LED forward voltage estimation in pulse oximeter

• US 20050187450A1
• Filed: 02/25/2004
• Published: 08/25/2005
• Est. Priority Date: 02/25/2004
• Status: Active Grant

• Current Assignee: Covidien LP (Medtronic Plc)
• Original Assignee: Nellcor Puritan Bennett Incorporated (Medtronic Plc)
• Inventors: Petersen, Ethan, Chew, Bradford B., Shea, William
• Granted Patent: US 7,120,480 B2
• Time in Patent Office: Days
• US Class Current: 600/323
• CPC Class Codes: A61B 5/02427 Details of sensor
• CPC Class Codes: A61B 5/14546 for measuring analytes not otherwise provided for, e.g. ions, cytochromes
• CPC Class Codes: A61B 5/14552 Details of sensors specially adapted therefor
• CPC Class Codes: A61B 5/1495 Calibrating or testing of in-vivo probes

Abstract

An apparatus and method for determining if a forward voltage of an LED in a pulse oximeter is within a predetermined range. This is accomplished by measuring the current through the LED, and also by knowing the duty cycle of the pulse width modulator (PWM) drive signal to the LED.

Claims

1. A pulse oximeter comprising:

at least one light emitting diode (LED) drive circuit;

a current measuring circuit for measuring a current through said LED;

a controller for generating a pulse width modulator (PWM) drive signal to said LED; and

a processor, coupled to said controller and to said current measuring circuit, configured to determine if a forward voltage of said LED is within a predetermined range using a measurement of said current and said PWM signal.

2. The pulse oximeter of claim 1 wherein said processor is configured to provide an error signal if said forward voltage is outside said range.

3. The pulse oximeter of claim 1 wherein said processor is configured to compare said PWM drive signal to said measurement of said current to determine if the ratio is within an acceptable voltage range.

4. The pulse oximeter of claim 1 wherein said processor includes a proportional integral (PI) loop which generates said PWM signal from a current error signal reflecting a difference between said measurement of said current and a desired current.

5. A pulse oximeter comprising:

at least one light emitting diode (LED) drive circuit;

a current measuring circuit for measuring a current through said LED;

a controller for generating a pulse width modulator (PWM) drive signal to said LED;

a processor, coupled to said controller and to said current measuring circuit, configured to determine if a forward voltage of said LED is within a predetermined voltage range using a measurement of said current and said PWM signal, by comparing said PWM drive signal to said measurement of said current to determine if the ratio is within an acceptable voltage range;

wherein said processor is configured to provide an error signal if said forward voltage is outside said voltage range; and

wherein said processor includes a proportional integral (PI) loop which generates said PWM signal from a current error signal reflecting a difference between said measurement of said current and a desired current.

6. A method for operating a pulse oximeter comprising:

providing a current using at least one light emitting diode (LED) drive circuit;

measuring a current through said LED;

generating a pulse width modulator (PWM) drive signal to said LED; and

determining if a forward voltage of said LED is within a predetermined range using a measurement of said current and said PWM signal.

7. The method of claim 6 further comprising providing an error signal if said forward voltage is outside said range.

8. The method of claim 6 further comprising comparing said PWM drive signal to said measurement of said current to determine if the ratio is within an acceptable voltage range.

9. The method of claim 6 further comprising using a proportional integral (PI) loop to generate said PWM signal from a current error signal reflecting a difference between said measurement of said current and a desired current.

10. A method for operating a pulse oximeter comprising:

providing a current using at least one light emitting diode (LED) drive circuit;

measuring a current through said LED;

generating a pulse width modulator (PWM) drive signal to said LED;

determining if a forward voltage of said LED is within a predetermined voltage range using a measurement of said current and said PWM signal, by comparing said PWM drive signal to said measurement of said current to determine if the ratio is within an acceptable voltage range;

providing an error signal if said forward voltage is outside said voltage range; and

using a proportional integral (PI) loop to generate said PWM signal from a current error signal reflecting a difference between said measurement of said current and a desired current.