Power Supply Subsystems for Photoplethysmography Remote Patient Vital Sign Monitors—Part 1

2024-12-18

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Power Supply Subsystems for Photoplethysmography Remote Patient Vital Sign Monitors—Part 2

摘要

This two-part article presents validated switch-mode power circuit designs for remote patient vital sign monitoring applications, including biosensors with excellent system signal-to-noise performance. “Power Supply Subsystems for Photoplethysmography Remote Patient Vital Sign Monitors—Part 1” shows a discrete solution for best performance and “Power Supply Subsystems for Photoplethysmography Remote Patient Vital Sign Monitors—Part 2” features an integrated solution for space-constrained applications.

➤ Learn how to select a power supply configuration based on photoplethysmogram (PPG) system requirements.

➤ Review the implementation of switch-mode power supply reference circuits for both discrete (Part 1) and integrated designs (Part 2).

➤ See a power supply performance test methodology that validates the system over different device use cases and transient loading conditions.

➤ Get a checklist to validate the implementation.

➤ Gain troubleshooting knowledge to address implementation issues.

Introduction

This two-part article presents prevalidated power supply circuit designs for PPG remote patient vital sign monitoring applications, including biosensors with excellent system signal-to-noise performance. A PPG device can be implemented to measure blood volume changes from which vital sign information such as blood oxygen levels and heart rates are derived. In Part 1, we provide discrete power supply circuit design solutions for best performance using the MAX86171 optical pulse oximeter and heart-rate sensor analog front end (AFE). In Part 2, we will show an integrated solution for space-constrained applications.

Switch-mode power supplies (also referred to as SMPS or DC-to-DC converters) are commonly used in wearable medical and healthcare applications for reasons such as size considerations and power efficiency. Designers can use these power supplies to create battery-operated products that achieve longer lifetimes. Unfortunately, designers are still left to select the appropriate SMPS device followed by creating a suitable circuit board layout that preserves the performance of a biosensing device in their system.

To simplify and speed the process, Analog Devices offers power supply subsystem circuit designs that have been prevalidated (that is, designed, built, and tested) to ensure the signal-to-noise ratio (SNR) performance of each biosensing AFE device. This article provides details for the power supply circuits with each example being complemented with a validation checklist and troubleshooting guide to aid circuit designers if needed. Figure 1 shows a standard power block diagram encountered in many remote-patient monitoring applications. Table 1 reveals the design limits, and Table 2 shows the design considerations for discrete and integrated solutions.

Figure 1. A block diagram of a typical PPG remote patient vital sign monitor.

Table 1. Design Limits
Input		Output (V_DIG, V_ANA, V_LED)		Noise, RTO
V_IMIN	V_IMAX	V_OMIN	V_OMAX	V p-p_(max)
3.0 V¹	4.2 V¹	1.6 V	2.0 V	30 mV p-p
2.0 V²	3.4 V²	1.6 V	2.0 V	30 mV p-p
		4.7 V	5.3 V	20 mV p-p

Note:
¹Secondary cell battery (LiPo)
²Primary cell battery (Li coin cell)

Table 2. Design Configuration
Design Configuration	Battery Implementation	Board Area Layout Considerations
Discrete	Primary (coin cell) Secondary (Li and LiPo)	Implements separate discrete circuits
Integrated	Secondary (Li and LiPo)	Uses single integrated circuit for minimal board area requirements Supports secondary batteries only

Discrete Design Description

This DC-to-DC converter design regulates the three-output power supply rails for use in a remote patient vital sign monitor subsystem. The circuit provides proper line and load regulation while maintaining low output noise levels to preserve biosensing SNR performance, which is powered by a rechargeable lithium polymer (LiPo) battery or a primary Li cell battery. Figure 2 shows the PPG subsystem using discrete power supply devices and Table 3 lists the main components.

Figure 2. A block diagram of a PPG subsystem using discrete power supply devices.

