Patent Application
20060033551
The present invention relates generally to a system and method integrated circuits, and more particularly to a system and method for turning on an internal voltage rail in an integrated circuit while avoiding a large transient current.
Providing power to circuitry in an integrated circuit (via turning on an internal voltage rail) can be a difficult proposition, especially during power-on (turn on) and power-off (turn off). For example, during power-on, when power is initially provided to the circuitry, the power-on current can increase rapidly and result in a large transient current. Large transient currents can be damaging to the transistors in the circuitry. This problem can be further exacerbated when high-gain transistors, such as input/output (I/O) PMOS transistors, are used as the switch for the power supply. In such a situation, the fast turn on could result in large transient currents while these high-gain transistors charge up the internal rail.
One commonly used technique to help reduce the magnitude of the transient current is to use multiple switches arranged sequentially with respect to one another and coupled in between the power supply and the circuitry. Each of the switches in the sequence of switches can be smaller than the single switch that they replace. Then, the switches can be turned on in single fashion, with a small delay between consecutive switches. Since each switch is smaller than the switch that is replaced, the amount of current is smaller. Furthermore, with a delay being present between consecutive switches being turned on, the transient current is spread out over time. This can effectively reduce the severity of the transient current.
One disadvantage of the prior art is that even with smaller transistors being used in the sequence of switches, as each switch is being turned on, a transient current of significant magnitude can still occur. Therefore, with the use of multiple switches, a sequence of transient currents can be produced, each having a magnitude that can cause problems.
Another disadvantage of the prior art is that since the large transient current will typically occur rapidly after the switch is closed, it can be desired to accelerate the power ramp-up after the danger of the transient current has passed. The use of the sequence of switches does not allow for the acceleration of the power ramp-up.
These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by preferred embodiments of the present invention which provides a system and method for providing power to circuitry in an integrated circuit while avoiding a large transient current.
In accordance with a preferred embodiment of the present invention, a circuit for adjusting a power ramp-up across a first transistor comprising a potential adjust circuit and a rate adjust circuit is provided. The potential adjust circuit is coupled to the first transistor and to a first power supply and to a second power supply, while the rate adjust circuit is coupled between a first source/drain terminal of the first transistor and the second power supply. The potential adjust circuit is configured to change a voltage potential at a gate terminal of the first transistor in a controlled manner when the first transistor is turned on and the rate adjust circuit is configured to accelerate the power ramp-up across the first transistor when a voltage potential at the first source/drain terminal of the first transistor reaches a threshold.
In accordance with another preferred embodiment of the present invention, a distributed switch comprising a plurality of switches and a plurality of pre-driver circuits is provided. Each switch in the plurality of switches is coupled between a first power supply and a circuit, and each pre-driver circuit in the plurality of pre-driver circuits is coupled to a switch in the plurality of switches. Each switch in the plurality of switches selectively couples the first power supply to the circuit and can be independently controlled. A pre-driver circuit is configured to adjust a voltage potential at the switch to which it is coupled and to adjust a power ramp-up rate.
In accordance with yet another preferred embodiment of the present invention, a method for reducing transient current magnitude, the method comprising turning on a switch, wherein the switch is one of a plurality of switches and retarding a current ramp-up across the switch once a voltage potential at the switch reaches a first threshold is provided. Once the voltage potential at the switch reaches a second threshold, the current ramp-up across the switch is accelerated. All the while, a pre-determined amount of time is allowed to pass before repeating the turning, retarding, accelerating, and waiting for remaining switches in the plurality of switches.
An advantage of a preferred embodiment of the present invention is that the Miller capacitance effect is used to effectively increase the rise time of a current flowing across a transistor being used as a switch. By increasing the rise time of the current, the magnitude and abruptness of the current can be reduced.
A further advantage of a preferred embodiment of the present invention is that while the Miller capacitance effect allows the increased current rise time and results in the power applied to the circuitry ramping up slowly, the power ramp up can speed up as the power approaches full power.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.
For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
a and 5b are diagrams of a portion of the switch arrangement with emphasis detail placed upon a pre-driver circuit, according to a preferred embodiment of the present invention;
a and 6b are diagrams of a switch control generator circuit, according to a preferred embodiment of the present invention;
The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.
The present invention will be described with respect to preferred embodiments in a specific context, namely a header switch for providing power to logic circuitry in an integrated circuit. The invention may also be applied, however, to other power supply applications, wherein there is an interest in reducing power-on transient currents and there is sensitivity to large transient currents.
With reference now to
The arrangement shown in
When the control signal “CNTRL” causes the transistor 105 to close, the rapid change of state can create a large transient current to flow through the transistor 105 and to the circuitry 100. A cause for the large transient current is the transistor's high gain, wherein a relatively small current can be amplified into a large current. If the transient current is of sufficient magnitude, problems can result in the circuitry 100, including possible damage to delicate transistors in the circuitry 100.
