards lower voltages in order to reduce power consumption and to increase speed in digital sections, the voltage of a coin cell may need to be stepped down to a lower voltage, such as a voltage range of less than 2 volts, depending upon the process voltage. The process of converting a DC voltage to a lower voltage consumes some amount of power, thereby reducing the length of time that the system can maintain the system time. Further, a certain minimum voltage is needed to operate the supplied circuit. Because the voltage of a battery or capacitor power source will fall over time as power is consumed, the voltage response of the DC-to-DC converter has an impact on the operation of the supplied circuit.
The invention may be best understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention. In the drawings:
A method and apparatus are described for a low power DC-to DC converter.
DC-to-DC Converter
According to an embodiment of the invention, a simplified DC-to-DC converter based on linear regulation is implemented. The simplicity of the DC-to-DC converter can provide for reduced current draw and less complicated circuitry, as compared to sophisticated voltage regulators. Sophisticated voltage regulators, though often providing good voltage regulation, may draw significant amounts of current and involve complicated circuitry, including feedback arrangements.
Under an embodiment of the invention, a DC-to-DC converter can provides for a relatively flat voltage response, providing a stable reference voltage for a wide variety of supply voltages. For this reason, an output from the converter output may not vary greatly as input voltage drops. In one example, a power source, such as a battery, provides a voltage that drops over a period of time. The voltage provided by the power source is converted by a DC-to-DC converter and utilized as a source voltage for a circuit, such as an RTC circuit maintaining a system time. Using an embodiment of the invention, the voltage level provided to the circuit is maintained at a relatively constant level as the voltage of the power source drops over a period of time, until the voltage reaches a minimum level. By maintaining the voltage level, the system may allow extended operation of the circuit. When the power source has a higher voltage (such as with a new battery), a circuit will generally consume more power than is necessary. When the power source has a lower voltage (such as with an older battery), the circuit may not receive a sufficiently high voltage to operate properly.
Under an embodiment of the invention, the provision of a relatively constant voltage output may simplify design of the circuit that receives the voltage. If a voltage source provides a relatively wide range of voltages, then a circuit, such as an RTC circuit, must accommodate the power supply swing as voltages change. If a voltage source provides a narrower range of voltages, then the circuit is not required to accommodate as wide of a power supply swing.
According to an embodiment of the invention, a DC-to-DC converter utilizes a bias voltage that is provided by another circuit. The use of the bias voltage provides for process, voltage and temperature (PVT) compensation for the output voltage. The DC-to-DC converter can dynamically provide a minimum required output voltage for a wide range of process and temperature conditions. Under an embodiment of the invention, the circuit may also allow an option for tuning the output voltage of DC-to-DC converter in post-silicon (after fabrication) state by use of configurable register settings that are based on post-silicon performance.
According to an embodiment of the invention, a DC-to-DC converter provides a voltage with minimal current draw. With the use of the converter, the life of a power source can be extended. In one illustration, the average useful life of a lithium coin cell battery life supplying an RTC circuit may be extended by 15% as compared to conventional operation. The extended lifetime allows for continued use of coin cell batteries or similar devices in computer operation, and reduces maintenance connected with battery replacement. If a charged capacitor is utilized in a personal computer in lieu of a coin cell battery (as is common in, for example, personal computers operated in Europe) the linear discharge of the capacitor results in a significant extension of operation. In certain circumstances, the allowable discharge time may be increased by 100% as compared to conventional operations.
An embodiment of a DC-to-DC converter may leverage a known reference load to bias the output voltage to a level that is very near the minimum voltage required. The maintenance of voltage at this level helps to compensate for PVT variation. Further, the circuit accomplishes this purpose while consuming small amounts of current, which may be in the range of only a few hundred nano-Amps of current. In one example, the DC-to-DC converter may provides a reference voltage while consuming less than 0.5 μA of current consumption, or under 1.5 μWatts of power consumption.
In the illustration shown in
The current generated by the Q2-Q3 current mirror through Q3 315, IQ3, is passed through a reference load. The reference load may vary depending on the characteristics needed, as described more fully below. In the illustration shown in
Vout≅Vref−K1·√{square root over (IQp)} (1)
Where K1 is a constant that is dependent on the characteristics of device Qp and on the magnitude of the total current draw.
