Low-headroom constant current source for high-current applications

A low-headroom current driver does not use an op amp or resistor. A sensing transistor having its source connected to a drain of an output transistor senses variations in an output current. The gate, source, and drain voltages of the sensing transistor are mirrored to a sense mirror transistor to control a sense current. The sense current is mirrored to a reference source transistor to generate a mirrored sense current. An error between the mirrored sense current and a fixed reference current is stored as charge on an error-storing capacitor. The stored error charge creates a negative-feedback compensation current that adjusts a gate voltage generated by a feedback-driving transistor. The adjusted gate voltage controls the gate of the output transistor to compensate for the sensed variation in output current. The sensing current is also compensated using a sense-mirror tail transistor connected to the sense mirror transistor.

FIELD OF THE INVENTION

This invention relates to current driver circuits, and more particularly to low-headroom high-current drivers.

BACKGROUND OF THE INVENTION

Many applications require that a high current is driven to an external device. A high-current driver may be used to drive a relatively large current to a Light-Emitting Diode (LED), liquid crystal display (LCD), motor, actuator, etc. Health-care applications requiring a large current include heart-rate monitoring, SPO2 monitors, and other sensors.

FIG. 1Ashows a high-current application. Current source106provides a large current that is switched by switch104to turn on and off LED102.FIG. 1Bis a waveform of the high-current switching application. The switch closes when switch voltage VSW is high. The source current switched through LED102pulses high when the switch is closed. Some low-power applications may save power by reducing the turn-on time. A large current may be needed to provide a rapid settling time for the source current so that the waveform is not distorted.

As semiconductor process technology improves, devices sizes shrink. These smaller devices use reduced power-supply voltages to prevent damage to the tiny devices. The lowered power-supply voltage produces a low-headroom environment for the circuit where smaller voltages are applied across transistors. The smaller voltages in a low-headroom environment produce lower currents, which is opposite of the design goal for a high-current driver circuit. Therefore careful circuit design is needed.

A traditional current driver might use a complementary metal-oxide-semiconductor (CMOS) current mirror. However, the output current varies with changes in the drain-source voltage. Cascode current mirror may be used to reduce the current variation with drain-source output voltage, but a large voltage drop occurs on a cascode transistor. In low-headroom environments, there may be insufficient available voltage drop for the cascode transistor.

When a second transistor is placed in series with a large output transistor, this second transistor also has to be large to carry the large current, increasing circuit size and cost. Also the second transistor may reduce the available voltage drop to the output transistors. Thus having one or more transistors in series with the output transistor is undesirable.

Many driver circuits use operational amplifiers (op amps). Op amps provide a quick response, but have a high gain and have high power consumption. It is thus desirable to avoid op amps in a high-current low-headroom driver circuit.

Other circuits use a resistor over a constant voltage to generate the constant current. However, the resistor's voltage drop reduces the available voltage for other transistors in the circuit, and the resistor burns power.

What is desired is a low-headroom high-current driver circuit. A current driver circuit that does not use an op amp is desirable. A current driver circuit that does not use a resistor to generate a constant current is also desirable. A driver circuit with a constant current mirror source for high-current applications in a low-headroom environment is desired.

DETAILED DESCRIPTION

FIG. 2is a conceptual diagram of a driver circuit that senses output-current variation to generate a compensating error current. Many prior designs attempt to have a mirror transistor match the gate, drain, and source voltages of the output transistor. A reference current flowing through the mirror transistor can then be scaled up by the transistor ratio to generate the output current.

In contrast, the inventors sense a variation or error in the output current. This error is used to adjust the reference current to compensate for the error. Current mirrors may still be used to scale the reference current to the output current.

A current mirror (not shown) scales a reference current i_REF from reference current source108to cause output current source106to generate output current i_OUT. Current sensing circuit112senses variation in the output current i_OUT from output current source106to generate an error current i_ERROR. Summer110subtracts the error current i_ERROR from the reference current i_REF to adjust generation of the output current i_OUT by output current source106. Thus variations in output current are sensed and a compensating current generated to adjust the output current.

