HK1069221B - Method of forming a low quiescent current voltage regulator - Google Patents

Description

Method of forming a low quiescent current voltage regulator and structure therefor

Background

The present invention relates generally to electronic devices and, more particularly, to methods and structures for forming semiconductor devices.

In the past, the semiconductor industry utilized various methods and structures to fabricate voltage regulators, including linear voltage regulators. During normal operation, when the output voltage produced by the voltage regulator reaches a desired operating value, the voltage regulator turns off (disable) the output transistor. The output transistor remains off until the output voltage drops to a value below the desired operating value. An external filter capacitor and a load are typically connected to the output of the regulator. During the off-period of the output transistor, the leakage current of the output transistor will flow through the external filter capacitor and continuously charge the filter capacitor. The leakage current charges the capacitor and the voltage across the capacitor increases in value and may reach a value that causes damage to the load. In some cases, a resistor is connected between the output transistor and ground so that the leakage current of the transistor flows through the resistor and not through the filter capacitor. A problem with this configuration is power dissipation. The leakage current through the resistor increases the quiescent current consumption and correspondingly increases the power consumption of the voltage regulator. Typically, a voltage regulator so constructed with a resistor has an average quiescent current consumption of at least about 55 microamps.

Accordingly, it is desirable to have a method of forming a voltage regulator that reduces quiescent current consumption and maintains the output voltage below a value that causes load damage.

Brief description of the drawings

FIG. 1 schematically illustrates a portion of an embodiment of a voltage regulator in accordance with the present invention; and

fig. 2 schematically illustrates a portion of an embodiment of a semiconductor device including the voltage regulator of fig. 1 in accordance with the present invention.

For simplicity and clarity of illustration, elements in the figures are not necessarily to scale, and the same reference numbers in different figures denote the same elements. Moreover, descriptions and details of well-known steps and components are omitted for brevity of the description. As used herein, current carrier means an element of a device that carries current through the device, such as a source or drain of a MOS transistor or an emitter or control electrode of a bipolar transistor, and a control electrode means an element of a device that controls current through the device, such as a gate of a MOS transistor or a base of a bipolar transistor.

Detailed description of the drawings

Fig. 1 schematically illustrates a portion of an embodiment of a voltage regulator 10, the voltage regulator 10 having low quiescent current consumption and low power consumption. Regulator 10 receives power from an external power source on a power input 11 and a power return 12 and has an output voltage between a voltage output 13 and a voltage return 14. A filter capacitor 34 and a load 33 are externally connected to regulator 10 between output 13 and return 14. Regulator 10 includes an error amplifier 26, an output device or output transistor 24, a feedback network 19, and a reference generator 16. Network 19, identified by a dashed box, includes a pair of feedback resistors 22 and 23 connected in series between output 13 and return 14 to form a resistor divider having a feedback node 21 formed by connecting resistor 22 and resistor 23. Error amplifier 26 receives a feedback voltage from node 21 and a reference voltage from output 17 of reference generator 16. Amplifier 26 receives the reference voltage and the feedback voltage and generates an error voltage in response at the output of amplifier 26. To control the value of the output voltage to the desired operating voltage, regulator 10 drives transistor 24 with the error voltage. The desired operating voltage depends on the value of the voltage divider and the value of the reference voltage. Those skilled in the art understand that the desired operating voltage typically has a desired operating range that includes an upper limit and a lower limit. For example, a desired operating voltage value of 2.5 volts (2.5V) may include a desired operating range including upper and lower limits of plus or minus two percent (± 2%). Therefore, the desired operating voltage range should have a nominal value of about 2.5 volts, a maximum value of about 2.55 volts, and a minimum value of about 2.45 volts. When the output voltage value is lower than the standard value, the feedback voltage value is lower than the reference voltage value and error amplifier 26 forms an error voltage that operates transistor 24. Transistor 24 provides a load current IL through load 33 and capacitor 34 and charges capacitor 34 to raise the output voltage to the desired operating value. When the output voltage value reaches the desired operating value, the feedback voltage value is higher than or equal to the reference voltage value on output 17 and error amplifier 26 generates an error voltage that turns off transistor 24. The characteristics and operation of network 19, generator 16, amplifier 26 and transistor 24 are well known to those skilled in the art.