With reference now to
With reference now to
When power is to be provided to the circuitry 300, each one of the switches can be turned into a closed state to couple the circuitry 300 to the voltage supply “VDD.” Note that each of the switches is turned into the closed state individually and that there is a delay between the closing of one switch and the closing of another switch. For example, the switch 305 may be the first switch to be closed and then after a delay, the switch 306 can be closed, and so forth. The sequential closing of the plurality of switches instead of the closing of the single switch can effectively reduce the magnitude of the power-on transient current as well as spread the power ramp-up over a period of time. The switches are shown to be coupled to a common power rail and the circuitry 300. However, it may be possible that each of the switches may be coupled to a different power rail and to a different portion of the overall circuitry 300.
Although the use of multiple switches in place of a single switch can reduce the power-on transient current by spreading it out over a period of time, it may still be possible to have transient currents of significant magnitude when each one of the switches are closed. Furthermore, as each of the switches are being closed and the power being provided to the circuitry 300 approaches the desired value, the delay in the power ramp-up remains substantially constant and therefore can cause a delay in the power ramp-up. This can result in a delay in the amount of time that it takes to reach the desired power for the circuitry 300, delaying the commencement of operations for the circuitry 300.
With reference now to
However, rather than having a control signal directly control the state of the switch to which it is coupled, such as control signal “CNTRL_1” and switch 405, the control signal can be provided to a pre-driver circuit, such as “PRE-DRIVER_1” 410 for switch 405 and “PRE-DRIVER_2” 411 and “PRE-DRIVER_N” 412 for switches 406 and 407. The pre-driver circuit can then be coupled to the switch. The pre-driver circuit can control the turn-on of the switch. According to a preferred embodiment of the present invention, the pre-driver circuit can make use of Miller capacitance feedback to control the rate of turn-on of the switch.
The Miller capacitance feedback can effectively delay the turn-on of the switch in this case, since the source terminals of the switches (such as switches 405, 406, and 407) are coupled together while the drain terminals of the switches are coupled together. When configured in such a fashion, each switch can see a full load capacitance on the internal rail, which can translate to a large Miller capacitance when the gate voltage of each of the switch is near the threshold voltage, which can help reduce the magnitude of the power-up transient current. Furthermore, since the Miller capacitance feedback can vary depending on a voltage difference seen at the switch, the power ramp-up can be accelerated as the power provided to the circuitry nears the desired level. For example, when the power provided to the circuitry is near the desired level, the voltage difference can be small. Therefore, the delay imparted by the Miller capacitance feedback on the turn-on of the switch can be relatively small, thereby decreasing the amount of time that it takes to reach the desired power level. A detailed description of the pre-driver circuit is provided below.
With reference now to
A second circuit, referred to as a rate adjust circuit 520, can be used to adjust the power ramp-up of the switch 405 after the initial turning on of the switch 405. The rate adjust circuit 520 can be coupled to the drain terminal of the switch 405, the potential adjust circuit 505, and a secondary voltage source. Note that the secondary voltage source to which the rate adjust circuit 520 is coupled to may be the same as the secondary voltage source to which the potential adjust circuit 505 is coupled or it may be different. Since the transient current is primarily seen in the early stages of the turning on of the switch 405, it can be possible to accelerate the power ramp-up of the switch 405 once a majority of the transient current has passed. By doing so, it can be possible to speed up the power ramp-up without incurring the risk of a large transient current.
With reference now to
Each transistor's gate terminals may be coupled together and to the control signal responsible for controlling the state of the switch 405. An output of the potential adjust circuit 505 may be taken at the drain terminal of the first transistor 510 and can be coupled to the gate terminal of the switch 405. Note that the three transistors 510, 512, and 514 are basically arranged in an inverting buffer configuration and depending upon the design of the control signal, an optional inverting buffer 516 can be used to maintain the desired polarity of the control signal. Note that the remaining pre-driver circuits 411 and 412 (and others not shown) may have a similar design, with differences possibly being in the sizing (geometry) of transistors to facilitate different turn-on rates.
The third transistor 514 may be used to decrease the effective size of the pull-down portion of the three-transistor chain (i.e., effectively creating a single small transistor from two larger transistors, the second and third transistors 512 and 514). Note that if it may be possible to fabricate a single transistor of desired size, a singe transistor can be used in place of the second and third transistors 512 and 514. Additionally, if there is a need for a yet smaller pull-down transistor, more than two pull-down transistors can be used. The use of a smaller transistor in the pull-down portion can more slowly place a voltage potential substantially equal to the secondary voltage source at the gate terminal of the switch 405. The rate of change in the voltage potential seen at the gate of the switch 405 can be regulated so that it rises slowly, and when it finally rises to above the threshold voltage, an increase in capacitance of the capacitor 530 (via the Miller effect) can help further by reducing the power ramp-up of the switch 405.
According to a preferred embodiment of the present invention, the rate adjust circuit 520 can be implemented with a single transistor 525, preferably an NMOS transistor. The transistor 525 may have its gate terminal coupled to the drain terminal of the switch 405 and its source terminal coupled to the secondary voltage source, while its drain terminal can be coupled to the drain of the transistor 514. As discussed previously, the secondary voltage source in the rate adjust circuit 520 may be the same as the secondary voltage source used in the potential adjust circuit 505. Configured as such, the transistor 525 will begin to conduct as the voltage at its gate terminal (which is at the same voltage potential as the drain terminal of the switch 405) reaches a certain threshold voltage. Once the transistor 525 begins to conduct, it provides an additional current path, increasing the rate of pull-down seen at the gate terminal of the switch 405.
