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21 Apr 2020, 11:50

Ventilator design solutions from Microchip

In the next part of our series featuring supplier solutions for the design of medical ventilator products we are looking at the offering from Microchip. In this article we will consider the main building blocks of a ventilator design, delving deeper into the power and sensing sub-systems for which Microchip have many well-suited products to complete your design.

Microchip Ventilator Reference Design - Block Diagram

Sensing – Analog Frontend

Selecting a sensor can often be a tricky task, you must balance the operating conditions of the device, required accuracy and precision, size and in most cases - cost. You may choose a fully integrated sensor with digital output via an I2C or SPI interface, which is very convenient if a suitable device exists however this can often limit your options. An alternative is to opt for an analog sensor and design a custom sensor conditioning circuit before interfacing to an ADC, to better meet the needs of your application. If you are not sure how to get started with this approach Microchip provide some useful Application Notes, a good starting point isAN990 Analog Sensor Conditioning Circuits - An Overview.

Instrumentation amplifiers are particularly useful in sensor conditioning circuits as they offer a very high input impedance and common mode rejection, this means they are robust to interference that may be encountered when acquiring a signal from a remote sensor transducer. The Microchip MCP6N11 (755-9546) is a popular choice and available as a development kit (749-6451) for you to experiment with before completing your final design.

Digital potentiometers are a common component of many sensor conditioning circuits to enable offsets and gains to be adjusted whilst in operation and provide capability for automated in-factory calibration. The MCP4561 Digital Potentiometer is available in multiple configurations including 10kΩ(687-8449) and 50kΩ (817-3588) .

Pressure is one of the most import factors to control in a ventilator design – poor control of pressure applied to a patient’s lungs can in fact cause them significant harm. Microchip provide an application note specifically guidinganalog interface approaches for pressure sensors.

It is sometimes useful to monitor the current flowing through a part of a circuit, for example in power management situations to protect against over-current, and in motor control. Typically, a sense resistor (or ‘shunt’) is placed in series with the load and the potential across this resistor measured. To minimise losses in the circuit a very low impedance resistor is used, which whilst minimising power consumption does mean the potential measured across the resistor is small and therefore a high level of amplification is needed. Microchip a range of Current Sense Amplifiers optimised for this specific purpose, the MCP6C02T range is available in 20V/V (193-5515) , 50 V/V (192-5517) , 100V/V (193-5520) variants in VDFN package, 100V/V is also available as a SOT-23 (187-6267) device.

Power

To ensure a stable supply of power to the ventilator it is common to include provision for battery power, even if it is to provide only temporary power whilst a device is moved to a new location to be resilient to local power failures. Li-Ion and Li-Po cells are the most commonly used rechargeable cell chemistry’s in modern devices, however, can be very dangerous if mistreated, and therefore a specialised charge controller is often desired. Microchip offer a popular device used in many designs, MCP73831 Li-Ion Li-Po battery charge controller, it is available in SOT-23 and DFN packages with a range of battery charge regulation voltages from 4.2-4.5V. For example, the SOT-23 4.2V (738-6360) , 4.4V (738-6367) devices. For space-constrained designs a DFN package is also available for example the 4.2V charge regulation DFN device MCP73831-2ACI/MC (823-1035) .

In medical applications it is often preferable to use a more stable battery cell chemistry such as Lithium Iron Phosphate (LiFePO4), these cells require different charge parameters and therefore charge controllers need to be optimised to this application. The Microchip MCP73123 (687-8679) is designed specifically for Lithium Iron Phosphate cells and provides integrated input overvoltage protection.

You may wish to operate the device whilst charging, in which scenario it is necessary to consider the power requirements of the device in operation and charge current of the cell with appropriate priority given to system power, whilst ensuring that external power supply or battery charge is not overloaded. The Microchip MCP73871 offers an integrated solution for load sharing and battery management, it is available on an evaluation board (687-2700) to speed up your development time and demonstrated with Voltage Proportional Current Charge functionality (681-0875) . The device is available in its standard form (669-6187) , with safety timer (669-6196) and low battery status indicator (669-6199) .

To maximise the usable capacity of rechargeable cells it is often necessary to use a boost converter to achieve the required voltage for system operation. The Microchip MCP16251 Boost Converter (177-3216) offers a flexible solution with configurable output voltage from 1.8-5.5V with minimum input voltage down to 0.35V, this device is available on an evaluation board (798-3081) and with under-voltage lockout (UVLO) circuitry for evaluation (146-3391) . UVLO ensures that the converter is shut down after the cell voltage reaches a low level and will not restart until it has risen above the hysteresis.

It is important that power-on of a system is controlled and only attempted when sufficient power is available. Where the input voltage is not considered in the power-up sequence of a device there is the risk of brown-out or uncontrolled shutdowns occurring during operation. Voltage supervisor devices ensure that power is only available to the system from the input (battery or external) if the voltage is sufficient for normal operation of the device. The Microchip MCP131x series offers a wide range of devices for different trip voltages and power-on/reset strategies, i.e push-pull, open-drain, active-high and active-low configurations. Full details of available options are available in the datasheets. A common variant is MCP1316T-29LE/OT featuring a 2.9V trip voltage (738-6285) and MCP1316T-31RE/OT in 3.1V trip-voltage (188-0215) .

Motor

An essential factor in ventilator design is motor control – effective motor control ensures that the pressure applied to the lungs and rate of inhalation and exhalation is optimised for effective respiration. A number of techniques can be used to control a brushless DC motor powering a fan, the MCP8024 BLDC Motor Gate Driver with Power Module (799-0269) integrates three half-bridge drivers to drive external NMOS/NMOS transistor pairs configured to drive a 3-phase brushless DC motor. This device has the added functionality of providing up to 20mA of regulated power for powering a companion MCU.

Wireless Connectivity

You may wish to consider wireless connectivity for your ventilator design, either to facilitate patient monitoring or for configuration and control of the machine. The RN4870 series of Bluetooth Low Energy modules offer a convenient solution to integrate Bluetooth Low Energy (BLE) connectivity into your design. These devices can interface with the device microcontroller via UART interface and act as a direct wireline replacement for a traditional UART interface. Provided as a pre-certified Bluetooth 5.0 module (123-8535) , the complexities of wireless certification and Bluetooth software development are greatly reduced. The RN4870 is also available as a daughter board (123-8526) for devices supporting the PICtail Plus interface such as the Explorer series, or as a MikroElecktronika Click Board (168-3002) .

Are you involved in medical device design and manufacture?

Let us know in the comments if you are involved in the response to the COVID-19 pandemic in medical ventilator design.

Lead Inspiration Engineer @ RS Components and DesignSpark

21 Apr 2020, 11:50