Abstract:
New devices and methods for producing a precision current source or sink with programmable slew rate are disclosed. For example, an electronic circuit capable of providing precision current control including a programmable slew rate is disclosed. For example, the electronic circuit can include a constant current circuit configured to provide a constant current, and a transient current circuit coupled to the constant current circuit at a common electrical node, the transient current circuit configured to sample the constant current of the constant current circuit during a sampling phase, then provide a turn-on programmable slew rate based on the sampled constant current during an active phase.

Description:
INCORPORATION BY REFERENCE 
     This application claims the benefit of U.S. Provisional Application No. 61/714,997 entitled “PRECISION CURRENT WITH PROGRAMMABLE SLEW RATE CONTROL” filed on Oct. 17, 2012, the content of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Current supply devices, i.e., current sources and current sinks, are used for a large variety of circuits, such as analog amplifiers and data acquisition devices. Often these devices are required to provide a highly precise reference current while at the same time be restrained by other factors. 
     SUMMARY 
     Various aspects and embodiments of the invention are described in further detail below. 
     In an embodiment, electronic circuit capable of providing precision current control including a programmable slew rate is disclosed. The electronic circuit includes a constant current circuit configured to provide a constant current, and a transient current circuit coupled to the constant current circuit at a common electrical node, the transient current circuit configured to sample the constant current of the constant current circuit during a sampling phase, then provide a turn-on programmable slew rate based on the sampled constant current during an active phase following the sample phase. 
     In another embodiment, a method for providing precision current control including a programmable slew rate is disclosed. The method includes providing a constant current, sampling the constant current to produce a sampled current during a sampling phase, producing a transient current according to a predetermined waveform based on the sampled current during an active phase, and subtracting the transient current from the constant current to provide an output current having a turn-on slew rate that varies according to the predetermined waveform. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of this disclosure that are proposed as examples will be described in detail with reference to the following figures, wherein like numerals reference like elements, and wherein: 
         FIG. 1  is a generalize example of a current source with programmable slew rate. 
         FIG. 2  is an example of a realizable transient current circuit usable for the current source of  FIG. 1  and capable of producing a linear slew. 
         FIG. 3  depicts various waveforms generated by the circuitry of  FIGS. 1 and 2 . 
         FIG. 4  is an example of a second type of transient current circuit usable for the current source of  FIG. 1  and capable of producing an exponential slew. 
         FIG. 5  is an example of a current source with programmable slew rate. 
         FIG. 6  is a second example of a precision current source with programmable slew rate having improved linearity as compared to the circuit examples of  FIGS. 1 and 5 . 
         FIG. 7  is a flowchart outlining a set of example operations useful for producing a precision current device with programmable slew rate. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The disclosed methods and systems below may be described generally, as well as in terms of specific examples and/or specific embodiments. For instances where references are made to detailed examples and/or embodiments, it is noted that any of the underlying principles described are not to be limited to a single embodiment, but may be expanded for use with any of the other methods and systems described herein as will be understood by one of ordinary skill in the art unless otherwise stated specifically. 
       FIG. 1  is a generalize example of a precision current source  100  with programmable slew rate. The current source  100  includes a control circuit  110 , a constant current circuit  120 , a transient current circuit  130 , and an open/closed output switch SW 1 . 
     The control circuit  110  of  FIG. 1  includes an assortment of electronic components, including timing components, delays and drivers, capable of manipulating any number of switches. The precise makeup of the control circuit  110  can vary from embodiment to embodiment to encompass different types of functional components as well as different types of technologies, such as analog and digital electronics, optical components and/or any other viable technology capable of controlling a plurality of switches. 
     The constant current circuit  120  of  FIG. 1  includes an idealized current source  122 . The idealized current source  122  can take any number of forms, such as one or more transistors acting as a current mirror, as is readily known to those of ordinary skill in the art in light of this disclosure. The constant current circuit  120  produces a constant current I 1 . 
     The transient current circuit  130  of  FIG. 1  includes an idealized variable current source  132  controlled by a transient waveform circuit  134 , and two open/closed sampling switches SW 2  and SW 2 B. The constant current circuit  120  produces a transient current I 2 . 