The combination of the potential adjust circuit 505 and the rate adjust circuit 525 can operate as follows: after the switch 405 is turned on by the control signal “CNTRL_1,” current begins to flow. Note that at substantially the same instant that the switch 405 is turned on, pull-down transistors (the second and third transistors 512 and 514) can begin to change the voltage potential at the gate terminal of the switch 405 to the secondary voltage supply. When the voltage potential reaches the threshold of switch 405, the Miller capacitance seen as the capacitor 530 effectively increases and retards the power ramp-up rate across the switch 405 to a greater extent. As the current across the switch 405 increases, the potential at the gate terminal of the transistor 525 increases until it reaches a second threshold. Once the potential at the gate terminal of the transistor 525 reaches the second threshold, the transistor 525 turns on and current begins to flow across the transistor 525. According to a preferred embodiment of the present invention, the second threshold can be a voltage potential that is some small delta from the value of the power supply. Once the potential at the gate terminal of the transistor 525 exceeds the second threshold, the voltage potential provided to a power rail in to the circuitry has become close enough to the voltage potential of the power supply so that the danger of the transient current has passed. Therefore, it can be safe to accelerate the power ramp-up across the switch 405. The additional current path across the transistor 525 can help to accelerate the power ramp-up across the switch 405. The acceleration of the power ramp-up is effected by the switch 405 operating in a linear mode, wherein the gain of the switch 405 is smaller than when switch 405 is operating in a saturation mode.
With reference now to
With reference now to
One possible configuration of the switch control generator 605 can be as follows. The control signal for switch number 1, “CNTRL_1,” can be the control signal “CNTRL” itself. Therefore, when a logic true value is asserted on the control signal “CNTRL,” then the control signal “CNTRL_1” also immediately assumes the logic true value. Then, after a delay equal to the delay element 655, the control signal for switch number 2, “CNTRL_2,” will assume the logic true value. This continues until the control signal for switch number N, “CNTRL_N,” assumes the logic true value. Note that the individual delays due to the delay elements can be substantially equal to each other. Alternatively, the delay of each delay element can be individually adjusted to provide the desired transient current suppression. For example, the delay of the first few initial delay elements can be longer than the delay of the later delay elements. This can be due to the fact that the transient current is typically more pronounced for the first few switches.
With reference now to
Running horizontally across the switch arrangement are lines 705 of the power grid. Since the switch 405 is made up of a plurality of transistors, when the switch 405 is turned on, the transistors closer to the pre-driver 405 will turn on prior to the transistors further away from the pre-driver 405. Similarly, the lines 705 of the power grid that are closer to the pre-driver 410 will be able to conduct sooner. Therefore, to help more evenly distribute the turn on of the transistors in the switches 405 and the lines 705 of the power grid, the pre-drivers 410 can be arranged in such a way that the order that transistors in the switches 405 are turned on and lines 705 in the power grid are energized is varied. Note that it may be possible to line the pre-drivers 410 up in a horizontal line with no resulting change in the spirit of the present invention.
With reference now to
When a switch structure such as the switch structure 400 is used with properly sized transistors, the transient current achieves its maximum magnitude of approximately 550 milli-amps at a time of approximately 20 nano-seconds. When compared with the results shown in
With reference now to
With reference now to
The sequence of events 1000 may begin when a first switch in the distributed switch is turned on (block 1010). According to a preferred embodiment of the present invention, the distributed switch can be made up of a plurality of switches arranged in parallel and each switch can be controlled by a control signal. After the first switch is turned on, the voltage potential at the drain of the switch 405 increases and power ramp-up of the switch 405 begins. The power ramp-up of the switch continues until the voltage potential at the drain of the switch reaches a first specified threshold (block 1015). If the voltage potential has reached the pre-determined threshold, the current ramp up is retarded by the Miller capacitance since the switch now is in the saturation region (block 1020).
If the voltage potential has not reached the first threshold, then the voltage potential will be permitted to rise. However, a timer can be checked to see if a delay corresponding to a period of time between when a first switch is turned on and a second switch is turned on has expired. If the delay (a time period) has not expired (block 1025), then the voltage potential will be permitted to continue to rise. If the time period has expired, then a check to determine if there are any remaining switches to be turned on (block 1030) can be performed. Note that the time period between the turning on of the switches can be implemented as delay elements (such as delay elements 655 and 660 (
As the current ramp-up is being accelerated, the potential of the voltage being provided by the distributed switch can be compared with a second specified threshold (block 1035). The purpose of the second specified threshold can be to determine if the voltage being provided by the distributed switch is close enough to the desired voltage potential and if it is, then the transient current has substantially passed and the current ramp-up can be accelerated (block 1040). The current ramp-up can be accelerated by creating an additional current path via the use of a rate adjust circuit, such as by the rate adjust circuit 520 (
Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.
Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.