In a particular embodiment of the invention, various circuit relationships and factors affect the voltage level of Vref and therefore affect the output voltage. In the design of a voltage converter for a particular implementation, these elements may be adjusted to provide a desired output. The elements include the device size and type of the elements of the reference load, such as load devices Q4 335 and Q5 340; the size and device type of Q1 325; the Vbias 365 voltage level, which is affected by the choice of bias generator 355; and the device size ratio of the current mirror devices Q2 310 and Q3 315.
The output Vout then is applied to the bias generator 355 and to any supplied devices. In the example shown in
Bias Generator
The bias generation circuit receives a supply voltage, which is shown as Vout 535, such as the output of a DC-to-DC voltage generator under and embodiment of the invention. The bias generation circuit comprises four transistor devices, M1 505, M2 510, M3 515 and M4 520, and a bias resistor, Rbias 525. The gates of M1 505 and M2 510 are connected together, with the gate and source of M1 505 being tied together. The gates of M3 515 and M4 520 are connected together, with the gate and drain of M4 520 being tied together. Bias resistor Rbias 525. is connected between the drain of M2 510 and ground 530. The constant reference current i1 may be determined as:
The voltage produced at the gates of M1 505 and M2 510 is Vbias 540, which may be used as the bias voltage in an embodiment of the invention.
Reference Load Devices
Under an embodiment of the invention, many different reference load devices may be utilized in a DC-to-DC converter. The choice of a reference load device can have a significant impact on circuit operation. As shown in
The load being powered by a DC-to-DC converter will draw a certain range of current at a given voltage. In a certain implementation, a DC-to-DC converter circuit powers CMOS logic elements in an ultra-low power state, with a few micro-amps of current being consumed per 10 k gates. In order to minimize excess leakage current, a voltage is supplied that is sufficiently high to allow the logic to operate correctly, but is low enough to prevent unnecessary power consumption.
In one example, an absolute minimum voltage to be supplied for all allowable process variation and temperature range may be chosen. If an absolute minimum voltage level is chosen, the reference load devices could comprise a low tolerance resistor. A minimum value for Vref applied to the output transistor can be found based upon the maximum power well current draw and using equation (1) or a more precise version of this calculation. With this minimum value, corresponding values for the resistor load and minimum IQ3 can be designed or chosen. A disadvantage to choosing a constant reference load is that the excess voltage margin (the amount of headroom above low-VCC inducted failing point) may be low in the worst case process/temperature analysis, and be excessive in the opposite best case process/temperature analysis.
Other reference loads can be chosen for different applications of embodiments of a converter circuit. Elements affecting the choice of reference loads include the allowable current draw of the circuit, the required accuracy of the Vout supply, and the desired process/temperature compensation, if any. For example, if a Vbias signal from a bias generation circuit is relatively constant (such a signal from a band gap or similar circuit), and the Vout voltage is thus relatively independent of process, voltage, and temperature variation, then a precision resistance load may be a desirable reference load. In an application in which a portable device is running using the power from a battery, the digital logic requires sufficient power, although not to a wasteful level. The choice of reference loads in this case may be a sample piece of logic which is connected in a manner to draws a reference based the device's own required switching current. For a given switching current, the required voltage needed is generally less for devices with low thermal energy than devices with high thermal energy. Therefore, passing a desired quantity of switching current through a circuit that is representative of the logic may save power by reducing an excessive voltage margin. In designing target levels for output voltage, system noise and transistor device may account for some of the required voltage margin that will be needed.
In some applications, logic being powered by a DC-to-DC converter may have a low power state (sleep state) and a high power state (active state). According to an embodiment of the invention, logic may be powered by a battery or similar power source in a sleep state, and by a standard power source (such as power from a wall outlet) in the active state. For CMOS logic, the power consumed is proportional to the voltage levels being applied. A reduction in the voltage level applied in the sleep state thus can reduce power consumption. While the voltage reference generator may vary and is not limited to any particular design, the voltage reference generator may include an adjustable voltage reference generator described in U.S. patent application Ser. No. 10/609,513, issued as U.S. Pat. No. 6,924,692.