The closed-loop feedback of sensing output-current variation and generating a compensating current does not use an op amp. A voltage-generating resistor is also not used. Thus power consumption is reduced.

FIG. 3is a schematic of a current-mirror circuit with sensing of output-current variation and current compensation. Output transistor56draws output current i_OUT from output node VO and from external LED102. Reference current source108sinks reference current i_REF from node V3, which is mirrored to feedback-driving transistor36to generate gate voltage V4to output transistor56. The ratio of sizes of feedback-driving transistor36and output transistor56allows i_OUT to be much larger than i_REF, since output transistor56is M times larger than feedback-driving transistor36.

Variations in the output current i_OUT are sensed by transistors50,54,40,42,44,46. The sensed current variation is then compensated for by transistors20,22,30,32,36. The sensed variation or error in current is stored on error-storing capacitor60. Transistors20,22,30,32,40,42,50are p-channel transistors, while feedback-driving transistor36, sense mirror transistor44, sense-mirror tail transistor46, sensing transistor54, and sense output transistor56are n-channel transistors.

A first bias voltage VB1is applied to the gates of reference middle transistor22, feedback middle transistor32, and sense-mirror middle transistor42. A slightly higher second bias voltage VB2is applied to the gate of sense-source transistor50. The gate voltages of reference source transistor20and sense-mirror source transistor40are voltage V2generated between the drains of sense-mirror middle transistor42and sense mirror transistor44. The gate voltage V3of feedback source transistor30is generated by the drain of reference middle transistor22as i_REF is pulled through it by reference current source108.

Transistors54,44,46form a current sensing circuit. The current and size of transistors54and44may be the same so that their gate-to-source voltages (Vgs) are the same. Therefore, the drain voltage of transistor46is same as the output voltage VO. When the output voltage VO changes, the drain voltage of transistor46changes. Transistor46always senses the output current with a ratio determined by the ratio of the sizes of transistors46,56, since transistors46,56have the same gate/source/drain voltages.

Sensing transistor54has its source connected to output VO. The size of sensing transistor54is much smaller than output transistor56, so that a very small or negligible amount of output current i_OUT is diverted through sensing transistor54. The current through output transistor56is approximately equal to i_OUT since the current through sensing transistor54is small.

The output voltage VO variations sensed by sensing transistor54cause V1to vary. V1is applied to the gate of sense mirror transistor44, which causes the drain voltage of sense-mirror tail transistor46to vary. Voltage V2also varies as the current through sense-mirror tail transistor46is varied due to its changing drain voltage. Transistor40has the same current as transistor46. V2varies and is applied to the gate of reference source transistor20. The current of transistor20is same as the current of transistor40(assuming the sizes of transistors20,40are the same, and that the sizes of transistors22,42are the same). They form a cascode current mirror source.

The current sourced into node V3is varied and stored on error-storing capacitor60since i_REF is fixed by reference current source108. The size of error storing capacitor60can be selected to provide a desired amount of smoothing or averaging of current-variation adjustments due to an R-C time constant. Capacitor60can be used as current error-storing to form a voltage, and it is also used as the compensation capacitor for the feedback current source transistor30.

The error charge stored on error-storing capacitor60adjusts V3and V4, causing a feedback current through feedback source transistor30,32, and into feedback-driving transistor36to adjust V4, the gate and drain voltage of feedback-driving transistor36and the gate voltage of output transistor56. The error current sensed by sensing transistor54is subtracted from the output current to adjust the output current through output transistor56back to a constant, stable value. Thus the output current through output transistor56is adjusted back to a constant, stable value to compensate for sensed variations by sensing transistor54. The small current from sensing transistor54also helps to settle changes in i_OUT faster.

