MCU Based Medical Alarm Reference Design With Supercapacitor Backup

System Description

Medical alarm systems are a required subsystem for most medical devices, especially those used in an intensive care unit (ICU). For patient safety, these medical devices must comply with the requirements established by the International Electrotechnical Commission (IEC). The IEC 60601-1-8 standard details the necessary alarm-related elements for these systems, including a primary alarm, a redundantly powered backup alarm, and a visual alarm indicator. This design provides an IEC 60601-1-8 based medical alarm utilizing the MSPM0G1507 or MSPM0G3507 microcontroller to offer primary alarm, backup alarm, and visual alarm functionality. Figure 2-1 depicts the design block diagram.

System Overview

Block Diagram

The system block diagram illustrates the flow of audio and power signals. A microphone captures audio, which is processed by the MSPM0G150x MCU. The MCU outputs audio waveforms via a 12-bit DAC to an audio amplifier (TPA6211A1) driving a speaker for the primary alarm. Visual alarms are indicated by three LEDs (high, medium, low priority). For backup, a supercapacitor powers a piezo buzzer and the MCU, managed by a TPS61094 charger and a TPL5010 watchdog timer. The backup alarm can be triggered by the MCU, loss of primary power (Vin), or watchdog reset (RSTn).

Primary Alarm Circuit - Current Sensing

The primary alarm circuit uses the TPA6211A1 class AB audio amplifier. Low-side current sensing is implemented using a shunt resistance (R20) that is voltage-amplified by the MSPM0 MCU's internal op-amp with a gain of 32. The MCU's 12-bit ADC digitizes this value to detect speaker connection. During testing, a maximum current of approximately 0.6 A was measured with a high-alarm state, a 4-Ω speaker, and 3.3-V input. A 20-mΩ shunt resistor resulted in approximately 496 ADC steps for 0.6 A. The maximum ADC output voltage swing and step count are calculated as:

Vshunt = Rshunt × Ishunt

VADC = 32 × (Rshunt × Ishunt)

With Rshunt = 20 mΩ and Ishunt_max = 0.6 A, VADC max = 0.384 V.

Max # ADC Steps = (VADC max / VREF) × 4095. With VREF = 3.3 V, Max # ADC Steps = 496.

Microphone Circuit - Coincidence Detection

An optional microphone is included for environmental feedback, such as ambient noise or acoustic feedback from the primary alarm. The microphone circuit connects to the MSPM0's configured op-amp for signal conditioning.

Backup Alarm Circuit

The backup alarm circuit triggers the backup alarm if the primary power is lost or if the MCU triggers it. The TPL5010 external watchdog timer resets the MCU if the system stops responding. The MCU can override the backup alarm via a secondary alarm override signal to the NAND gate U4, allowing the MCU to disable the alarm after a required duration (e.g., 3 minutes). Triggers for the backup alarm include:

• MCU_Secondary_Alarm_trigger: If the MCU detects improper functioning of another component.
• RSTn: If the watchdog timer does not receive a response from the MCU.
• Vin: When the main power drops below a logic threshold.

Supercapacitor Charging Circuit

The TPS61094 is used for supercapacitor charging and backup power. The circuit schematic shows the bidirectional buck/boost converter and supercapacitor charging path. The TPS61094 requires a minimum supercapacitor voltage of 0.7 V to operate. The usable supercapacitor energy storage (EJoules) for supercapacitors rated for less than 5 V is calculated as:

EJoules = ½ × C × V² - ½ × C × (0.7V)² = ½ × C × V² - 0.245 × C

For supercapacitors rated for 5 V or greater, the usable energy storage is:

EJoules = ½ × C × (5V)² - 0.245 × C = 12.255 × C

To enter auto buck mode and charge the supercapacitor, VIN must be 100 mV greater than the target VOUT. The target VOUT was set to 3 V by connecting a 3-kΩ resistor to the OSEL pin. Boost operation begins when VIN drops below the target voltage.

Operation Modes

Software Flow Chart

The software flow begins with device initialization, followed by declaration of audio file data and alarm delay timings. The MCU then configures the DAC, volume, and alarm state. An interrupt service routine (ISR) for the DAC is entered. The system then enters the selected alarm state and enters a playback loop. After outputting the next DAC value, it checks if pulse playback is complete. If not, it waits for the next interrupt. If complete, it introduces a delay between alarm pulses before repeating the loop. The system also monitors timer interrupts and can trigger a watchdog reset if it stops responding.

Highlighted Products

MSPM0G150x

The MSPM0G150x microcontrollers (MCUs) are part of the MSP family, featuring an enhanced Arm Cortex-M0+ core operating up to 80-MHz. They offer high-performance analog peripheral integration, extended temperature ranges (-40°C to 125°C), and operate with supply voltages from 1.62 V to 3.6 V. These MCUs include up to 128KB embedded flash memory with ECC, 32KB SRAM with ECC, a memory protection unit, DMA, math accelerator, multiple ADCs, DACs, op-amps, and various digital peripherals like timers and communication interfaces (UART, I2C, SPI). The MSPM0 MCU platform combines the Arm Cortex-M0+ with an ultra-low-power system architecture.