The regulator 10 also includes a compensation circuit 20, generally identified by a dashed box, that helps to reduce the quiescent current and power consumption of the regulator 10. Circuit 20 includes a selectable current source 28, a fixed current source 29, a compensation comparator 27, and a reference offset 18. Regulator 10 is configured to selectively operate selectable current source 28 to produce a compensation current that flows from transistor 24, through current source 28, and to return 12 when the output voltage value is equal to or greater than the first voltage value or the compensation voltage value. Typically the compensation voltage value is above the maximum value of the desired operating voltage range and below a value that may damage the load 33. As will be seen below, the offset 18 forms an offset reference voltage that is equal to the reference voltage value from the generator 16 plus the offset voltage value. Comparator 27 receives the offset reference and the feedback voltage and responsively enables or disables selectable current source 28.

Fixed current source 29 sinks a fixed current value from transistor 24. The fixed current value is typically formed as an approximate leakage current value that is expected from transistor 24 under standard process conditions including temperature and standard operating conditions. Under normal operating and process conditions, when transistor 24 is off, current source 29 sinks leakage current from transistor 24 and no leakage current from transistor 24 flows through capacitor 34 or load 33. However, if the process conditions used to form transistor 24 vary from standard process parameters or if the operating conditions vary from standard operating conditions, the leakage current of transistor 24 will exceed the current drawn by fixed source 29 when transistor 24 is off. This additional leakage current or excess leakage current is greater than the leakage current that can be absorbed by fixed source 29 and will flow through capacitor 34. This excess leakage current begins to charge capacitor 34 causing an increase in the value of the output voltage. The value of the output voltage increases until a compensation value is reached that is determined by the offset reference voltage value from the offset 18 and the feedback voltage. The compensation comparator 27 receives the feedback voltage and the offset reference voltage and responsively operates the current source 28 when the value of the output voltage reaches the value of the compensation voltage. The offset current plus the fixed current should be at least equal to and preferably higher than the worst case leakage current of transistor 24. In a preferred embodiment, the individual compensation currents are set at least equal to or higher than the worst case leakage current of transistor 24. This provides a safety margin for the variation in the worst case leakage current. Absorbing the excessive leakage current by the operation source 28 suppresses the increase of the output voltage value beyond the compensation value and prevents the damage of the load 33. Since current source 28 operates only to sink current when the output voltage exceeds the offset voltage value, current source 28 is not always operating, and therefore, selectively operating source 28 to sink the excess leakage current reduces the quiescent current consumption of regulator 10.

Comparator 27 is typically formed with hysteresis to ensure that selectable current source 28 does not oscillate back and forth between on and off. In a preferred embodiment, comparator 27 has a hysteresis characteristic of twenty millivolts, such that comparator 27 enables current source 28 to operate when the feedback voltage is at or above the offset reference voltage value and disables current source 28 when the feedback voltage value is twenty millivolts below the offset reference voltage value.

It should be noted that current source 29 may be omitted in some embodiments but the output voltage may oscillate between the desired voltage value and the compensation voltage value even under standard conditions. However, the resistor divider of the resistors 22 and 23 may be formed to have a fixed current value and the fixed current source 29 may be omitted. In other embodiments, comparator 27 may be replaced with an amplifier that selectively operates current source 28 to form a compensation current responsive to the analog output signal of the amplifier. In addition, the regulator 10 may include other well-known circuit functions including overcurrent protection and temperature protection. Such circuitry is not shown in fig. 1 for clarity of explanation.

In one embodiment, regulator 10 is formed to have a standard desired operating value of approximately 2.5 volts (2.5V), plus or minus two percent (+ -2%) of which is about 2.45 volts to about 2.55 volts of the desired operating range. The maximum voltage value that will not damage load 33 is a value close to 2.7 volts. The value of the capacitor 34 is about 1 microfarad. The standard leakage current of transistor 24 is approximately two (2) microamps at approximately 25 degrees celsius (25 ℃) and standard process parameters. The worst-case leakage current for transistor 24 under the worst process parameters and worst operating conditions is approximately fifteen (15) microamps. The fixed current value is selected to be equal to the standard leakage current or about 2 microamps. The current that current source 28 is capable of sinking is chosen to be forty microamperes to ensure that current source 28 is capable of sinking all of the worst case transistor 24 leakage current. But the actual current drawn by current source 28 is an actual value that exceeds the leakage current of transistor 24. The value of the compensation voltage is chosen to be about 2.6 volts (2.6V). The offset voltage value is 100 millivolts to ensure that the output voltage value of output 13 is no greater than 100 millivolts above the desired operating value of 2.5V. When the output voltage on output 13 reaches a value close to 2.5V, amplifier 26 turns off transistor 24 to maintain the output voltage at this value. Since the value of the leakage current from transistor 24 exceeds 2 microamperes, the value of the voltage on capacitor 34 increases to a value of about 2.6 volts, and comparator 27 operates selectable current source 28 to sink the excess leakage current of transistor 24. The value of the voltage on capacitor 34 slowly drops to a value below 2.6 volts and the output of comparator 27 again disables current source 28. During the exemplary circuit evaluation, during the period that transistor 24 is off, current source 28 is not operating for approximately two (2) milliseconds, while capacitor 34 is charging and operating for approximately six hundred fifty (650) microseconds, while capacitor 34 is discharging, so that current source 28 operates for approximately twenty-five percent (25%) of the off time of transistor 24. In this embodiment, the average quiescent current of the regulator 10 is about thirty-five microamps, which is thirty-six percent (36%) lower than the average quiescent current of the fifty-five microamps of prior regulators. This current saving is important in certain applications, such as battery operation.

