“Switch-mode power supplies (also known as SMPS or DC-DC converters) are often used in wearable healthcare applications for reasons including size considerations and energy efficiency. Designers can use these power supplies to create battery-powered products that last longer. Unfortunately, designers still need to select the appropriate SMPS device and then create a proper board layout to protect the performance of the biosensing device in the system.

“

By Felipe Neira and Marc Smith

Summary

This two-part article presents a proven switch-mode power supply circuit design for remote patient vital sign monitoring applications, including biosensors with excellent system signal-to-noise ratio performance. The first part describes discrete solutions that provide excellent performance, and the second part describes integrated solutions for space-constrained applications.

What you will learn:

• Learn how to select a power configuration based on PPG system requirements.
• Review implementation of switch-mode power supply reference circuits for discrete (Part 1) and integrated design (Part 2).
• Understand power supply performance test methods to validate the system under different device use cases and transient loading conditions.
• Obtain a checklist to verify implementation.
• Gain troubleshooting knowledge to resolve implementation issues.

This two-part article presents a pre-validated power circuit design for photoplethysmography (PPG) remote patient vital sign monitoring applications, including biosensors with excellent system signal-to-noise ratio performance. PPG devices can be used to measure changes in blood volume, from which vital sign information such as blood oxygen levels and heart rate can be obtained. Part 1 describes a discrete power-supply circuit design solution that provides excellent performance using the MAX86171 optical pulse oximeter and heart-rate sensor analog front-end (AFE). The second section describes an integrated solution for space-constrained applications.

Switch-mode power supplies (also known as SMPS or DC-DC converters) are often used in wearable healthcare applications for reasons including size considerations and energy efficiency. Designers can use these power supplies to create battery-powered products that last longer. Unfortunately, designers still need to select the appropriate SMPS device and then create a proper board layout to protect the performance of the biosensing device in the system.

To simplify and speed up the development process, Analog Devices offers pre-validated (i.e., designed, built, and tested) power subsystem circuit designs to guarantee the signal-to-noise ratio (SNR) performance of each biosensing AFE device. This article describes these power supply circuits in detail, with each example accompanied by a verification checklist and troubleshooting guidelines to assist the circuit designer when needed. Figure 1 shows a block diagram of a standard power supply found in many remote patient monitoring applications.

design limit

enter		Output (VDIG, VANA, VLED)		noise, RTO
VIMIN	VIMAX	VOMIN	VOMAX	VPP(max)
3.0V1	4.2V1	1.6V	2.0V	30mVPP
2.0V2	3.4V2	1.6V	2.0V	30mVPP
		4.7V	5.3V	20mVPP

Notes:

Secondary battery (LiPo)
Primary battery (lithium button cell)

design configuration

design configuration	battery implementation	Board Area Layout Considerations
separate	One time (button battery) Quadratic (Li & LiPo)	Implement separate discrete circuits.
integrated	Quadratic (Li & LiPo)	A single integrated circuit is used to minimize board area requirements. Only secondary batteries are supported.

Discrete Design Description

This DC-DC converter design regulates three output power rails for remote patient vital sign monitoring subsystems. The circuit provides proper line and load regulation while maintaining a low output noise level to maintain biosensing SNR performance, powered by a rechargeable Li-polymer battery or a primary Li-ion battery. Figure 2 shows the PPG subsystem using discrete power devices.

key components

logo	element	describe
U1	DC-DC Converter	Power Conversion Devices (MAX38640A and MAX20343H)
L1	2.2μH Inductor	Low Equivalent Series Resistance (ESR) Inductive (Energy) Storage Element1
C1	22μF capacitor	Low ESR capacitive (energy) storage element1

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

1.8V SMPS circuit using nanoPower buck converter

The following circuit, based on the MAX38640A nanoPower step-down converter (Figure 3), shows typical input and output supply levels for proper operation of an SMPS device in a remote patient vital-signs monitoring application. As shown in Figure 3, the input and output ports can be probed with a digital multimeter (DMM) to verify the supply voltage level. Power supply output levels can vary due to various factors, such as:

The battery is discharged.

Load change (device mode change, device wake-up from sleep mode, etc.).

