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

2024-12-19

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

摘要

This two-part article presents validated switch-mode power circuit designs for remote patient monitoring applications, including biosensors with excellent system signal-to-noise performance. “Power Supply Subsystems for Photoplethysmography Remote Patient Vital Sign Monitors—Part 1“ presents 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 provided 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 now show an integrated solution for space-constrained applications using the MAX86141 optical pulse oximeter and heart-rate sensor AFE (can be used with the MAX86171 as well).

As mentioned in Part 1 of this article, Analog Devices simplifies and speeds the development process by offering power supply subsystem circuit designs that have been prevalidated (that is, designed, built, and tested) ensuring the signal-to-noise ratio (SNR) performance of each biosensing AFE device. We provided a discrete option in Part 1 and now we will provide an integrated solution.

To reiterate, below are details for the power supply circuits with each example being complemented with a validation checklist and troubleshooting guide to aid circuit designers if needed. Here, Figure 1 shows a standard power block diagram encountered in many remote patient monitoring applications.

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	Vp-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

Integrated Design Description

This DC-to-DC power management integrated circuit (PMIC) design is used to regulate three output power supply rails for use in a remote patient vital sign monitor subsystem. The IC features a single-inductor multiple-output (SIMO) buck-boost regulator that provides the power rails from a single inductor to minimize total solution size while maintaining high efficiency.

This circuit provides proper line and load regulation while maintaining low output noise levels needed to preserve biosensor SNR performance, which is powered by a rechargeable lithium polymer battery. Figure 2 shows the PPG subsystem using an integrated power supply device.

Figure 2. A block diagram of a PPG subsystem using an integrated MAX77642 power supply device.

Table 3. Key Components
Designator	Component	Description
U1	DC-to-DC converter	Power converter device (MAX77642)
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/1.8 V/5.0 V SMPS Circuit Using a PMIC

The following circuit based on the MAX77642 PMIC shows typical input and output power supply levels for a properly operating 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 several factors such as:

➤ Discharging battery

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

Figure 3. An integrated 1.8 V/1.8 V/5.0 V MAX77642 SMPS circuit for remote patient vital sign monitor applications.

Figure 4. Integrated (MAX77642) power supply for remote patient vital sign monitors.

1.8 V/1.8 V/5.0 V SMPS Circuit Validation Checklist

Figure 4 shows the integrated MAX77642 PMIC for remote patient vital sign monitoring.

Integrated 1.8 V/1.8 V/5.0 V SMPS Circuit Validation Checklist

Table 4 can be used as a checklist to validate the operation of the 1.8 V/1.8 V/5.0 V SMPS circuit using the MAX77642 device while connected to a biosensing circuit load.

Table 4. MAX77642 Circuit Validation Checklist
Step	Action	Procedure	Measurement	Need Help?
1	Check the input DC power supply LP401230 LiPo Batt	Measure voltage across the battery	Reading range: 3.0 V to 4.2 V	For troubleshooting instructions, see below
2	Check the input DC power supply LP401230 LiPo batt	Measure voltage across C_IN	Reading range: 3.0 V to 4.2 V
3	Check V_OUTDC level	Measure the SBB0 output DC voltage with reference to GND	Digital 1.8 V reading range: 1.71 V to 1.89 V
4		Measure the SBB1 output DC voltage with reference to GND	Analog 1.8 V reading range: 1.71 V to 1.89 V
5		Measure the SBB2 output DC voltage with reference to GND	V_LED 5.0 V reading range: 4.75 V to 5.25 V
6	Check the analog 1.8 V output noise level	Use pigtail 10× single-ended probe or differential active probe across C₅	Ripple noise level should be < 20 mV p-p
			Switch spikes should be < 30 mV p-p
7	Check the digital 1.8 V output noise level	Use pigtail 10× single-ended probe or differential active probe across C₄	Ripple noise level should be < 20 mV p-p
			Switch spikes should be < 30 mV p-p
8	Check the analog 5.0 V output noise level	Use pigtail 10× single-ended probe or differential active probe across C₆	Ripple noise level should be < 20 mV p-p
			Switch spikes should be < 30 mV p-p

1.8 V/1.8 V/5.0 V SMPS Circuit Troubleshooting Guide

The following circuit troubleshooting instructions (Figure 5) will help designers if operational issues arise with operation of the 1.8 V/1.8 V/5.0 V SMPS circuit. This guide addresses the most common problems that arise in implementing these integrated switch-mode power supplies.

