Abstract:
A light emitter drive circuit for an oximeter which utilizes a single inductor for driving multiple light emitters. The inductor is connected to a switching circuit to multiple energy storage circuits, such as capacitors. These are alternately charged up, using the same inductor. Subsequently, the capacitors are alternatively discharged for their corresponding light emitters through the same inductor. Also, the magnetic susceptibility of the LED drive circuit is reduced by using magnetic flux canceling in the inductor. In one embodiment, a toroidal inductor is used with geometric symmetry and its magnetic flux. In other embodiment, a dual core closed bobbin shielded inductor is used.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 10/787,852, filed on Feb. 25, 2004. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to oximeters, and in particular to LED drive circuits in pulse oximeters. 
     Pulse oximetry is typically used to measure various blood chemistry characteristics including, but not limited to, the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and the rate of blood pulsations corresponding to each heartbeat of a patent. Measurement of these characteristics has been accomplished by use of a non-invasive sensor which scatters light through a portion of the patient&#39;s tissue where blood perfuses the tissue, and photoelectrically senses the absorption of light in such tissue. The amount of light absorbed at various wavelengths is then used to calculate the amount of blood constituent being measured. 
     The light scattered through the tissue is selected to be of one or more wavelengths that are absorbed by the blood in an amount representative of the amount of the blood constituent present in the blood. The amount of transmitted light scattered through the tissue will vary in accordance with the changing amount of blood constituent in the tissue and the related light absorption. For measuring blood oxygen level, such sensors have typically been provided with a light source that is adapted to generate light of at least two different wavelengths, and with photodetectors sensitive to both of those wavelengths, in accordance with known techniques for measuring blood oxygen saturation. 
     Known non-invasive sensors include devices that are secured to a portion of the body, such as a finger, an ear or the scalp. In animals and humans, the tissue of these body portions is perfused with blood and the tissue surface is readily accessible to the sensor. 
     The light sources, typically light emitting diodes (LEDs), need to be driven with current to activate them. Because of the significant amount of current required, this can interfere with reducing power consumed by an oximeter. One solution is shown in U.S. Pat. No. 6,226,539. There, an inductor and capacitor circuit is used to first store charge in a first switch position, and then subsequently, in a second switch position, deliver that stored charge to the LED. Two different inductor and capacitor circuits are used, one for each LED. It would be desirable to reduce the number of components required in the circuit of this patent. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a light emitter drive circuit for an oximeter which utilizes a single inductor for driving multiple light emitters. The inductor is connected through a switching circuit to multiple energy storage circuits, such as capacitors. These are alternately charged up, using the same inductor. Subsequently, the capacitors are alternately discharged to activate their corresponding light emitters through the same inductor. 
     In another aspect of the present invention, the magnetic susceptibility of the LED drive circuit is reduced by using magnetic flux canceling in the inductor. In one embodiment, a toroidal inductor is used with geometric symmetry in its magnetic flux. In another embodiment, a dual core closed bobbin shielded inductor is used. This embodiment has windings of both cores in series that are used to cancel the effect of an external magnetic field. 
     For a further understanding of the nature and advantages of the present invention, reference should be made to the following description taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an oximeter incorporating the present invention. 
         FIG. 2  is a circuit diagram of a LED drive circuit according to an embodiment of the present invention. 
         FIG. 3  is a block diagram of one embodiment of the logic for generating the timing and control signals for the circuit of  FIG. 2 . 
         FIG. 4  is a diagram of a toroidal inductor used in one embodiment of the present invention. 
         FIGS. 5 and 6  are diagrams of a dual core inductor according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Oximeter System 
       FIG. 1  illustrates an embodiment of an oximetry system incorporating the present invention. A sensor  10  includes red and infrared LEDs and a photodetector. These are connected by a cable  12  to a board  14 . LED drive current is provided by an LED drive interface  16 . The received photocurrent from the sensor is provided to an I-V interface  18 . The IR and red voltages are then provided to a sigma-delta interface  20  incorporating the present invention. The output of sigma-delta interface  20  is provided to a microcontroller  22 . Microcontroller  22  includes flash memory for a program, and SRAM memory for data. The processor also includes a microprocessor chip  24  connected to a flash memory  26 . Finally, a clock  28  is used and an interface  30  to a digital calibration in the sensor  10  is provided. A separate host  32  receives the processed information, as well as receiving an analog signal on a line  34  for providing an analog display. 
     LED Drive Circuit 
       FIG. 2  is a circuit diagram of the LED drive circuit according to an embodiment of the invention, which forms a portion of LED drive interface  16  of  FIG. 1 . A voltage regulator  36  provides a voltage separate from the voltage supply for the overall oximeter circuitry. The output is provided as a 4.5 volt signal on line  38 , with the level being set by the feedback resistor divider of resistors R 89  and R 90 . The voltage on line  38  is provided to a FET transistor Q 11  to an inductor L 6 . The current through inductor L 6  is provided by a switch  40  to one of capacitors C 65  and C 66 , which store charge for the red and IR LEDs, respectively. A red/IR control signal on line  42  selects the switch position under control of the oximeter processor. A control signal LED PWM gate on line  44  controls the switching of transistor switch Q 11 . 
     Once the capacitors are charged up, the control signal on line  44  turns off switch Q 11  and current is provided from either capacitor C 65  or C 66 , through switch  40  and inductor L 6  to either the red anode line  46  or the IR anode line  48  by way of transistors Q 5  and Q 6 , respectively. A signal “red gate” turns on transistor Q 5 , while its inverse, “/red gate” turns off transistor Q 7 . This provides current through the red anode line  46  to the back to back LEDs  50 , with the current returning through the IR anode to transistor Q 8  and through resistor R 10  to ground. Transistor Q 8  is turned on by the signal “/IR gate” while the inverse of this signal, “IR gate” turns off transistor Q 6 . The signals are reversed when the IR anode is to be driven, with the “IR gate” and “red gate” signals, and their inverses, changing state, so that current is provided through transistor Q 6  to IR anode  48  and returns through red anode  46  and through transistor Q 7  to resistor R 10  and ground. The “LED current sense” signal is read for calibration purposes not relevant to the present invention. 