Fig. 2 schematically illustrates a partially enlarged plan view of an embodiment of a semiconductor device 40 formed on a semiconductor module 41. The regulator 10 is formed on the module 41. The module 41 may also comprise other circuits, which are not shown in fig. 2 for the sake of simplicity of the drawing.

While the present invention has been described with reference to certain preferred embodiments, it is evident that many alternatives and variations will be apparent to those skilled in the semiconductor arts. For example, the offset reference voltage may be formed elsewhere including as a separate output from the generator 16. The comparator 27 may be an analog amplifier instead of a comparator. Further, the fixed current source 29 may be omitted. Also, the present invention describes a particular P-channel output transistor, although the method is directly applicable to other MOS transistors, but also to bipolar transistors, BiCMOS, metal-semiconductor FETs (MESFETs), HFETs, and other transistor structures.

In view of the foregoing, it is apparent that a new method and apparatus is disclosed. Among other features is forming a voltage regulator to selectively generate a flowing compensation current to prevent leakage current from an output transistor from increasing an output voltage of the voltage regulator to a value that may damage a load. Selectively flowing current reduces quiescent current consumption of the regulator.

Claims (9)

1. A method of forming a voltage regulator, comprising:

forming a voltage regulator (10) to provide an output voltage having a first value and a load current on a voltage output; and

a voltage regulator (10) is formed to selectively generate a compensation current that flows from an output device of the voltage regulator to a voltage return of the voltage regulator but not through the voltage output, wherein the voltage regulator is configured to selectively generate the compensation current after the output device is inactive and when an output voltage of the voltage regulator exceeds a second value that is higher than the first value.

2. The method of claim 1, wherein forming the voltage regulator to selectively generate the compensation current comprises: the compensation current is discontinued when the output voltage decreases to a third value, which is lower than the second value and higher than the first value.

3. The method of claim 1, wherein forming the voltage regulator to selectively generate the compensation current to flow comprises: a voltage regulator is formed to selectively generate a compensation current that flows through the output device but not through an external load (33) or an external filter capacitor (34).

4. A method of forming a regulated voltage, comprising:

generating an output voltage having a desired operating range between a first desired value and a second desired value that is lower than the first desired value;

-deactivating the output device (24) when the output voltage reaches a first desired value; and

a compensation current (28) is selectively caused to flow from the output device to the voltage return when the output device is not operating and when the output voltage exceeds a compensation value (18 plus 17) that is higher than the first desired value.

5. The method of claim 4, further comprising: when the output voltage drops to another value below the compensation value and above the first desired value, the compensation current is discontinued (28).

6. The method of claim 4, wherein the step of generating the output voltage comprises connecting the output voltage to an output (13, 14) of the voltage regulator, and wherein the step of selectively causing the compensation current to flow from the output device to the voltage return comprises: diverting current from flowing through the output.

7. A voltage regulator, comprising:

an output device (24) for receiving an input voltage and forming an output on an output of the voltage regulator;

a selectable current source connected between the output device and a voltage return;

a feedback network (19) for forming a feedback voltage representative of the output voltage;

an error amplifier (26) for receiving the first reference voltage and the feedback voltage and responsively driving the output device; and

a compensation amplifier (27) for receiving the feedback voltage and a second reference voltage higher than the first reference voltage and responsive to generating a compensation current that flows from the output device through the selectable current source to the voltage return but not through the output of the voltage regulator.

8. The voltage regulator of claim 8, wherein the compensation amplifier is a hysteretic comparator.

9. The voltage regulator of claim 8, further comprising a fixed current source for generating a fixed current flowing from an output device, wherein a value of the fixed current is approximately equal to a value of a leakage current of the output device.