Power on reset circuit

The present application claims the benefit of U.S. Provisional Application No. 60/487,752, filed on Jul. 16, 2003 by Julian Jenkins, which is incorporated herein by reference in its entirety.

1. Field of the Invention

The present invention relates generally to electrical circuits, and more particularly but not exclusively to power on reset circuits.

2. Description of the Background Art

Power on reset circuits prevent invalid conditions in an electrical circuit by ensuring that the electrical circuit has sufficient power before allowing it to operate. In an integrated circuit chip, for example, a power on reset circuit may be employed to ensure that sufficient power supply voltage is available to the chip before the chip is enabled. The power on reset circuit enables the chip only when the power supply powering the chip is within a specified range. Otherwise, the power on reset circuit keeps the chip disabled. The voltage at which the power on reset circuit enables the chip is referred to as a “trip point.” The signal used to enable or disable the chip is also referred to as a “power on reset signal.”

FIG. 1 schematically shows an example power on reset circuit 100 . Circuit 100 includes resistors R 1 , R 2 , R 3 , R 4 , and R 5 , capacitors C 1 and C 2 , transistors M 1 and M 2 , and an inverter 110 . Circuit 100 is powered by a power supply labeled as V SUPPLY . V SUPPLY is typically the same power supply providing power to the chip controlled by circuit 100 .

Resistors R 1 , R 2 , R 3 , and R 4 form a resistor ladder that produce a scaled version of V SUPPLY . The scaled version of V SUPPLY appears on node vtrip, thereby controlling the voltage on the gate of transistor M 1 . When the scaled version of V SUPPLY reaches the threshold voltage of transistor M 1 , transistor M 1 will turn ON, thereby pulling the input voltage to inverter 110 (see the node labeled as “vout”) to ground. This results in the output 112 of inverter 110 (see the node labeled as “por_xl”) to be at a logical HIGH state. A logical HIGH on output 112 enables the chip controlled by circuit 100 . When V SUPPLY rises from ground to its operating level (e.g., after V SUPPLY has been turned OFF then ON), circuit 100 thus keeps the chip disabled until V SUPPLY rises to a level that is sufficient to turn ON transistor M 1 . The trip point of circuit 100 is the V SUPPLY voltage required to turn ON transistor M 1 .

The chip controlled by circuit 100 is disabled when output 112 is at a logical LOW. When the V SUPPLY voltage is below the trip point, transistor M 1 is OFF and resistor R 5 pulls the input voltage to inverter 110 to the V SUPPLY voltage. Inverter 110 will interpret that V SUPPLY voltage as a logical HIGH, thereby placing output 112 at a logical LOW. The logical LOW on output 112 serves as an active LOW reset signal, which resets the chip and keeps it disabled.

Capacitors C 1 and C 2 and transistor M 2 help reduce the sensitivity of circuit 100 to noise. Capacitors C 1 and C 2 slow down the slew rate of nodes vtrip and vout, respectively, so that V SUPPLY has to exceed the trip point for a minimum amount of time before the voltage on V OUT crosses the threshold of inverter 110 .

The V SUPPLY voltage below which transistor M 1 turns OFF is referred to as a “falling trip point.” Transistor M 2 provides some hysteresis by modifying the falling trip point of circuit 100 . After V SUPPLY rises above the trip point, transistor M 1 turns ON, which in turn turns ON transistor M 2 . This results in transistor M 2 shorting out resistor R 4 , thereby lowering the V SUPPLY voltage required to keep transistor M 1 ON and lowering the falling trip point at which M 1 turns OFF.

A problem with circuit 100 is that its trip point may vary widely due to variations in the properties of its components, most importantly transistor M 1 . These variations in component properties (e.g., transistor threshold voltage) are typically caused by unavoidable variations in the manufacturing process and changes in the temperature of the chip. The chip is designed to work across an expected range of processes and temperature. For example, the trip point of circuit 100 may vary from 0.35 volts to 0.9 volts from circuit to circuit. This wide trip point variation may consume a considerable portion of the available voltage range where the chip is to be disabled and enabled. Also, at process corners where transistor M 1 has a low threshold voltage, there may not be sufficient voltage to turn ON transistor M 2 at the rising trip point (i.e., trip point crossed by a rising V SUPPLY ), preventing hysteresis. This is specially problematic when circuit 100 is fabricated using low voltage processes (e.g., 1.2 volt or 1.5 volt processes).

In one embodiment, a power on reset circuit includes a process dependent element. The process dependent element may be designed to compensate for process and temperature effects that may vary a trip point of the power on reset circuit. The process dependent element may be configured to lower or raise the trip point. The process dependent element may be a diode connected transistor, for example.

These and other features of the present invention will be readily apparent to persons of ordinary skill in the art upon reading the entirety of this disclosure, which includes the accompanying drawings and claims.

FIG. 1 schematically shows an example power on reset circuit.

FIG. 2 schematically shows a power on reset circuit in accordance with an embodiment of the present invention.

FIG. 3 schematically shows example process dependent elements in accordance with an embodiment of the present invention.

FIG. 4 schematically shows a power on reset circuit in accordance with an embodiment of the present invention.