1.8V SMPS Circuit Verification Checklist

The following circuit verification checklist (Figure 4) is intended to assist designers in performing various electrical reference checks on 1.8V SMPS circuit printed circuit board assemblies. This checklist can also be used as a template for product testing.

The following table can be used as a checklist to verify the operation of an analog or digital 1.8V SMPS circuit using the MAX38640A device connected to a biosensing circuit load.

step	operate	program steps	Measurement	need help?
1	Check input DC power LP401230 LiPo battery CR2032 lithium button battery	Measure the voltage across the battery	Reading range: 3.0VC 4.2V 2.0VC 3.4V	Troubleshooting Instructions
2	Check input DC power Coin Batt CR2032 lithium button battery	Measure the voltage across CIN	Reading range: 3.0VC 4.2V 2.0VC 3.4
3	Check VOUT DC level	Measure the voltage across COUT	Reading range: 1.71V 1.89V
4		Measure the voltage across the load	Reading range: 1.71V 1.89V
5	Check output noise level	10x single-ended probes or differential active probes using pigtail leads	The ripple noise level should be

MAX38640A (1.8V output) SMPS circuit troubleshooting

The following circuit troubleshooting instructions (Figure 5) can help the designer if there is a problem with the operation of the 1.8V SMPS circuit. This guide addresses the most common issues you may encounter when implementing this type of switch mode power supply.

Troubleshooting the MAX38640A SMPS Circuit:

Step 1C Check the input voltage: Measure the voltage at the input of the MAX38640A device using a digital multimeter (DMM) with an internal impedance of 1MΩ or greater, such as the Fluke 87. Be sure to connect the negative “black” lead to ground and the positive “red” lead to the input “IN” pin of the device. If the input pins are not easily accessible, route the leads through the input capacitor CIN.

Use the following table to diagnose and resolve related issues:

Input voltage reading	potential cause	operate	note
Zero Volts/No Reading	The battery is not charged. The battery is defective.	Disconnect the battery and check the voltage. If it reads 0V, charge the battery.	If it won’t charge, replace the battery.
	No battery connection (IN or GND wire).	With the battery disconnected, test the conductivity from the battery connector to the device input.	The PCB may have an open circuit.
	Input capacitor shorted to ground	Disconnect the battery and check the continuity of the capacitor.	Capacitor damage; There may be a short on the PCB.
	The EN pin is grounded.	With the battery disconnected, test the conductivity from the EN pin to ground.	The EN pin needs to be tied high for proper operation.
Reading (LiPo battery) Reading (Lithium-ion battery)	battery is low The battery is defective.	Disconnect the battery and check the voltage. If the reading is below 2.8V, recharge the battery.	If it won’t charge, replace the battery.
3.0V ≥ reading ≤ 4.2V (LiPo battery) 2.0V ≥ reading ≤ 3.4V (Lithium Ion Battery)		No action.	If the input voltage is normal, go to step 2.
Reading ≥ 4.2V (LiPo battery) Reading ≥ 3.4V (Lithium Ion Battery)	The battery is defective.	Replacement battery.

Step 2C Examine the inductor signal waveform: Use an oscilloscope or digital storage oscilloscope (DSO) to probe the LX pin on the MAX38640A device. If the input pins are not easily accessible, place the probe on the inductor terminal capacitance.

Note: An oscilloscope and probes with a minimum bandwidth of 200MHz are recommended.

If the circuit is running at a light load (i.e. less than 50mA), the waveform should look like Figure 6.

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

The square wave amplitude should be approximately equal to the input battery voltage. The square wave bottom voltage should be about 200mV to 300mV below ground (eg -250mV). The duty cycle is directly proportional to the output voltage. Therefore, an input battery voltage of 3.6V will have a duty cycle of approximately 50% when generating an output voltage of 1.8V. Figure 8 shows the relationship between duty cycle and output voltage.

Deviations from an ideal square wave can be used to effectively diagnose and solve many problems.

