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Power Supply Subsystems for Remote Patient Vital Sign Monitors

This application note is part of a series providing guidance for its readers to implement and validate a proven switch-mode power supply circuit for use in remote vital sign monitoring devices using Maxim’s biosensing analog front ends (AFEs). The following section focuses on an overview of the intended audience.

Figure 1 below shows a block diagram where various biosensing AFE devices require multiple power channels to power the digital supply, analog supply, and external LED(s) supply.

Typical Wearable Biosensing Subsystem.

Figure 1. Typical Wearable Biosensing Subsystem.

Switching mode power supply offers high efficiency and compact footprint optimal for remote patient monitors. However, designs must ensure the signal-to-noise ratio (SNR) performance of each Maxim biosensing device is not adversely affected by high frequency switching noise which can be aliased in-band where vital sign signals are sampled to avoid performance degradation.

This cookbook offers pre-validated power supply circuits for use with biosensing devices. In addition, each power supply circuit example is complemented with a validation checklist and troubleshooting guide to aid circuit designers if needed.

Who Should Read This?

Designers developing remote patient vital sign monitors and consumable diagnostic devices using Maxim Integrated biosensing AFEs:

Part Number Function
MAX30001CWV+
MAX30003CWV+
MAX30004CWV+
ECG, R-to-R, Pace, and BioZ Biosensor
Ultra-Low Power, Single-Channel Integrated Biopotential (ECG, R to R Detection) AFE
Ultra-Low Power, Single-Channel Integrated Biopotential HR Detection AFE
MAX86140ENP+
MAX86141ENP+
Optical Pulse Oximeter and Heart Rate Biosensor (MAX86140 Single Channel; MAX86141 Dual Channel)
MAX86176ENX+ ECG, Optical Pulse Oximeter, and Heart Rate Biosensor
MAX86131CWA+ Electrochemical Biosensor
MAX30208CLB+ ±0.1°C Accurate, I2C Digital Temperature Sensor

Retrieving actionable information from biosensor data requires excellent system signal-to-noise performance. Adopting Maxim AFEs is the first step toward this goal (Designing Accurate, Wearable Optical Heart Rate Monitors). The next step should be complemented with sound power supply design.

This cookbook guides designers to:

  • Select a power supply configuration based on system requirements
  • Use reference circuits and layouts of both discrete and/or integrated designs
  • Adopt a power supply performance test methodology to validate the system over different device use cases and transient loading conditions
  • Utilize a comprehensive checklist to validate their implementation
  • View test data expected from a successful implementation
  • Apply system integration guidelines
  • Follow troubleshooting instructions to address issues

Remote Patient Monitor and Medical Wearable System Configurations

This cookbook is applicable to designs with the following requirements:

  • Small system form factor and low weight
  • Maintains sufficient battery life with low battery weight and cost. Most of the systems are kept in standby or low power state,
  • Very high signal-to-noise performance at sampling rates below 1kHz
  • Uses one or more (primary or secondary) batteries with a nominal voltage ranging from 0.9V to 4.2V. For example:
    • LP401230 3.7V 105mAh Secondary (Rechargeable) Cell LiPo Battery
    • BR2032 3.0V 190mAh Primary (Non-Rechargeable) Cell Battery
    • CR2032 3.0V 235mAh Primary (Non-rechargeable) Cell Li Battery
  • Includes devices which require one or more voltage rails, for example:
    • 1.8V for digital devices (VDIG) with fast transitions and high operating current
    • 1.8V analog supply (VANA) where power supply noise affects sensor data integrity
    • 5V supply for LED currents (VLED) in optical systems

Maxim’s rechargeable power system configuration is designed to work with input voltages ranging from 3.0V to 4.2V, typical of Li-Ion or LiPo rechargeable batteries. Three outputs are generated: two 1.8V supplies (VANA & VDIG) one 5V supply (VLED). Figure 1 shows a block diagram for reference. One supply is a tightly regulated 1.8V digital supply to power the digital sections of a microcontroller where noise is typical with fast transitions of digital signals.

Battery Choices for Wearable Applications

Common wearable battery types can be categorized into two basic groups: primary cell (non-rechargeable) and secondary cell (rechargeable) batteries. Examples of primary cell batteries include alkaline, Li-ion, zinc-air, and silver-oxide varieties, where secondary cell battery examples include lithium Ion and lithium polymer (LiPo or LiPoly) cells. Currently, lithium ion and lithium polymer batteries for wearable applications are favored due to size, weight, rechargeability, energy density, and eco-friendliness.

The designer should be aware that each battery type will have its own electrical characteristics (e.g., voltage output level, energy storage level, charge/discharge behavior, etc.). Thus, an appropriate switch-mode power supply (SMPS) circuit will need to be implemented for each battery type utilized. In addition, as newer battery types are deployed, the engineer will need to evaluate, characterize, and possibly re-design their power supply circuits accordingly.

The DC-DC converter circuits (or SMPS) presented in this document identify all battery types used.

Switch-Mode Power Supply (DC-DC Converter) Circuits

The below chapter and subsequent ones will highlight operational details of known good reference designs including:

  • Circuit description including web links to applicable design files (schematic/BOM/layout)
  • Validation checklist to confirm the implemented circuit function
  • Selecting test data plots highlighting secondary typical operating characteristics

Both discrete and integrated switch-mode power supply options are offered to help designers accommodate their specific PCB layout requirements.

 

Power Supply Subsystem for MAX30001-Based ECG Remote Patient Vital Sign Monitor


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