Table 3. Key Components
Designator	Component	Description
U1	DC-to-DC converter	Power converter device (MAX38640A and MAX20343H)
L1	2.2 μH inductor	Low equivalent series resistance (ESR) inductor (energy) storage component¹
C1	22 μF capacitor	Low ESR capacitor (energy) storage component¹

¹L1 and C1 are specially selected passive components that are critical to the performance of the DC-to-DC converter (also known as, switch-mode power supply).

1.8 V SMPS Circuit Using a Nanopower Buck Converter

The following circuit based on the MAX38640A nanopower buck converter (Figure 3) shows the typical input and output power supply level to properly operate the SMPS device in remote patient vital sign monitor applications. As shown in Figure 3, a digital multimeter (DMM) can be used to probe the input and output ports to validate the supply voltage levels. The power supply output levels can vary due to various factors such as:

➤ Discharging battery

➤ Changing loads (that is, device mode changes, devices waking up from sleep mode, etc. )

Figure 3. A 1.8 V_DC MAX38640A SMPS circuit for remote patient vital sign monitoring applications

1.8 V SMPS Circuit Validation Checklist

The circuit validation checklist (Table 4) is intended to help designers with their electrical bench checkout of the 1.8 V SMPS circuit post circuit board assembly (see Figure 4). This checklist can also be used as a template for product testing.

Figure 4. Tools for troubleshooting the MAX38640A SMPS circuit.

Table 4 can be used as a checklist to validate the operation of the analog or digital 1.8 V SMPS circuit using the MAX38640A device while connected to a biosensing circuit load.

Table 4. MAX38640A Validation Checklist
Step	Action	Procedure	Measurement	Need Help?
1	Check the input DC power supply	Measure voltage across the battery	Reading range:	For troubleshooting instructions, see below
	LP401230 LiPo batt		3.0 V to 4.2 V
	CR2032 Li coin batt		2.0 V to 3.4 V
2	Check the input DC power supply	Measure voltage across C_IN	Reading range:
	LP401230 LiPo batt		3.0 V to 4.2 V
	CR2032 Li coin batt		2.0 V to 3.4 V
3	Check V_OUT DC level	Measure voltage across C_OUT	Reading range: 1.71 V to 1.89 V
4		Measure voltage across the load	Reading range: 1.71 V to 1.89 V
5	Check output noise level	Use pigtail 10× single-ended probe or differential active probe	Ripple noise level should be < 20 mV p-p

Troubleshooting the MAX38640A (1.8 V Output) SMPS Circuit

The following circuit troubleshooting instructions will help designers if operational issues arise with the operation of the 1.8 V SMPS circuit (see Figure 5). This guide addresses the most common problems that arise in implementing these switch-mode power supplies.

Troubleshooting the MAX38640A SMPS Circuit

Step 1—Check the input voltage: Using a digital multimeter (DMM) with an internal impedance of 1 MΩ or larger (for example, Fluke 87), measure the voltage across at the input to the MAX38640A device. Be sure to connect the negative black lead to the ground and the positive red lead to the input IN pin of the device. If the input pin is not easily accessible, place the leads across the input capacitor, C_IN.

Use Table 5 to diagnose and fix associated problems:

Table 5. Diagnosing MAX38640A Input Voltage Issues
Input Voltage Reading	Potential Cause	Action	Notes
Zero volts/no reading	Battery uncharged or battery defective	Disconnect the battery and check the voltage; if it reads 0 V, recharge the battery	Replace the battery if it doesn’t charge
	No connection from the battery (IN or GND line)	Disconnect the battery and test for conductivity from the battery connector to the device input	PCB may have an open
	Input capacitor shorted to ground	Disconnect the battery and check for continuity across the capacitor	Bad capacitor; PCB may have short
	EN pin connected to ground	Disconnect the battery and test for conductivity from the EN pin to ground	EN pin needs to be tied high for normal operation
Reading < 3.0 V (LiPo battery) Reading < 2.0 V (Li-Ion battery)	Low battery charge or battery defective	Disconnect the battery and check the voltage; if it reads below 2.8 V, recharge the battery	Replace the battery if it doesn’t charge
3.0 V ≥ reading ≤ 4.2 V (LiPo battery) 2.0 V ≥ reading ≤ 3.4 V (Li-Ion battery)		No action	Input voltage OK, proceed to Step 2
Reading ≥ 4.2 V (LiPo battery) Reading ≥ 3.4 V (Li-Ion battery)	Defective battery	Replace the battery