Use the following table to diagnose and resolve related issues:

input waveform	potential cause	operate	note
Incorrect amplitude	The inductor is open. IN pin open circuit EN open circuit or ground	Disconnect the battery and check all connections to the DMM.	Repair the PCB if needed.
Incorrect duty cycle (not related to output voltage)	The value of RSEL is incorrect (768KΩ). The external resistor is damaged.	Disconnect battery, check RSEL with DMM (R measure)	Replace with resistor of correct value.
	Leave the RSEL pin open (Vo = 2.5V).	Check the 2.5V output. With the battery disconnected, test the conductivity from the resistor to the RSEL pin.	The PCB may have an open circuit.
	RSEL pin is shorted to ground (Vo=0.8V)	Check the 0.8V output. Disconnect the battery and measure the resistance across the capacitor.	There may be a short on the PCB.
wave distortion circular rising edge	Poor inductor connection	Reconnect the inductor. Replace inductor.	Poor connections can result in high line resistance

Step 3A-C Check output DC voltage: Measure the voltage at the output of the MAX38640A device using a DMM with an internal impedance of 1MΩ or greater (such as a Fluke 87). Be sure to connect the negative “black” lead to ground and the positive “red” lead to the output “OUT” pin of the device. If the output pins are not easily accessible, route the leads through the output capacitor COUT.

Use the following table to diagnose and resolve related issues:

Output voltage reading	potential cause	operate	note
Zero Volts/No Reading	No connection from SMPS to COUT	Disconnect the battery and test the conductivity from output to COUT	The PCB may have an open circuit.
	The output capacitor is shorted to ground	Disconnect the battery and check the continuity of the capacitor.	There may be a short on the PCB.
low reading (	Wrong inductance value Inductor saturation Wrong RSEL value	Disconnect the battery and check the inductance and/or resistance values.
1.71V ≥ reading ≤ 1.89		No action.	can work.
reading too high (> 1.89 VDC)	Wrong RSEL value	Disconnect the battery and check the RSEL value.

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

Note: An oscilloscope and probes with a minimum bandwidth of 200MHz are recommended.

If the circuit is working properly, the waveform should be a 1.8VDC output with a small ripple waveform superimposed on it. Figure 9 shows the ripple waveform.

Use the following table to diagnose and resolve related issues:

input waveform	potential cause	operate	note
Ripple amplitude is too high (> 20mVpp)	Wrong capacitor value; capacitor is defective.	Disconnect battery and check all connections to DMM; measure capacitance.
Ripple Frequency vs. VLXSquare wave frequency mismatch	light load	check load
Broadband noise is too high	Excessive load; ambient noise.	Check load and ambient noise.	Use pigtail 10x probes or active differential probes at the output to reduce ambient noise.
Transition spikes too high (> 30mVp)	load inductance; Insufficient input current.	Check line inductance; check input current with oscilloscope.

5.0V SMPS Circuit Using Low Noise Buck-Boost Converter

The following circuit, based on the MAX20343H low-noise buck-boost converter, shows typical input and output supply levels for proper operation of the SMPS device in a remote patient vital-signs monitoring application. As shown in Figure 10, a DMM can be used to probe the input and output ports to verify supply voltage levels. Power supply output levels can vary due to various factors, such as:

The battery is discharged.
Load change (device mode change, device wake-up from sleep mode, etc.).

5.0V SMPS Circuit Verification Checklist

The following circuit verification checklist (Figure 10) is intended to assist the designer in performing various electrical reference checks for 5.0V SMPS circuit printed circuit board assemblies. This checklist can also be used as a template for product testing.

The following table can be used as a checklist to verify the operation of an analog 5.0V SMPS circuit using the MAX20343H device connected to a biosensing circuit load.

step	operate	program steps	Measurement	need help?
1	Check input DC power LP401230 LiPo battery CR2032 lithium button battery	Measure the voltage across the battery	Reading range: 3.0VC 4.2V 2.0VC 3.4	Troubleshooting Instructions
2	Check input DC power LP401230 LiPo battery CR2032 lithium button battery	Measure CINvoltage across	Reading range: 3.0VC 4.2V 2.0VC 3.4
3	Check Vout DC level	Measure Coutvoltage across	Reading range: 4.75V 5.25V
4	Check Vout DC level	Measure the voltage across the load	Reading range: 4.75V 5.25V
5	Check output noise level	10x single-ended probes or differential active probes using pigtail leads	The ripple noise level should be

5.0V SMPS Circuit Troubleshooting Guide

The following circuit troubleshooting instructions (Figure 11) can assist the designer if there is a problem with the operation of the 5.0V SMPS circuit. This guide addresses the most common issues you may encounter when implementing this type of switch mode power supply.

