TUTORIAL 4685

How Continuous Positive Airway Pressure (CPAP) Respiratory Ventilation Systems Function

By:  John Gosson

Abstract: This application note introduces how a continuous positive airway pressure (CPAP) ventilation system operates. The main subfunctions of CPAP respiratory ventilation include air-hose-environment sensing, compressor motor-drive feedback, motor-drive excitation, and a communication interface to a technician/doctor. These subfunctions are explained and a functional block diagram showing system components is also detailed.

Continuous positive airway pressure (CPAP) is a type of respiratory ventilation originally developed for combating sleep apnea, which remains its primary use. It is also useful in providing ventilation for newborns and anyone suffering respiratory failure.

As airway muscles relax during sleep, the airway can become partially obstructed. This can lead to lower blood oxygenation and cause awakening or arousal from deep sleep. Maintaining positive air pressure by supplying a continuous source of compressed air, the face mask forms a seal to the face. It is only this air pressure that maintains the open airway, and not the actual movement of air. A sleep physician usually determines the required air pressure after completing a sleep study.

Pressure sensors supply feedback of the applied air pressure in the mask/delivery hose to the microprocessor controller. This microprocessor controller manages the motor-drive stage of a compressor to maintain the correct fan velocity necessary to generate the required air pressure.

Continuous positive airway pressure (CPAP) respiratory ventilation system.

The main subfunctions that the system is required to monitor and control can be divided as follows:
  1. Air-hose-environment sensing This covers air pressure, but may also include air temperature, humidity, and flow rate.

  2. Compressor motor-drive feedback Similar to all motor-drive systems, some feedback must be provided to maintain torque and/or velocity control. Typically, phase currents or shunt current and rotor feedback must be provided.

  3. Motor-drive excitation This is the generation of the waveforms necessary to both induce current in the electric motor and produce the torque that causes motion.

  4. Communication interface to technician/doctor This requires the ability to display information as well as input commands and controls from the medical team. This can include LCD drivers and touch-screen controllers, as well as a means for audio communication alerts, such as beeps and tones.
Functional block diagram of a CPAP system. For a list of Maxim's recommended solutions for CPAP designs, please go to: www.maximintegrated.com/CPAP.
Functional block diagram of a CPAP system. For a list of Maxim's recommended solutions for CPAP designs, please go to: www.maximintegrated.com/CPAP.

Given the time and expense required to achieve FDA approval, manufacturers must select a supplier with a customer-oriented discontinuance policy to ensure that system components will be available for many years.

Medical customers rely on Maxim products because, over the years, we have carefully avoided discontinuing parts. We realize how devastating product discontinuance can be to a customer, so we work diligently to transfer some products to newer production lines, create wafer buffers, allow last-time purchases, or develop upgrade devices. Very few Maxim parts have ever been discontinued while demand still existed. Maxim's Discontinuance Policy is one of the most flexible among our peer supplier companies.

Related Parts
DS600 ±0.5°C Accurate Analog-Output Temperature Sensor Free Samples  
DS7505 High-Precision Digital Thermometer and Thermostat Free Samples  
DS75LV Digital Thermometer and Thermostat Free Samples  
MAX11800 Low-Power, Ultra-Small Resistive Touch-Screen Controllers with I²C/SPI Interface  
MAX11801 Low-Power, Ultra-Small Resistive Touch-Screen Controllers with I²C/SPI Interface  
MAX11802 Low-Power, Ultra-Small Resistive Touch-Screen Controllers with I²C/SPI Interface  
MAX11803 Low-Power, Ultra-Small Resistive Touch-Screen Controllers with I²C/SPI Interface  
MAX11811 TacTouch™, Low-Power, Ultra-Small, Resistive Touch-Screen Controller with Haptic Driver  
MAX1228 12-Bit 300ksps ADCs with FIFO, Temp Sensor, Internal Reference Free Samples  
MAX1229 12-Bit 300ksps ADCs with FIFO, Temp Sensor, Internal Reference Free Samples  
MAX15036 2.2MHz, 3A Buck or Boost Converters with an Integrated High-Side Switch Free Samples  
MAX16056 125nA nanoPower Supervisory Circuits with Capacitor-Adjustable Reset and Watchdog Timeouts Free Samples  
MAX16814 Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller Free Samples  
MAX16826 Programmable, Four-String HB LED Driver with Output-Voltage Optimization and Fault Detection Free Samples  
MAX16838 Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller Free Samples  
MAX3232E ±15kV ESD-Protected, Down to 10nA, 3.0V to 5.5V, Up to 1Mbps, True RS-232 Transceivers Free Samples  
MAX4238 Ultra-Low Offset/Drift, Low-Noise, Precision SOT23 Amplifiers Free Samples  
MAX5064 125V/2A, High-Speed, Half-Bridge MOSFET Drivers Free Samples  
MAX5556 Low-Cost Stereo Audio DAC  
MAX6034 Precision, Micropower, Low-Dropout, SC70 Series Voltage Reference Free Samples  
MAX6129 Ultra-Low-Power Series Voltage Reference Free Samples  
MAX8902A Low-Noise 500mA LDO Regulators in a 2mm x 2mm TDFN Package Free Samples  
MAX9617 High-Efficiency, 1.5MHz Op Amps with RRIO Free Samples  
MAX9634 nanoPower, 4-Bump UCSP/SOT23, Precision Current-Sense Amplifier Free Samples  
MAX9860 16-Bit Mono Audio Voice Codec Free Samples  
MAX9918 -20V to +75V Input Range, Precision Uni-/Bidirectional, Current-Sense Amplifiers Free Samples  
MAX9922 Ultra-Precision, High-Side Current-Sense Amplifiers Free Samples  


Next Steps
EE-Mail Subscribe to EE-Mail and receive automatic notice of new documents in your areas of interest.
Download Download, PDF Format (76kB)  

© May 10, 2010, Maxim Integrated Products, Inc.
The content on this webpage is protected by copyright laws of the United States and of foreign countries. For requests to copy this content, contact us.

APP 4685: May 10, 2010
TUTORIAL 4685, AN4685, AN 4685, APP4685, Appnote4685, Appnote 4685