Step 2—Check the inductor signal waveform: Using an oscilloscope or digital storage scope (DSO), probe the LX pin on the MAX38640A device. If the input pin is not easily accessible, place the probe on the inductor end capacitor.

Note: It is recommended that the oscilloscope and probes used have a minimum bandwidth of 200 MHz.

If the circuit is operating with a light load (that is, less than 50 mA), the waveform should appear as shown in Figure 6.

Figure 6. Oscilloscope screen capture of a typical MAX38640A VLX waveform with light load.

If the circuit is operating with a heavy load, the waveform should be a square wave with minimal ringing on the rise and falling edges as shown in Figure 7.

Figure 7. Oscilloscope screen capture of a switching waveform for the MAX38640A.

The square wave amplitude should be approximately equal to the input battery voltage. The square wave floor voltage should be about 200 mV to 300 mV below ground (for example, –250 mV). The duty cycle is proportional to the output voltage. Thus, a 3.6 V input battery voltage will have an approximate 50% duty cycle when producing an output voltage of 1.8 V. Figure 8 shows the relationship between the duty cycle and the output voltage.

Figure 8. An illustration of the MAX38640A duty cycle vs. output voltage.

Deviations from the ideal square wave can be used to effectively diagnose and fix many problems.

Use Table 6 to diagnose and fix associated problems:

Table 6. Diagnosing MAX38640A Inductor Signal Waveform Issues
Input Waveform	Potential Cause	Action	Notes
Amplitude is not correct	Inductor open; IN pin open EN is open or ground	Disconnect the battery and check all connections with DMM	Repair PCB if needed
The duty cycle is not correct (doesn’t correlate to the output voltage)	R_SEL (768 kΩ) is not the correct value; bad external resistor	Disconnect the battery and check R_SEL with a DMM (R-measurement)	Replace resistor with correct value resistor
	R_SEL pin open (V_O = 2.5 V)	Check the output for 2.5 V; disconnect the battery and test for conductivity from the resistor to R_SEL pin	PCB may have an open
	R_SEL pin shorted to ground (V_O = 0.8 V)	Check the output for 0.8 V; disconnect the battery and measure the resistance across the capacitor	PCB may have short
Waveform distortion Rounded rising edge	Bad inductor connection	Reconnect the inductor; replace the inductor	Bad connection can cause higher line resistance

Step 3A—Check the output DC voltage: Using a DMM with an internal impedance of 1 MΩ or larger (for example, Fluke 87), measure the voltage at the output of the MAX38640A device. Be sure to connect the negative black lead to the ground and the positive red lead to the output OUT pin of the device. If the output pin is not easily accessible, place the leads across the output capacitor, C_OUT.