Troubleshooting the MAX20343H SMPS Circuit:

Step 1C Check the input voltage: Measure the voltage at the input of the MAX20343H device using a DMM with an internal impedance of 1MΩ or greater (such as a Fluke 87). Be sure to connect the negative “black” lead to ground and the positive “red” lead to the input “IN” pin of the device. If the input pins are not easily accessible, route the leads through the input capacitor CIN.

Use the following table to diagnose and resolve related issues:

Input voltage reading	potential cause	operate	note
Zero Volts/No Reading	The battery is not charged. The battery is defective.	Disconnect the battery and check the voltage. If it reads 0V, charge the battery.	If it won’t charge, replace the battery.
	No battery connection (IN or GND wire)	With the battery disconnected, test the conductivity from the battery connector to the device input.	The PCB may have an open circuit.
	Input capacitor shorted to ground	Disconnect the battery and check the continuity of the capacitor.	There may be a short on the PCB.
	EN pin (SDA/EN) grounded	With the battery disconnected, test the conductivity from the battery connector to the device input.	The EN pin needs to be tied high for proper operation.
reading	battery is low battery is defective	Disconnect the battery and check the voltage. If the reading is below 2.8V, recharge the battery.	If it won’t charge, replace the battery.
2.8V ≥ reading ≤ 4.2V		No action.	The input voltage is normal. Continue to step 2.
Reading ≥ 4.2V	battery is defective	Replacement battery.

Step 2C Examine the inductor signal waveform: Use an oscilloscope or DSO to probe the HVLX pin on the MAX20343H device. If the input pins are not easily accessible, place the probe on the inductor terminal capacitance.

Note: An oscilloscope and probes with a minimum bandwidth of 200MHz are recommended.

If the circuit is working properly, the waveform should be pulsed with minimal ringing on the rising and falling edges, as shown in Figure 12.

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

Deviations from an ideal square wave can be used to effectively diagnose and solve many problems.

Use the following table to diagnose and resolve related issues:

input waveform	potential cause	operate	note
Incorrect amplitude	The inductor is open. IN pin open circuit EN open circuit or ground	Disconnect the battery and check all connections to the DMM.	Repair the PCB if needed.
Incorrect duty cycle (not related to output voltage)	RSELIncorrect value (6.65KΩ). The external resistor is damaged.	Disconnect battery, check R with DMM (R measurement)SEL	Replace with resistor of correct value.
	RSEL pin open circuit (Vo=3.3V)	Check the output of 3.3V With the battery disconnected, test the conductivity from the resistor to the RSEL pin.	The PCB may have an open circuit.
	RSEL pin is shorted to ground (Vo=5.5V)	Check the output of 5.5V Disconnect the battery and measure the resistance across the capacitor.	There may be a short on the PCB.
wave distortion circular rising edge	Bad inductor connection.	Reconnect the inductor. Replace inductor.	Poor connections can result in high line resistance

Step 3A-C Check output DC voltage: Measure the voltage at the output of the MAX20343H device using a DMM with an internal impedance of 1MΩ or greater (such as a Fluke 87). Be sure to connect the negative “black” lead to ground and the positive “red” lead to the output “OUT” pin of the device. If the output pins are not easily accessible, route the leads through the output capacitor COUT.

Use the following table to diagnose and resolve related issues:

Output voltage reading	potential cause	operate	note
Zero Volts/No Reading	No connection from SMPS to COUT	Disconnect the battery and test the conductivity from output to COUT	The PCB may have an open circuit.
	The output capacitor is shorted to ground	Disconnect the battery and check the continuity of the capacitor.	There may be a short on the PCB.
low reading (	Wrong inductance value Inductor saturation RSELwrong value	Disconnect the battery and check the inductance and/or resistance values.
4.75V ≥ reading ≤ 5.25V		No action.	can work.
reading too high (> 5.25 VDC)	Wrong RSEL value	Disconnect the battery and check the RSEL value.

