Patent Application
20080231240
1. Field of the Invention
This invention relates generally to voltage regulators, and more specifically to voltage regulators in an integrated circuit used for supplying voltage to load circuits in the integrated circuit.
2. Related Art
In an integrated circuit (IC) fabricated using a dual-oxide, complementary metal oxide semiconductor (CMOS) process, there is at least one circuit whose components are built with thick oxide devices and at least one circuit whose components are built with thin oxide devices. The thin oxide devices consume less power than do the thick oxide devices; however, the thin oxide devices cannot withstand as high of a voltage without damage as can the thick oxide devices. An IC typically includes at least one voltage regulator. The regulator is built with thick oxide devices because the regulator is biased off the highest voltage available, which is typically the voltage of a battery that supplies power to the IC. The voltage of the battery is typically 1.7 v to 3.6 v, depending on the process used for fabricating the IC. The regulator provides a lower voltage power supply to circuits that are constructed with thin oxide devices, and a higher voltage power supply to circuits that are constructed with thick oxide devices. Depending on the process for manufacturing the IC, the regulated level of the voltage at the output of the regulator is typically 0.9 v to 1.6 v for thin oxide devices. Any occasion that the voltage at the output of the regulator goes above the regulated level, the thin oxide devices of the IC are at risk of being damaged, possibly leading to a circuit failure. The output of the regulator going above the regulated level is more likely to occur during connection of the battery to the IC (“battery insertion”), which is when the voltage supply increases abruptly from zero volts.
One known regulator 108 comprises an operational amplifier (not shown) and an analog drive device (not shown). Through a feedback loop, the operational amplifier maintains the voltage at the output 130 of the regulator 108 at the regulated level. At time of insertion of the battery 120, the analog drive device becomes active prior to the operation of the operational amplifier becomes established. As a result, the voltage of the output 130 of the regulator 108 can momentarily reach the voltage level of the battery 120, i.e., the output of the regulator overshoots the regulated voltage. The higher voltage level of the battery 120, while not high enough to damage circuits made from thick oxide devices, can damage circuits made from thin oxide devices.
Some ICs have a battery-save circuit. A battery-save circuit may be useful, during normal start-up to help prevent the voltage of the output of the regulator from going above the regulated level. This is because, during normal start-up, the digital circuitry that controls the battery-save analog transistor is always on because such digital circuitry does not turn-off during a normal turn-off procedure. However, such digital circuitry does turn-off as a result of removal of the battery, and, therefore, is not operating immediately prior to battery insertion. The battery-save circuit does not turn on fast enough to be effective at time of battery insertion. A typical battery-save circuit comprises digital circuitry whose output is coupled to a gate of an analog battery-save transistor. The analog drive device of the regulator turns on before the battery-save circuit turns on. Therefore, during battery insertion, the battery-save circuit cannot be relied upon to prevent the voltage of the output of the regulator to go above the regulated level.
If the load circuit 110 of the regulator 108 has a significant amount of capacitance, such as capacitor 140, the capacitance of the load can help absorb any voltage overshoot at the output 130 of the regulator that may occur immediately after battery insertion. Some ICs 101 save die area by not having a significant internal capacitance. If the load circuit 110 of the regulator 108 lacks a significant amount of capacitance, an excessive voltage overshoot during battery insertion is more likely to occur. When the load circuit 110 lacks sufficient capacitance to absorb a voltage overshoot, most known IC packages 102 use a power-out pin 132 for allowing a capacitor 140 external to the IC package to be coupled to the output 130 of the regulator 108.
Some IC packages save pin-out by not having the power-out pin. The outputs of regulators in such IC packages are more susceptible to voltage overshoots during battery insertion because such IC packages do not have any provision for allowing additional, external capacitance to be coupled to the output of such regulators.
Known protection circuits for ICs only address voltage overshoots that occur during normal start-up, that is, when the battery has remained continuously connected to the device during the period that the device was turned off.
Known protection circuits for ICs do not address protection from voltage overshoots that occur immediately after battery insertion. Known protection circuits for ICs do not address protection from voltage overshoots caused by the insertion of the battery 120.