Troubleshooting the MAX77642 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 MAX77642 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 MAX77642 Input Voltage Issues
Input Voltage Reading	Potential Cause	Action	Notes
Zero volts/no reading	Battery uncharged; battery defective	Disconnect the battery and check the voltage; if it reads 0V, 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	PCB may have short
Reading < 2.8 V	Low battery charge; battery defective	Disconnect the battery and check voltage; if it reads below 2.8 V, recharge the battery	Replace the battery if it doesn’t charge
2.8 V ≥ reading ≤ 4.2 V		No action	Operational
Reading ≥ 4.2 V	Defective battery	Replace the battery

Step 2—Check the inductor signal waveform: Using an oscilloscope or digital storage scope (DSO), probe the LXA pin on the MAX77642 device. If the input pin is not easily assessable, place the probe on the (LXA) 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 series of pulse waves with minimal ringing on the rise and falling edges as shown in Figure 6.

Figure 6. Typical MAX77642 LXA waveform with SBB0 and SBB1 I_OUT = 1.2 mA and SBB2 I_OUT = 126.1 mA load conditions.

The pulse waveforms demonstrate the time multiplexing of three switch-mode power supplies sharing a single inductor (also known as SIMO power supply).

Deviations from the ideal series of pulse waves can be used to effectively diagnose and fix many problems.

Use Table 6 to diagnose and fix associated problems:

Table 6. Diagnosing MAX77642 Inductor Signal Waveform Issues
Input Waveform	Potential Cause	Action	Notes
Amplitude is not correct	Inductor open; IN pin open	Disconnect the battery and check all connections with DMM	Repair PCB if needed
Duty cycle is not correct (missing pulses)
SBB0 pulse missing	EN0 shorted to GND	Check SBB0 output for 0 V; disconnect battery and test for conductivity from EN0 pin to GND	PCB may have short
SBB1 pulse missing	EN1 shorted to GND	Check SBB1 output for 0 V; disconnect battery and test for conductivity from EN1 pin to GND	PCB may have short
SBB2 pulse missing	EN2 shorted to GND	Check SBB2 output for 0 V; disconnect battery and test for conductivity from EN2 pin to GND	PCB may have short
Duty cycle is not correct (pulse widths are not correct)	Output voltage select resistors; defective device	Identify SBBx channel associated with incorrect PW and follow associated steps below
SBB0 PW incorrect	RSET_SBB0 shorted to GND (SBB0 V_O = 0.5 V)	Disconnect the battery and test for 40.2 kΩ to GND	Bad/wrong resistor; PCB may have short
	RSET_SBB0 pin open (SBB0 V_O = 5.2 V)	Disconnect the battery and test for conductivity from resistor to RSET_SBB0 pin	PCB may have an open; bad solder connection
	Wrong RSET_SBB0 resistor value	Disconnect the battery and test for 40.2 kΩ to GND	Bad/wrong resistor installed
SBB1 PW incorrect	RSET_SBB1 shorted to GND (SBB1 V_O = 0.5 V)	Disconnect the battery and test for 28 kΩ to GND	Bad (shorted) resistor; PCB may have short
	RSET_SBB1 pin open (SBB1 V_O = 5.2 V)	Disconnect the battery and test for conductivity from resistor to RSET_SBB1 pin	PCB may have an open; bad solder connection
	Wrong RSET_SBB1 resistor value	Disconnect the battery and test for 28 kΩ to GND	Bad/wrong resistor installed.
SBB2 pulse missing	RSET_SBB2 shorted to GND (SBB2 V_O = 0.5 V)	Disconnect the battery and test for 536 kΩ to GND	Bad (shorted) resistor; PCB may have short
	RSET_SBB2 pin open (SBB2 V_O = 5.5V)	Disconnect the battery and test for conductivity from resistor to RSET_SBB2 pin	PCB may have an open; bad solder connection
	Wrong RSET_SBB2 resistor value	Disconnect the battery and test for 536 kΩ to GND	Bad/wrong resistor installed
Waveform distortion rounded rising edge	Bad inductor connection	Reconnect inductor; replace 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 three outputs of the MAX77642 device. Be sure to connect the negative black lead to the ground and the positive red lead to the associated SBBx channel output OUT pin of the device. If the output pin is not easily accessible, place the leads across the associated output capacitor, C_OUT.