FIGS. 4A-Ehighlight operation of the sensing and compensating circuit ofFIG. 3. InFIG. 4A, when the current through output transistor56suddenly increases, its drain voltage VO drops slightly. The current of transistor54is constant because its current is from transistor50. The constant current causes Vgs to remain constant. When voltage VO changes suddenly or slowly, V1is always higher than VO by one Vgs. Sense mirror transistor44receives V1on its gate and causes its source voltage, which is also the drain voltage of sense-mirror tail transistor46, to always be equal to VO. A rise in V1increases the sense current i_SEN through sense mirror transistor44. Thus sense-mirror tail transistor46always senses the output current.

When additional parallel LED's are suddenly connected to VO and draw a surge in current, the output voltage VO is pulled high suddenly. The output current increases suddenly, so the sense current of sense-mirror tail transistor46also increases. Using transistors44,54to force the drain voltage of transistor46to be equal to VO, limits VO to be no higher than VDD minus one Vgs and two Vdsat.

InFIG. 4B, transistors20,40,22,42form a cascode current mirror source. The current through sense-mirror source transistor40is mirrored to reference source transistor20. The increased sense current i_SEN through sense-mirror tail transistor46is also pulled through sense-mirror middle transistor42and sense-mirror source transistor40, causing V2to drop slightly to increase the current through transistors40,42. The lower V2is applied to the gate of reference source transistor20, causing sense current i_SEN to be mirrored to reference source transistor20to generate mirrored sense current MIRROR i_SEN.

InFIG. 4C, the larger mirrored sense current flowing through reference source transistor20flows through reference middle transistor22and onto error-storing capacitor60, since reference current source108is a fixed current source that does not allow for variation in the reference current i_REF. The error or difference between the mirrored sense current and reference current is stored as charge on error-storing capacitor60.

Voltage V3increases as error-storing capacitor60is charged by the excess mirrored sense current. The higher V3applied to the gate of feedback source transistor30reduces the feedback compensation current i_FBC through feedback middle transistor32. Feedback middle transistor32is a cascode transistor that can have the same size and bias voltages as reference middle transistor22and sense-mirror middle transistor42.

InFIG. 4D, the lower feedback compensation current i_FBC flows through feedback-driving transistor36, causing its gate and drain to lower to reduce its current drive. Thus voltage V4to the gate of output transistor56is lowered, reducing the output current to compensate for the sudden increase in output current.

As V4is lowered, the current through sense-mirror tail transistor46is also reduced. The mirrored sense current through sense-mirror tail transistor46is thus compensated for the sensed error so that the tail current through sense-mirror tail transistor46is equal to i_REF. Once there is any error between this sensed current and i_REF, the feedback current will compensate it.

InFIG. 4E, since the gate/source/drain voltage of sense-mirror tail transistor46and output transistor56are the same, the output current of transistor56is adjusted back to a constant value because the current of transistor46is adjusted to be the same as i_REF. A similar but inverse mechanism occurs when the output current suddenly is reduced.

FIG. 5is an alternative sensing and compensating circuit. The varying voltage V3is applied to feedback middle transistor32rather than to feedback source transistor30. In this alternative, a bias voltage drives the gate of feedback source transistor30rather than feedback middle transistor32.

The gate of feedback source transistor30is driven by second bias voltage VB2rather than by V3. The gate of feedback middle transistor32is not a fixed bias voltage but is directly connected to V3. Error-storing capacitor62is connected between V3and the fixed power-supply VDD.

Variations in V3adjust the current through feedback middle transistor32, causing the node between feedback source transistor30and feedback middle transistor32to vary in voltage, causing the negative feedback compensation current i_FBC to vary. Feedback middle transistor32acts as a source follower, which may ease compensation.

FIG. 6is another alternative circuit without an error-storing capacitor. Voltage V3directly drives the gate of feedback source transistor30.

A second leg of second source transistor70and second reference middle transistor72are in parallel with the leg of reference source transistor20and feedback source transistor30. However, while the gate of second reference middle transistor72is biased by bias voltage VB1, the gate of second source transistor70is driven by V3while the gate of reference source transistor20is driven by V2.

The current from reference current source108can be double the reference current of other embodiments, or 2*i_REF.

