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profile must be determined. A load profile is a simple representation of the load duty cycle of the system. For the remote patient monitoring patch we are using, we will consider the three different operating modes that were previously presented: standard monitoring, temperature monitoring, and transmission modes. In standard monitoring mode, the current consumption of the patch shown in Figure 1 (including the 330 nA quiescent current of each buck converter and the current draw of the MCU) is 1.88 mA. In temperature monitoring mode, the current draw is 1.95 mA for 200 ms every 15 minutes. In transmission mode, the current draw is 7.90 mA for 30 seconds every two hours while the patch is transmitting data via BLE. These values can be found within the respective device data sheets by looking at active and quiescent current specifications. To start the load profile analysis, a duty cycle calculation is determined using the time period for each operating mode during a day. Equation 1 is used:


operating mode, the average current consumption per day can be approximated from Equation 2:


Here is a sample calculation:


Standard Monitoring Mode Current per Day = Standard Monitoring Mode Current × Standard Monitoring Mode Duty Cycle × 24 Hours


Standard Monitoring Mode Current = 1.88 mA Standard Monitoring Mode Duty Cycle = 0.9956


Standard Monitoring Mode Current per Day = 1.88 mA × 0.9956 × 24 Hours = 44.92 mAh/Day


Once each operating mode’s current consumption per day is found, the lifetime of the battery can be determined via Equation 3:


and transmission modes. In standard monitoring mode, only the ECG and accelerometer are monitored. In temperature monitoring mode, an additional temperature sensor is monitored. In transmission mode, the BLE radio communicates the data while simultaneously monitoring the ECG and accelerometer data.


POWER SUPPLY CHALLENGES Designing an RPM, such as an ECG patch, poses multiple challenges for power supply designers. The design is typically space constrained and patches with several sensors may require multiple power rails. Because the RPM patch is usually intended to be a single- use item, a coin cell battery is typically the most cost-effective power source available to the designer. Using only a coin cell battery to supply power to the patch, a designer must also be cognisant of the efficiency of the power supply subsystem.


An often overlooked challenge for power supply designers is extending the shelf life of the product. Shutdown currents and battery self-discharge can shorten the life of any system. Therefore, it is important for a designer to determine if the RPM patch can meet operating time requirements after a typical shelf life period and, if it cannot, what steps can be taken to preserve battery life before the patch reaches the end user.


DETERMINING BATTERY RUN TIME To accurately determine if the power supply solution meets battery life requirements, a load


Instrumentation Monthly August 2023


Which gives us the duty cycles for our patch seen in Table 1 below. Using the load profile in Figure 2, we can calculate the current consumption of the patch. Taking the active current consumption of each


Here is a sample calculation: Battery Capacity = 235 mAh


Standard Monitoring Mode Current per Day = 44.92 mAh/Day


Temperature Monitoring Mode Current per Day = 0.01 mAh/Day Transmission Mode Current per Day = 0.79 mAh/Day


Figure 2. Load profile diagram.


Battery Life (Days) = 235 mAh/ (44.92 mAh/Day + 0.01 mAh/Day + 0.79 mA/Day) = 5.14 Days


TABLE 1. DUTY CYCLES FOR PATCH OPERATING MODES Operation Mode


Temperature Measurement Percentage (%/Day)


BLE Communication Percentage (%/Day) ECG Monitoring Duration (%/Day) Duty Cycle 0.02% 0.42% 99.56%


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