     In operation, there are two operational phases: a sampling phase followed by an active phase. 
     During the sampling phase, the control circuit  100  opens the output switch SW 1 , closes the sampling switch SW 2  and opens switch SW 2 B. This causes the output current IouT to equal zero, and current I 1  to equal current I 2 , thus allowing the transient current circuit  130  to accurately measure/sample the current I 1  provided by the constant current circuit  120 . Once the transient current circuit  130  has measured the current I 1  provided by the constant current circuit  120 , the control circuit  100  closes the output switch SW 1 , opens sampling switch SW 2  and closes switch SW 2 B such that the output current I OUT  equal current I 1  minus current I 2 , i.e., I OUT =I 1 −I 2 . 
       FIG. 2  is an example of a realizable transient current circuit  130  usable for the current source  100  of  FIG. 1  capable of producing a linear slew as will be further described in  FIG. 3 . As shown in  FIG. 2 , the realizable transient current circuit  130  includes a transistor Q 1  acting as a variable/controllable current source, and a capacitor C 1  in parallel with constant current source I C , which as known to those skilled in the relevant arts in view of this disclosure can be a current mirror or any number of known or later developed electronic circuits. 
     During the sampling phase when I 1 =I 2 , the voltage across capacitor C 1  is charged so as to bias the gate of transistor Q 1  until a steady state condition is reached, i.e., the voltage across the capacitor C 1  and the current though the transistor Q 1  are unchanging. Upon start of the active phase when the sampling switch SW 2  is opened and SW 2 B is closed, the voltage across the capacitor C 1  will drop as a linear function of time. Accordingly, the current through the transistor Q 1  will linearly decline as a function of the declining voltage across capacitor C 1  until I 2 =0. 
     Often, a circuit may have a settling requirement. This means that a current must settle to a certain accuracy. For the devices in this disclosure, however, current I 1  is settled long before needed such that as the circuit switches modes, current I 1  will settle 100% once the transient operation is completed 
       FIG. 3  depicts various waveforms of the circuitry of  FIGS. 1 and 2 . As is shown in  FIG. 3 , there is sampling phase that transitions to an active phase at time T 1 . Control lines CL 1  and CL 2  (generated by control circuit  110 ), which control the output switch SW 1  and the sampling switch SW 2  of  FIG. 1 , cause current I 2  to equal current I 1 , and output current I OUT  to equal zero prior to time T 1 . While control lines CL 1  and CL 2  appear inverted to one another, this is depicted so as to give an idea that some switches will be opened while others are closed. A single control line for all switches may be sufficient assuming that such switches are appropriately active high or low. 
     After time T 1 , however, current I 2  transitions linearly to zero according to a predetermined slope based on the value of the capacitor C 1  and the current source I C  of  FIG. 2 . As current I 2  transitions to zero, the output current I OUT  transitions to current I 1  according to the equation: I OUT =I 1 −I 2 . 
       FIG. 4  is a second example of transient current circuit  120 B usable for the current source of  FIG. 1  capable of producing an exponential slew. The circuitry is identical to that of  FIG. 2  except that current source I C  is replaced by resistor R C . Upon transition to an active phase, the waveform of current I 2  will decline according to the equation: I 2 =I 1 ×e (−t/RC) .  FIG. 4  demonstrates that an onset slew rate for the current source  100  of  FIG. 1  can be manipulated to nearly any type of waveform based by used of any combination of linear components, e.g., resistors and capacitors, and non-linear components, such as diodes. Accordingly, a wide variety of slew waveforms may be provided as may be found necessary, useful or otherwise desirable. 
       FIG. 5  is an example of a precision current sink  500  with programmable slew rate that can acts as a complement to the current source  100  of  FIG. 1 . The current sink  500  includes a control circuit  510 , a constant current circuit  520 , a transient current circuit  530 , and an output switch SW 51 . 
     The constant current circuit  520  includes transistor Q 52  acting as a current mirror to transistor Q 53 . 