Startup Circuit for DC-to-DC Converter
During startup conditions, a power supply, such as a 3V supply in a computer, is ramped up to the output voltage, and the output voltage is initially at ground or floating potential. While a bias voltage generator circuit is not properly powered, a bias voltage also generally will be at ground or floating potential. In the illustration provided in
Techniques described here may be used in many different environments. One possible environment is a computer with a backup power supply that is used to maintain the system clock.
Under an embodiment of the invention, a computer 600 comprises a bus 605 or other communication means for communicating information, and a processing means such as one or more processors 610 (shown as 611, 612 and continuing through 613) coupled with the bus 605 for processing information. The maintained system time may be utilized by the processors 610 in normal system operations.
The computer 600 further comprises a random access memory (RAM) or other dynamic storage device as a main memory 615 for storing information and instructions to be executed by the processors 610. Main memory 615 also may be used for storing temporary variables or other intermediate information during execution of instructions by the processors 610. The computer 600 also may comprise a read only memory (ROM) 620 and/or other static storage device for storing static information and instructions for the processor 610.
A data storage device 625 may also be coupled with the bus 605 of the computer 600 for storing information and instructions. The data storage device 625 may include a magnetic disk or optical disc and its corresponding drive, flash memory or other nonvolatile memory, or other memory deviceSuch elements may be combined together or may be separate components, and utilize parts of other elements of the computer 600.
The computer 600 may also be coupled via the bus 605 to a display device 630, such as a liquid crystal display (LCD) or other display technology, for displaying information to an end user. In some environments, the display device may be a touch-screen that is also utilized as at least a part of an input device. In some environments, display device 630 may be or may include an auditory device, such as a speaker for providing auditory information. An input device 640 may be coupled with the bus 605 for communicating information and/or command selections to the processor 610. In various implementations, input device 640 may be a keyboard, a keypad, a touch-screen and stylus, a voice-activated system, or other input device, or combinations of such devices. Another type of user input device that may be included is a cursor control device 645, such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor 610 and for controlling cursor movement on display device 630.
A communication device 650 may also be coupled with the bus 605. Depending upon the particular implementation, the communication device 650 may include a transceiver, a wireless modern, a network interface card, or other interface device. The computer 600 may be linked to a network or to other devices using the communication device 650, which may include links to the Internet, a local area network, or another environment.
General Matters
In the description above, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without some of these specific details. In other instances, well-known structures and devices are shown in block diagram form.
The present invention includes various steps. The steps of the present invention may be performed by hardware components or may be embodied in machine-executable instructions, which may be used to cause a general-purpose or special-purpose processor or logic circuits programmed with the instructions to perform the steps. Alternatively, the steps may be performed by a combination of hardware and software.
Portions of the present invention may be provided as a computer program product, which may include a machine-readable medium having stored thereon instructions, which may be used to program a computer (or other electronic devices) to perform a process according to the present invention. The machine-readable medium may include, but is not limited to, floppy diskettes, optical disks, CD-ROMs, and magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, magnet or optical cards, flash memory, or other type of media/machine-readable medium suitable for storing electronic instructions. Moreover, the present invention may also be downloaded as a computer program product, wherein the program may be transferred from a remote computer to a requesting computer by way of data signals embodied in a carrier wave or other propagation medium via a communication link (e.g., a modern or network connection).
Many of the methods are described in their most basic form, but steps can be added to or deleted from any of the methods and information can be added or subtracted from any of the described messages without departing from the basic scope of the present invention. It will be apparent to those skilled in the art that many further modifications and adaptations can be made. The particular embodiments are not provided to limit the invention but to illustrate it. The scope of the present invention is not to be determined by the specific examples provided above but only by the claims below.
It should also be appreciated that reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature may be included in the practice of the invention. Similarly, it should be appreciated that in the foregoing description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims are hereby expressly incorporated into this description, with each claim standing on its own as a separate embodiment of this invention.
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