     When the current from the capacitor C 65  or C 66  is provided through inductor L 6  to the LEDs, and that current is switched off at the desired time, transistor Q 11  is turned on so that the remaining current during the transition can be dumped into capacitor C 64 . This addresses the fact that the FET transistor switching is not instantaneous. Subsequently, C 64  will dump its current through Q 11  and inductor L 6  into the capacitors when they are recharged. 
     Resistor R 38  and capacitor C 67  are connected in parallel to inductor L 6  to protect against signal spikes, and provide a smooth transition. Connected to inductor L 6  is a sampling circuit with a switch  52  controlled by an LED sample hold signal on line  54  to sample the signals and provide them through an amplifier  56  to a “LED current” signal on line  58  which is read by the processor. Operational amplifier  56  operates between 4.5 volts and ground. Thus, a voltage reference slightly above ground, of 0.2 volts, is provided as a voltage reference on pin  3 . An integrating capacitor C 68  is provided in parallel to amplifier  56 . A switch  60  responds to a “clear LED sample” signal to operate the switch to short out the capacitor between samples. 
     The sample and hold circuit measures the voltage at node T 18 , between capacitor C 69  and inductor L 6 , to determine the current. Capacitor C 69  is 1/1000 of the value of capacitors C 65  and C 66 . Thus, a proportional current is provided through C 69 , which is injected through switch  52  to integrating capacitor C 68  to provide a voltage which can be measured at the output of amplifier  56  on line  58 . The voltage measured by the processor on line  58  is used as a feedback, with the processor varying the width of the pulse delivered to transistor Q 11  to selectively vary the amount of energy that&#39;s delivered to the capacitors  65  and  66 , and then is eventually discharged to the LEDs  50 . A PI (Proportional Integral) loop inside the processor then controls the PWM signal at Q 11 . This allows precise control of the LED intensity, allowing it to be maximized, if desired, without exceeding the desired limits (to avoid burning the patent, etc.). 
     The lower left of the diagram shows a “4.5 v LED disable” signal which is used by the microprocessor to turn off the voltage regulator  36  in certain instances. For example, diagnostics looking for shorts in a new sensor plugged in will turn off the voltage regulator if there is a problem with the LED line. 
       FIG. 3  illustrates processor  22 , from  FIG. 1 , connected to programmed logic  62 , which is in the LED drive interface  16  in  FIG. 1 . Programmed logic  62  provides the different control signals used by the circuit of  FIG. 2  in response to basic timing signals from the processor of a clock, a sync pulse, and a pulse width signal. 
     Thus, the present invention provides an improvement over the circuit shown in U.S. Pat. No. 6,226,539 by moving the switch position between the inductor and the capacitors to eliminate the need for two inductors. This not only reduces the part count, requiring only one inductor instead of two, but also provides better matching between the red and IR drive currents since both use the same inductor. 
     In another aspect of the invention, the LED drive circuit&#39;s susceptibility to magnetic interference is reduced. This magnetic interference can distort the detected pleth waveform. This is minimized by using magnetic flux canceling in the inductor. In one embodiment, this is a toroidal inductor as shown in  FIG. 4 . The toroidal inductor has a geometric symmetry in its magnetic flux. Another embodiment uses a dual core closed bobbin shielded inductor, such as shown in  FIGS. 5 and 6 . The windings of both cores in series are used to cancel the effect of an external magnetic field. These magnetic flux canceling inductors can be used either in the circuit of  FIG. 2 , or could be used in the dual inductor embodiment of the prior art.  FIG. 5  shows the dual core inductor with a bobbin  70  in a cylinder  72 . The wires are wound through gaps  76 , as shown in  FIG. 6 . A first winding  78  is clockwise, while a second winding  80  is counterclockwise. A top view  82  is also shown. Ideally, the combined inductance in one embodiment is 680 uH. 
     The invention as illustrated in the embodiment of  FIG. 2  enables the multiplexing of current, through an H-bridge topology, to back-to-back LEDs. Alternately, a different number of loads could be provided. The present invention is scalable to N-loads. The present invention is scalable to N-loads. The present invention provides significant efficiencies through reduction of support components, choice of components, and the properties of “loss-less” capacitor and inductor storage devices. The circuit of  FIG. 2  can handle a range of forward voltage drops across the LEDs. The voltage provided varies automatically in accordance with the LED voltage drop, and does not put out more energy than it needs to. 
     The circuit is dynamically controlled through a PI loop in the processor, with current feedback being provided by the capacitive current divider from each storage capacitor (C 65  and C 66 ), which provides isolation. The feedback can be calibrated with a traditional in-line sense resistor, R 10 . In addition, this technique allows adjustment of the peak current for optimal signal-to-noise during the sampling period. 
     The addition of the upstream linear regulator  36  enhances power supply rejection capability, while the PI loop provides additional power supply insensitivity (to draft, P-P, surge, etc.). 
     As will be appreciated by those with skill in the art, the present invention can be embodied in other specific forms without department from the essential characteristics thereof. For example, instead of two drive lines, three drive lines could be provided by adding another leg with FET transistor switches connected to the inductor. Additionally, this could be scalable to more than three legs connected in parallel, similar to the leg of Q 6 , Q 8 , and the leg of Q 5 , Q 7 . Accordingly, the foregoing description is intended to be illustrative, but not limiting, of the scope of the invention which is set forth in the following claims.