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 Integrated’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.
Figure 1. Typical Wearable Biosensing Subsystem.
This document offers descriptions of pre-validated power supply circuits for use with biosensing devices intended remote patient vital sign monitor applications. In addition, each power supply circuit example is complemented with a validation checklist and troubleshooting guide to aide circuit designers if needed.
All circuits have been designed and validated to ensure the SNR performance of each Maxim Integrated biosensing device (Refer to “How Power Supply Noise Affects Sensing Data” section).
Who Should Read This?
Designers developing remote patient vital sign monitors and consumable diagnostic devices using Maxim Integrated biosensing AFEs:
|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
|Optical Pulse Oximeter and Heart Rate Biosensor (MAX86140 Single Channel; MAX86141 Dual Channel)|
|MAX86176ENX+||ECG, Optical Pulse Oximeter, and Heart Rate 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 analog front ends (AFE) is the first step towards this goal (Maxim Integrated mgineer Blog: Designing Accurate, Wearable Optical Heart Rate Monitors; August 2017; Easson, Craig). 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
- Use a comprehensive checklist to validate their implementation
- View test data expected from a successful implementation
- Implement system integration guidelines
- Utilize troubleshooting instructions to address implementation 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
- Maintain 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 required 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
- LP401xxx 3.7V xxxmAh Secondary (Rechargeable) Multi-Cell LiPo 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 Integrated’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 for reasons of 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 (Switch-Mode Power Supplies or SMPS) presented in this document identify all battery types used.
Switch-Mode Power Supply (DC-DC Converter) Circuits
The following chapters 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
- Select test data plots highlightingsec typical operating characteristics
Both discrete and integrated switch-mode power supply options are offered to help designers accommodate their specific PCB layout requirements.