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

Note: An oscilloscope and probes with a minimum bandwidth of 200MHz are recommended.

If the circuit is working properly, the waveform should be a 1.8VDC output with a small ripple waveform superimposed on it. Figure 15 shows the ripple waveform.

Use the following table to diagnose and resolve related issues:

input waveform	potential cause	operate	note
Ripple amplitude is too high	Wrong capacitor value; defective capacitor	Disconnect battery and check all connections to DMM; measure capacitance
Ripple Frequency vs. VHVLXPulse wave frequency mismatch	light load	check load
Broadband noise is too high	Excessive load; ambient noise.	Check load and ambient noise.	Use pigtail 10x probes or active differential probes at the output to reduce ambient noise.
jump spike too high	load inductance; Insufficient input current	Check line inductance; check input current with oscilloscope.

epilogue

This article is divided into two parts. The above is the first part, which describes the pre-validated discrete power supply circuit for use with the MAX86171-based PPG remote vital signs monitor. These power circuits can also be used with MAX86141-based PPG devices.

The second part of this article describes a pre-verified integrated power supply circuit for use with MAX86171-based and MAX86141-based PPG remote vital-signs monitors.

For appropriate verification test data for discrete and integrated power supply implementations, visit the Maxim Integrated (now part of Analog Devices) website:

“Power Subsystems for Remote Patient Vital Signs Monitors”.

illustrate:

Figure 1. Block Diagram of a Typical PPG Remote Patient Vital Signs Monitor
Figure 2. Block Diagram of PPG Subsystem Using Discrete Power Devices
Figure 3. 1.8VDC MAX38640A SMPS Circuit for Remote Patient Vital Signs Monitoring Applications
Figure 4.1.1 Verification checklist for 8VDC MAX38640A SMPS circuit design
Figure 5. Troubleshooting tools for the MAX38640A SMPS circuit
Figure 6. Oscilloscope screenshot of typical MAX38640A VLX waveform at light load
Figure 7. Oscilloscope screenshot of switching waveforms for the MAX38640A
Figure 8. MAX38640A Duty Cycle vs. Output Voltage Plot
Figure 9. Oscilloscope screenshot of MAX38640A output ripple waveform
Figure 10. Block Diagram of the 5.0VDC MAX20343H SMPS Circuit for Remote Patient Vital Signs Monitoring Applications
Figure 11. Troubleshooting tools for the MAX20343H circuit
Figure 12. Oscilloscope screenshot of a typical MAX20343H HVLX waveform at a light load of 10mA
Figure 13. Oscilloscope screenshot of typical MAX20343H HVLX waveform at 125mA load
Figure 14. Oscilloscope screenshot of typical MAX20343H HVLX waveform at 246mA load
Figure 15. Oscilloscope screenshot of MAX20343H (5V) output ripple waveform

About Analog Devices

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about the author

Felipe Neira

Senior Member of the Applied Technology Team – Training and Technical Services
Maxim Integrated (now part of Analog Devices)
www.maximintegrated.com

About the author: Felipe Neira is an applications engineer at Maxim Integrated (now part of Analog Devices). He enjoys working on portable and wearable solutions with a focus on battery power management for health sensors. In addition, he provides technical support for all of ADI’s broad market products. Felipe joined the company shortly after graduating from the University of California, Santa Cruz with a Bachelor of Science in Electrical Engineering (BSEE).

Marc Smith

Key members of the application technology team
Maxim Integrated (now part of Analog Devices)
www.maximintegrated.com

About the author: Marc Smith is a member of the Health and Medical Biosensing Applications Technology Team at Maxim Integrated (now part of Analog Devices). He is an industry expert in MEMS and sensor technology with over 30 years of experience in sensor product and electronics development for multiple markets. Marc holds 12 patents and has authored more than ten publications. He earned a Bachelor of Science in Electrical Engineering (BSEE) from the University of California, Berkeley and a Master of Business Administration (MBA) from Saint Mary’s College of California.