Use Table 7 to diagnose and fix associated SBB0 and SBB1 (1.8 V_DC) output problems:

Table 7. Diagnosing MAX77643 SBB0 and SBB1 Output DC Voltage Issues
Output Voltage Reading	Potential Cause	Action	Notes
SBB0/SBB1: zero volts/ no reading	No connection from SBB0/SBB1 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
SBB0/SBB1: reading too low (< 1.71 V_DC)	Inductor wrong value; inductor saturated; RSET_SBB0/ RSET_SBB1 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 has wrong value	Disconnect the battery and check R_SEL value

Use Table 8 to diagnose and fix associated SBB2 (5.0 V_DC) output problems:

Table 8. Diagnosing MAX77643 SBB2 Output DC Voltage Issues
Output Voltage Reading	Potential Cause	Action	Notes
SBB2 zero volts/ no reading	No connection from SBB2 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
SBB2: reading too low (< 4.75 V_DC)	Inductor wrong value; inductor saturated; RSET_SBB2 has wrong value	Disconnect the battery and check for the inductor and/or resistor values
4.75 V ≥ reading ≤ 5.25 V		No action	Operational
SBB2 reading too high (> 5.259 V_DC)	R_SEL 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 three outputs of the MAX77642 device. It is recommended to use a differential technique to properly measure the output and avoid RF pickup.

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

If the circuit is operating correctly, the SBB0 waveform should be a 1.8 V_DC (digital) output with a small ripple waveform superimposed on it. Figure 7 shows the ripple waveform.

Figure 7. Oscilloscope screen capture of a MAX77642 SBB0 (digital = 1.8 V) ripple waveform (V_IN = 4.2 V, I_OUT = 100 mA).

Use Table 9 to diagnose and fix associated problems for SBB0, SBB1, and SBB2:

Table 9. Diagnosing MAX77643 SBB0, SBB1, and SBB2 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
Broadband noise is too high	Load too large; environmental noise	Check load and environmental noise	Use differential probing on output to reduce environmental noise
Transition spikes too high	Load inductance too large; input current not adequate	Check line inductance; check input current with scope

If the circuit is operating correctly, the SBB1 waveform should be a 1.8 V_DC (analog) output with a small ripple waveform superimposed on it. Figure 8 shows the ripple waveform.

Figure 8. Oscilloscope screen capture of a MAX77642 SBB1 (analog = 1.8 V) ripple waveform (V_IN = 4.2 V, I_OUT = 100 mA).

If the circuit is operating correctly, the SBB2 waveform should be a 5.0 V_DC (for LEDs) output with a small ripple waveform superimposed on it. Figure 9 shows the ripple waveform.

Figure 9. Oscilloscope screen capture of a MAX77642 SBB2 (5.0 V) ripple waveform (V_IN = 4.2 V, I_OUT = 100 mA).

Conclusion

This concludes the article series in which prevalidated power supply circuits, both discrete and integrated, for use with the MAX86171-based and MAX86141-based PPG remote patient vital sign monitors were presented. While both the integrated and discrete switch-mode power circuit designs support PPG performance, the integrated solution offers a smaller footprint and reduced part count and is recommended for size-constrained applications.

References

“Designing Accurate, Wearable Optical Heart Rate Monitors.” Analog Devices, Inc., August 2017. Frenzel, Lou. “Improving Remote Patient Monitoring toOvercome 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)。