The current through transistor70is 2*i_REF minus the current of transistor22. If the current through transistor20is i_REF−i_ERROR, the current through transistor70is i_REF+i_ERROR. Since transistors70,30and72,32form the cascode current mirror source, the current through transistor30is the current through transistor70, which is i_REF+i_ERROR. This current is used to compensate the current of transistors46and56. Since all of the paths use current mirror sources without any compensation capacitors, the settling time may be fast with this embodiment

ALTERNATE EMBODIMENTS

Several other embodiments are contemplated by the inventors. For example, various combinations of the alternative embodiments are possible. Error-storing capacitor60could have it's back terminal connected to VDD or to ground rather than to node V4. Parasitic and gate capacitances could reduce the size of error-storing capacitor60or eliminate it. Various ratios of transistor sizes could be used, or exact matching of transistor sizes, shapes, and orientations. The output current may be at least ten times larger than the small sense current through sensing transistor54.

While n-channel transistors have been shown, the circuits could be flipped over and p-channel transistors substituted. The LED could be external to the circuit or they could be integrated together.

N-channel transistors have been shown. The substrate or bulk connections may be tied to the highest voltage, such as VDD or VCC, or to a substrate or back-bias voltage, or to the transistor sources.

The current source could be implemented as n-channel transistors having gates receiving a fixed voltage. Bias voltages could be generated by bias generators such as a resistor divider or a series of transistors.

Various theories of operation have been presented to try to explain operation. These theories are approximations of real, often complex, physical behaviors. These theories may be incorrect, although useful for designing driver circuits. The invention is not limited by these theories and does not depend on these theories being correct.

The circuit designer may choose resistors, capacitors, transistors, and other components to have a ratio that produces the desired voltages. While Complementary-Metal-Oxide-Semiconductor (CMOS) transistors have been described, other transistor technologies and variations may be substituted, and materials other than silicon may be used, such as Galium-Arsinide (GaAs) and other variations. DMOS, LDMOS, and diffusion-enhanced transistors may be used. Bipolar transistors could also be used, such as for output transistor56.

Timings may be adjusted by adding delay lines or by controlling delays in leading-edge blocking units. Pulse generators could also be added. The outputs or control signals may be swapped to add an inversion. Inverting and non-inverting inputs may be swapped and the polarity of the output reversed.

Separate power supplies and grounds may be used for some components. The bulk or substrate nodes may be tied to power for p-channel transistors, and to ground for n-channel transistors, or a substrate bias generate be used to generate bulk voltages. Various filters could be added. Active low rather than active high signals may be substituted. The signals applied to the gates of p-channel and n-channel transistors may be switched to power or ground to power down the circuit.

The bias voltages may be fixed, or may be adjustable, such as to track temperature, process, or power-supply voltage. The reference current i_REF from reference current source108may likewise be fixed, or may be adjustable to track temperature, process, or supply voltage. Band-gap references may be used.

While positive currents have been described, currents may be negative or positive, as electrons or holes may be considered the carrier in some cases. Source and sink currents may be interchangeable terms when referring to carriers of opposite polarity. Currents may flow in the reverse direction.

Additional components may be added at various nodes, such as resistors, capacitors, inductors, transistors, etc., and parasitic components may also be present. Enabling and disabling the circuit could be accomplished with additional transistors or in other ways. Pass-gate transistors or transmission gates could be added for isolation.

Inversions may be added, or extra buffering. The final sizes of transistors and capacitors may be selected after circuit simulation or field testing. Metal-mask options or other programmable components may be used to select the final capacitor, resistor, or transistor sizes.

The background of the invention section may contain background information about the problem or environment of the invention rather than describe prior art by others. Thus inclusion of material in the background section is not an admission of prior art by the Applicant.

Any methods or processes described herein are machine-implemented or computer-implemented and are intended to be performed by machine, computer, or other device and are not intended to be performed solely by humans without such machine assistance. Tangible results generated may include reports or other machine-generated displays on display devices such as computer monitors, projection devices, audio-generating devices, and related media devices, and may include hardcopy printouts that are also machine-generated. Computer control of other machines is another tangible result.