Use Table 7 to diagnose and fix associated problems:

Table 7. Diagnosing MAX38640A Output DC Voltage Issues
Output Voltage Reading	Potential Cause	Action	Notes
Zero volts/no reading	No connection from SMPS to C_OUT	Disconnect the battery and test for conductivity from output to C_OUT	PCB may have an open
	Output capacitor shorted to ground	Disconnect the battery and check for continuity across the capacitor	PCB may have short
Reading too low (< 1.71 V_DC)	Inductor wrong value; inductor saturated R_SEL(768 kΩ) has wrong value	Disconnect the battery and check for the inductor and/or resistor values
1.71 V ≥ reading ≤ 1.89 V		No action	Operational
Reading too high (> 1.89 V_DC)	R_SEL(768 kΩ) has wrong value	Disconnect the battery and check R_SEL value

Step 3B—Check the output AC voltage: Using an oscilloscope or DSO, now measure the output ripple (AC) by probing the OUT pin on the MAX38640A device. To properly measure the output and minimize RF pickup, it is recommended that 10× pigtail probes be used. Differential active probes can also be used to reduce ambient noise further.

Note: It is recommended that the oscilloscope and probes used have a minimum bandwidth of 200 MHz.

If the circuit is operating correctly, the waveform should be a 1.8 V_DC output with a small ripple waveform superimposed on it. Figure 9 shows the ripple waveform.

Figure 9. Oscilloscope screen capture of the MAX38640A output ripple waveform.

Use Table 8 to diagnose and fix associated problems:

Table 8. Diagnosing MAX38640A Output AC Voltage Issues
Input Waveform	Potential Cause	Action	Notes
Ripple amplitude is too high (> 20 mV p-p)	Wrong capacitor value; defective capacitor	Disconnect the battery and check all connections with DMM; measure capacitor value
Ripple frequency doesn’t match V_LX square wave frequency	Light load	Check load
Broadband noise is too high	Load too large; environmental noise	Check load and environmental noise	Use pigtail 10× probe or active differential probing on output to reduce environmental noise
Transition spikes too high (> 30 mV p-p)	Load inductance; input current not adequate	Check line inductance; check input current with scope

5.0 V SMPS Circuit Using a Low Noise Buck-Boost Converter

The following circuit based on the MAX20343H low noise buck-boost converter shows the typical input and output power supply levels for a properly operating SMPS device in remote patient vital sign monitor applications. As shown in Figure 10, a DMM can be used to probe the input and output ports to validate the supply voltage levels. The power supply output levels can vary due to various factors such as:

➤ Discharging battery

➤ Changing loads (that is, device mode changes, devices waking up from sleep mode, etc.)

Figure 10. A block diagram of the 5.0 V_DC MAX20343H SMPS circuit for remote patient vital sign monitoring applications.

5.0 V SMPS Circuit Validation Checklist

The following circuit validation checklist (Table 9) is intended to help designers with their electrical bench checkout of the 5.0 V SMPS circuit (see Figure 10) post circuit board assembly. This checklist can also be used as a template for product testing.

Table 9 can be used as a checklist to validate the operation of the analog 5.0 V SMPS circuit using the MAX20343H device while connected to a biosensing circuit load.

Table 9. MAX20343H Validation Checklist
Step	Action	Procedure	Measurement	Need help?
1	Check the input DC power supply	Measure voltage across the battery	Reading range:	Troubleshooting instructions
	LP401230 LiPo batt	3.0 V to 4.2 V
	CR2032 Li coin batt	2.0 V to 3.4 V
2	Check the input DC power supply	Measure voltage across C_IN	Reading range:
	LP401230 LiPo batt	3.0 V to 4.2 V
	CR2032 Li coin batt	2.0 V to 3.4 V
3	Check V_OUT DC level	Measure voltage across C_OUT	Reading range: 4.75 V to 5.25 V
4	Check V_OUT DC level	Measure voltage across the load	Reading range: 4.75 V to 5.25 V
5	Check output noise level	Use pigtail 10× single-ended probe or differential active probe	Ripple noise level should be < 20 mV p-p

5.0 V SMPS Circuit Troubleshooting Guide

The following circuit troubleshooting instructions (Figure 11) will help designers if operational issues arise with the operation of the 5.0 V SMPS circuit. This guide addresses the most common problems that arise in implementing these switch-mode power supplies.