     The transient current circuit  530  includes transistor Q 51  acting as a variable current source, sampling switches SW 52  and SW 52 B, and a capacitor C 51  in parallel with current source I C5 . 
     As with the current source  100  of  FIG. 1 , there are two operational phases: a sampling phase followed by an active phase. One of ordinary skill in the art viewing this disclosure will readily see that the operation of the current sink  500  of  FIG. 5  is analogous to that of the current source  100  of  FIG. 1 . As such, a detailed description of the operation of the current sink  500  is omitted here. 
       FIG. 6  is another example of a current source  600  with programmable slew rate having improved linearity as compared to the devices of  FIGS. 1 and 5 . The current source  600  includes control circuitry (not shown so as to reduce clutter in  FIG. 6 ), an output switch SW_B 2 , a constant current circuit  620 , and a transient current circuit  630 . 
     The constant current circuit  620  includes an idealized current source  622  that produces current steady current I 1 . 
     The transient current circuit  630  includes a number of switches {SW_A 1 , SW_A 1 , SW_A 1 , SW_B 1 , SW_B 3 }, a first capacitor C 61  switchably in parallel with a first resistor R 61  and a third resistor R 63 , an amplifier A fed by a current limiting source I C62 , a second capacitor C 62 , a transistor Q 61  and a second resistor R 62 . 
     During the sampling phase, switches SW_B 1 , SW_B 2  and SW_B 3  are open and the remaining switches {SW_A 1 , SW_A 2 , SW_A 3 } are closed. In operation of the sampling phase, the voltage across the first capacitor C 61  charges, while amplifier A, transistor Q 61  and resistor R 62  act as a voltage-to-current converter to the charge across capacitor C 61 . The current limiting source I C62  and second capacitor C 62  provide stability to the voltage-to-current converter. A load (not shown) above switch SW_A 3  provides a compensation current to counteract a measurement current consumed during sampling. Eventually, the transient current circuit  630  will reach a steady state whereby I 2 =I 1 . 
     During the active phase, switches SW_B 1 , SW_B 2  and SW_B 3  are closed and the remaining switches {SW_A 1 , SW_A 2 , SW_A 3 } are opened. The RC constant of the first capacitor C 63  and first resistor R 61  provide an exponential decay, which in turn causes current I 2  to decay proportionally. As with the previous examples, the first capacitor C 61  and first resistor R 61 , which for this example constitute a transient waveform circuit, can be replaced with any combination of circuitry to provide a large variety of different onset slew waveforms. For example, by replacing the first resistor R 61  with a constant current source, a linear slew rate is produced. 
       FIG. 7  is a flowchart outlining a set of example operations useful for producing a precision current device with programmable slew rate. It is to be appreciated to those skilled in the art in light of this disclosure that, while the various functions of  FIG. 7  are shown according to a particular order for ease of explanation, that certain functions may be performed in different orders or in parallel. The functions below are applicable to any number of current sources or current sinks having a desirable onset slew rate/waveform, including any of those devices described above for  FIGS. 1-6 . 
     The process starts at S 702  where a current level I 1  for a constant current source is set/determined. At S 704 , a transfer function/waveform for an onset slew rate is determined. As discussed above, such a slew rate waveform can be linear, exponential or any of a large variety of designs as may be found necessary, useful or otherwise desirable. Control continues to S 706 . 
     At S 706 , a number of switches, such as switches SW 1  and SW 2  of  FIG. 1 , (and possibly other sampling circuitry) is set so as to enable some form of transient current circuit to sample/mirror current level I 1  until a transient current I 2  settles to a constant state and I 1 =I 2 . Control continues to S 708 . 
     At S 708 , the state of the switches and sampling circuitry is reconfigured so as to put the current source/sink into an active phase. Then, at S 710  the transient current I 2  is provided according to the predetermined transfer function/waveform of S 704 , thus causing the output current of the source/sink to transition from zero to current level I 1  according to the equation: I OUT =I 1 −I 2 . 
     While the invention has been described in conjunction with the specific embodiments thereof that are proposed as examples, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, embodiments of the invention as set forth herein are intended to be illustrative, not limiting. There are changes that may be made without departing from the scope of the invention.