Known protection circuits for ICs do not sense the supply voltage, such as the voltage of the battery 120, to limit the voltage of the output 130 of the regulator 108 to the regulated level.
The present invention is illustrated by way of example and is not limited by the accompanying figures, in which like references indicate similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale.
The protection circuit 260 is coupled to the output 230 of the voltage regulator 208. The protection circuit 260 comprises capacitor 312, resistor 324, and transistor 316. The protection circuit 260 works as follows. Before battery insertion, the supply voltage and the output 230 of the regulator 208 are at 0 v, and the time constant of the protection circuit 260 is zero because the capacitor 312 is discharged. Upon battery insertion, capacitor 312 starts in a discharged state, i.e., capacitor 312 has 0 v across it. When the battery 220 is inserted, the supply voltage ramps up to 3 v, the capacitor 312 becomes charged within a few nanoseconds, which causes the gate of transistor 316 to be pulled high, thus pulling down the voltage of the output 230 of the regulator 208. With the gate of transistor 316 at 3 v, the drain of transistor 316 pulls the voltage of the output of the regulator 208 low, thereby preventing the voltage from going above the regulated level. In the exemplary embodiment, the regulated level is 1.2 v. The capacitor 312 discharges to ground through resistor 324. Once the capacitor 312 is discharged, the gate of transistor 316 is pulled low and the regulator 208 is free to operate normally. Transistor 316 remains off during normal operation and has no impact on the performance of the regulator 208. The protection circuit 260 is completely autonomous, and limits the voltage level of the output 230 of the regulator 208 during battery insertion, thereby protecting the CMOS devices in the load circuit 210 from breaking down. The protection circuit 260 senses the supply voltage, such as the voltage of the battery 220, to limit the voltage of the output 130 of the regulator 208 to the regulated level. The protection circuit 260 senses when the regulator 208 first powers up and limits the output 230 of the regulator for a duration based on a time constant. The time constant is determined by the values of capacitor 312 and resistor 324. In the exemplary embodiment, the value of the time constant is approximately 10 μsec. Therefore, in the exemplary embodiment, transistor 316 remains on for 30-50 μsec after battery insertion. Consequently, the protection circuit 260 protects the load circuit 210 during the period that a voltage overshoot is most likely to occur.
Advantageously, an IC 201 with the protection circuit 260 prevents the regulator 208 from violating voltage limitations placed on thin oxide CMOS devices in the IC. The protection circuit 260 protects thin oxide, lower-voltage CMOS devices, such as used with digital circuits, from damage by limiting, during battery insertion, voltage transients on the output 230 of the regulator 208.
The protection circuit 260 is not limited for use with the regulator 208, but can be used with any regulator that is built out of thick oxide, higher voltage CMOS devices, and that has an output driving thin oxide, lower voltage CMOS circuitry.
The protection circuit 260 has no impact on current drain, little impact on die area. Advantageously, only if the power supply to the regulator 208 is toggled as a means of battery-save, the protection circuit 260 also provides protection during normal start-up conditions. A dual-oxide IC 201 having the protection circuit 260 coupled to the output of a regulator, has the reliability of a thick oxide IC without a current drain or a die area penalty of a thick oxide-only IC.
Advantageously, without using any external capacitance, the protection circuit 260 provides protection to ICs 201 that lack significant internal capacitance. Referring again to
It should be understood that all circuitry described herein may be implemented either in silicon or another semiconductor material or alternatively by software code representation of silicon or another semiconductor material.
Although the invention is described herein with reference to specific embodiments, various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. For instance, although the exemplary embodiment is fabricated using a dual-oxide process, the invention is equally applicable to ICs manufactured using a multiple-oxide process. Although the exemplary embodiment is fabricated using a 90-nm CMOS process, the invention is equally applicable to ICs manufactured using a larger or a smaller CMOS process.
Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention. Any benefits, advantages, or solutions to problems that are described herein with regard to specific embodiments are not intended to be construed as a critical, required, or essential feature or element of any or all the claims.
Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements.