Troubleshooting the MAX20343H SMPS Circuit

Step 1—Check the input voltage: Using a DMM with an internal impedance of 1 MΩ or larger (for example, Fluke 87), measure the voltage across at the input to the MAX20343H device. Be sure to connect the negative black lead to the ground and the positive red lead to the input IN pin of the device. If the input pin is not easily accessible, place the leads across the input capacitor, C_IN.

Use Table 10 to diagnose and fix associated problems:

Table 10. Diagnosing MAX20343H Input Voltage Issues
Input Voltage Reading	Potential Cause	Action	Notes
Zero volts/ no reading	Battery uncharged; battery defective	Disconnect the battery and check voltage; if it reads 0 V, recharge the battery	Replace battery if it doesn’t charge
	No connection from the battery (IN or GND line)	Disconnect the battery and test for conductivity from the battery connector to the device input	PCB may have an open
	Input capacitor shorted to ground	Disconnect the battery and check for continuity across the capacitor	PCB may have short
	EN pin (SDA/EN) connected to ground	Disconnect the battery and test for conductivity from the battery connector to the device input	EN pin needs to be tied high for normal operation
Reading < 2.8 V	Low battery charge; battery defective	Disconnect the battery and check the voltage; if it reads below 2.8 V, recharge the battery	Replace battery if it does not charge
2.8 V ≥ reading ≤ 4.2 V		No action	Input voltage OK; proceed to Step 2
Reading ≥ 4.2 V	Defective battery	Replace battery

Figure 11. Tools for troubleshooting the MAX20343H circuit.

Step 2—Check the inductor signal waveform: Using an oscilloscope or DSO, probe the HVLX pin on the MAX20343H device. If the input pin is not easily accessible, place the probe on the inductor end cap.

Note: It is recommended that the oscilloscope and probes used have a minimum bandwidth of 200 MHz.

If the circuit is operating correctly, the waveform should be a pulse wave with minimal ringing on the rise and falling edges as shown in Figure 12.

Figure 12. Oscilloscope screen capture of a typical MAX20343H HVLX waveform with 10 mA light load.

The 500 ns pulse wave amplitude should be approximately equal to the input battery voltage. The pulse wave floor voltage should be within 100 mV of the ground. The output frequency and duty cycle of the pulse wave are proportional to the load current. Figures 13 and 14 show the output wave and signal frequency under different load conditions.

Figure 13. Oscilloscope screen capture of a typical MAX20343H HVLX waveform with 125 mA load.

Deviations from the ideal square wave can be used to effectively diagnose and fix many problems.

Use Table 11 to diagnose and fix associated problems:

Table 11. Diagnosing MAX20343H Inductor Signal Waveform Issues
Input Waveform	Potential Cause	Action	Notes
Amplitude is not correct	Inductor open; IN pin open; EN is open or ground	Disconnect the battery and check all connections with DMM	Repair PCB if needed
Duty cycle is not correct (doesn’t correlate to the output voltage)	R_SEL (6.65 kΩ) is not the correct value; bad external resistor	Disconnect the battery and check R_SEL with a DMM (R-measurement)	Replace the resistor with the correct value resistor
	R_SEL pin open (V_O = 3.3 V)	Check the output for 3.3 V; disconnect the battery and test for conductivity from the resistor to R_SEL pin	PCB may have an open
	R_SEL pin shorted to ground (V_O = 5.5 V)	Check the output for 5.5 V; disconnect the battery and measure resistance across the capacitor	PCB may have short
Waveform distortion Rounded rising edge	Bad inductor connection	Reconnect the inductor; replace the inductor	Bad connection can cause higher line resistance

Figure 14. Oscilloscope screen capture of a typical MAX20343H HVLX waveform with 246 mA load.

Step 3A—Check the output DC voltage: Using a DMM with an internal impedance of 1 MΩ or larger (for example, Fluke 87), measure the voltage at the output of the MAX20343H device. Be sure to connect the negative black lead to the ground and the positive red lead to the output OUT pin of the device. If the output pin is not easily accessible, place the leads across the output capacitor, C_OUT.