FIG. 5 schematically shows a power on reset circuit in accordance with an embodiment of the present invention.

The use of the same reference label in different drawings indicates the same or like components.

In the present disclosure, numerous specific details are provided, such as examples of circuits, components, and methods, to provide a thorough understanding of embodiments of the invention. Persons of ordinary skill in the art will recognize, however, that the invention can be practiced without one or more of the specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the invention.

Referring to FIG. 1 , the threshold voltage of transistor M 1 may vary with the process used to fabricate it. More specifically, the variations in the threshold voltage of transistor M 1 may be caused by changes in process and temperature.

FIG. 2 schematically shows a power on reset circuit 200 in accordance with an embodiment of the present invention. Circuit 200 , and the later described circuits 400 and 500 , may be part of an integrated circuit fabricated using CMOS technology. Circuit 200 may include a process dependent element 220 coupled between the gate of transistor M 1 and ground. In the example of FIG. 2 , the process dependent element 220 is in parallel with resistor R 1 and capacitor C 1 , and changes the effective resistance between the gate of transistor M 1 and ground. Under certain process corners, the process dependent element 220 permits current to leak to ground from the nodes at the gate of transistor M 1 , thus lowering the ratio of the voltage on node vtrip to V SUPPLY (i.e., V TRIP /V SUPPLY ratio).

The resistance of the process dependent element 220 is process dependent to allow it to vary with process and temperature. The process dependent element 220 may be chosen such that its resistance tracks the threshold variation of transistor M 1 . The properties of the process dependent element 220 may be chosen such that under process and temperature corners where transistor M 1 has a low threshold voltage, the process dependent element 220 conducts sufficient current to lower the voltage on node vtrip when the V SUPPLY voltage is near the trip point, thereby raising the trip point of circuit 200 . Under process and temperature corners where transistor M 1 has a high threshold voltage, the process dependent element 220 may be optimized to conduct relatively little current to have minimal effect on the trip point. In the example of FIG. 2 , the process dependent element 220 thus effectively raises the trip point under process and temperature corners where it would be relatively low, while not appreciably affecting the trip point under process and temperature corners where the trip point would be relatively high. This reduces the overall trip point variation with process and temperature.

In one embodiment, the process dependent element 220 may be a diode connected NMOS transistor. FIG. 3 schematically shows example process dependent elements 220 in accordance with embodiments of the present invention. The example process dependent element in box 210 comprises a diode connected NMOS transistor M 3 , while that shown in box 211 comprises a diode connected NMOS transistor M 4 with a series resistance R 6 . The process dependent element 220 may be fabricated using the same process used to fabricate transistor M 1 and M 2 .

FIG. 4 schematically shows a power on reset circuit 400 in accordance with an embodiment of the present invention. Circuit 400 includes a process dependent element 420 connected in parallel with resistors R 3 and R 4 . In the example of FIG. 4 , the process dependent element 420 changes the effective resistance between the gate of transistor M 1 and Vsupply. Under certain process corners, the process dependent element 420 permits current to leak from V SUPPLY to the nodes at the gate of transistor M 1 , thus increasing the ratio of the voltage on node vtrip to V SUPPLY (i.e., V TRIP /V SUPPLY ratio).

The resistance of the process dependent element 420 is process dependent to allow it to vary with process and temperature. The process dependent element 420 may be chosen such that its resistance tracks the threshold variation of transistor M 1 . The properties of the process dependent element 420 may be chosen such that under process and temperature corners where transistor M 1 has a high threshold voltage, the process dependent element 420 conducts sufficient current to increase the voltage on node vtrip when the V SUPPLY voltage is near the trip point, thereby lowering the trip point of circuit 400 compared to circuit 100 . Under process and temperature corners where transistor M 1 has a low threshold voltage, the process dependent element 420 may be optimized to conduct relatively little current to have minimal effect on the trip point. In the example of FIG. 4 , the process dependent element 420 thus effectively lowers the trip point under process and temperature corners where it would be relatively high, while not appreciably affecting the trip point under process and temperature corners where the trip point would be relatively low. This reduces the overall trip point variation with process and temperature.

As in circuit 200 , the process dependent element 420 may be a diode connected NMOS transistor, such as those shown in boxes 210 and 211 of FIG. 3 . NMOS transistors are the preferred transistors for a process dependent element because their square law characteristics allow more current to be conducted at higher V SUPPLY voltages (high threshold).

FIG. 5 shows a power on reset circuit 500 in accordance with an embodiment of the present invention. Circuit 500 includes two process dependent elements namely, a process dependent element 520 and a process dependent element 540 . Process dependent elements 520 and 540 may be the same as process dependent element 220 . The process dependent element 520 raises the trip point under conditions where the trip point would be lower than expected (see circuit 200 ), while the process dependent element 540 lowers the trip point under conditions where the trip point would be higher than expected (see circuit 400 ).

While specific embodiments of the present invention have been provided, it is to be understood that these embodiments are for illustration purposes and not limiting. Many additional embodiments will be apparent to persons of ordinary skill in the art reading this disclosure.