TPS61094

The TPS61094 is a 60-nA IQ boost converter with supercapacitor management, designed for smart meter and supercapacitor backup power applications. It supports a wide input voltage range and an output voltage up to 5.5 V. In buck mode, it charges the supercapacitor, with programmable charging current and termination voltage. In boost mode, it regulates the output voltage from the supercapacitor. It offers true shutdown and forced bypass modes for low quiescent current.

TPA6211A1

The TPA6211A1 is a 3.1-W mono fully-differential amplifier designed to drive a speaker with at least 3-Ω impedance, consuming minimal PCB area. It operates from 2.5 V to 5.5 V and draws only 4 mA of quiescent supply current. Key features include high supply voltage rejection, improved RF rectification immunity, and a fast start-up time, making it suitable for applications like PDAs and smartphones.

Hardware, Software, Testing Requirements, and Test Results

Hardware Requirements

The required test equipment includes a DC Power Supply (3.3 V, 1 A), a Speaker (4 Ω), an SPI Flash Programmer, and an MSPM0 Programmer (e.g., LaunchPad™ or XDS110).

Software Requirements

Software Overview

Programming MSPM0 MCU

To program the MCU, connect the MSPM0 programmers GND, NRST, SWDIO, and SWCLK pins to the J3 connector. Connect an external 3.3-V DC power supply to the VIN and GND connections on the alarm board. Program the MCU using the Code Composer Studio™ integrated development environment (IDE).

Programming External SPI Flash

An SPI flash programmer is required to write audio to the external flash. Connect the programmer to the J2 connector and ensure it operates at 3.3 V before flashing.

Test Setup

The TIDA-010264 board connections for testing are as follows:

Test Results

Primary Alarm Waveforms

Figure 3-1 shows the waveform measured at the output of the MSPM0 12-bit DAC during a high-alarm state, detailing fundamental frequency, rise time, fall time, and pulse spacing to meet IEC 60601-1-8 requirements. Figure 3-2 shows the corresponding high-priority alarm waveform measured from the speaker terminals.

Custom Audio Waveform

Figure 3-3 displays an example custom audio waveform measured from the speaker terminals.

Primary Alarm Harmonic Testing

Figure 3-4 illustrates the harmonic content of the high alarm state. To comply with IEC 60601-1-8, at least 4 harmonics must be within +/- 15 dB of the fundamental frequency's amplitude. This test shows eight harmonics within the required range.

Coincidence Detection

The coincidence detection circuit was tested by measuring the MSPM0G150x internal op-amp output. Figure 3-5 shows the high alarm state waveform via the TPA6211A1 current consumption. A faulty speaker (e.g., disconnected) results in a significantly reduced current waveform. Figure 3-6 shows the analog signal output of the microphones during a high alarm state, which can be digitized by the MSPM0's ADC for acoustic measurements and ambient noise level adjustments.

Backup Power Transition

Figure 3-7 illustrates the transition from an external 3.3-V power source to the 3-V supercapacitor backup source, showing a voltage drop of 200.0 mV over 20.55 ms. Figure 3-8 shows the transition back from the 3-V supercapacitor backup mode to the external 3.3-V power source, with a voltage increase of 240.0 mV over 20.15 ms.

Alarm Sound Levels and Backup Alarm Runtime

Table 3-3 presents the primary and backup alarm sound levels in dBA measured one meter away, along with the alarm runtime from the supercapacitor.

Design and Documentation Support

Design Files

Schematics

To download the schematics, see the design files at TIDA-010264.

BOM

To download the bill of materials (BOM), see the design files at TIDA-010264.

Tools and Software

Tools: Code Composer Studio™ is an integrated development environment (IDE) for TI's microcontrollers and processors, used for developing and debugging embedded applications. It is available for Windows, Linux, and macOS, and can also be used in the cloud via the TI Developer Zone.

Software: TIDA-010264-MSPM0G150x-FW is downloadable firmware that assists in developing a medical alarm design according to the IEC 60601-1-8 standard.

Documentation Support

1. Texas Instruments, MSPM0-Based Medical Alarm Design application brief
2. Texas Instruments, Hardware-Based Smart DAC Medical Alarm Design application brief
3. Texas Instruments, Demystifying Medical Alarm Designs With Smart DACs application brief
4. Texas Instruments, Demystifying medical alarm designs, part 1: IEC60601-1-8 standard requirements TI E2E™ forum
5. Texas Instruments, Demystifying medical alarm design, part 2: Design inputs and existing techniques TI E2E™ forum
6. Texas Instruments, TPS61094 60-nA Quiescent Current Boost Converter With Supercap Management data sheet
7. Texas Instruments, MSPM0G150x Mixed-Signal Microcontrollers data sheet
8. Texas Instruments, TPA6211A1 3.1-W Mono Fully Differential Audio Power Amplifier data sheet
9. Texas Instruments, TPL5010 Nano-Power System Timer With Watchdog Function data sheet

Support Resources

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