Use Table 12 to diagnose and fix associated problems:

Table 12. Diagnosing MAX20343H Output DC Voltage Issues
Output Voltage Reading	Potential Cause	Action	Notes
Zero volts/ no reading	No connection from SMPS to C_OUT	Disconnect the battery and test for conductivity from the output to C_OUT	PCB may have an open
Zero volts/ no reading	Output capacitor shorted to ground	Disconnect the battery and check for continuity across the capacitor	PCB may have short
Reading too low (< 4.75 V_DC)	Inductor wrong value; inductor saturated R_SEL(6.65 kΩ) has the wrong value	Disconnect the battery and check for the inductor and/or resistor values
4.75 V ≥ reading ≤ 5.25 V		No action	Operational
Reading too high (> 5.25 V_DC)	R_SEL (6.65 kΩ) has the wrong value	Disconnect the battery and check R_SEL value

Step 3B—Check the output AC voltage: Using an oscilloscope or DSO, now measure the output ripple (AC) by probing the OUT pin on the MAX20343H device. To properly measure the output and minimize RF pickup, it is recommended that 10× pigtail probes be used. Differential active probes can also be used to reduce ambient noise further.

Note: It is recommended that the oscilloscope and probes used have a minimum bandwidth of 200 MHz.

If the circuit is operating correctly, the waveform should be a 1.8 V_DC output with a small ripple waveform superimposed on it. Figure 15 shows the ripple waveform.

Figure 15. Oscilloscope screen capture of the MAX20343H (5 V) output ripple waveform.

Use Table 13 to diagnose and fix associated problems:

Table 13. Diagnose MAX20343H Output AC Voltage Issues
Input Waveform	Potential Cause	Action	Notes
Ripple amplitude is too high	Wrong capacitor value; defective capacitor	Disconnect the battery and check all connections with DMM; measure capacitor value
Ripple frequency doesn’t match V_HVLX pulse wave frequency	Light load	Check load
Broadband noise is too high	Load too large; environmental noise	Check load and environmental noise	Use pigtail 10× probe or active differential probing on output to reduce environmental noise
Transition spikes too high	Load inductance; input current not adequate	Check line inductance; check input current with scope

Conclusion

This concludes part one of the two-part article in which prevalidated discrete power supply circuits were presented for use with the MAX86171-based PPG remote patient vital sign monitor. These power supply circuits can be used with MAX86141-based PPG devices as well. In Part 2, we present prevalidated integrated power supply circuits for use with both MAX86171-based and MAX86141-based PPG remote patient vital sign monitors.

References

“Power Supply Subsystems for Vital Sign Monitors.” Analog Devices, Inc., January 2022.

“Designing Accurate, Wearable Optical Heart Rate Monitors.” Analog Devices, Inc., August 2017.

Frenzel, Lou. “Improving Remote Patient Monitoring to Overcome Healthcare Limitations.” Electronic Design, November 2020.

关于作者

Felipe Neira

Felipe Neira是ADI公司的应用工程师。他喜欢钻研便携式和可穿戴解决方案，侧重于健康传感器的电池电源管理。此外，他为ADI公司的所有广泛市场产品提供技术支持。Felipe毕业于加利福尼亚大学圣克鲁斯分校，获电气工程学士学位(BSEE)，毕业后不久即加入本公司。

Marc Smith

Marc Smith是ADI公司健康和医疗生物传感应用技术人员。他是MEMS和传感器技术领域的行业专家，在针对多个市场的传感器产品和电子开发方面拥有超过30年的经验。Marc拥有12项专利，并撰写了十多份出版物。他获得了加利福尼亚大学伯克利分校的电气工程学士学位(BSEE)和加利福尼亚圣玛丽学院的工商管理硕士学位(MBA)。