Wearable Electronics
charging. If the system gets too warm, the regulated charge current and voltage may be reduced to allow the circuit to cool. This throttling of current protects the end user from discomfort and preserves the system’s reliability and lifetime. Unfortunately, throttling the charge current also means a slower charge cycle. The need to incorporate JEITA features represents a design trade-off that creates pressure on wearable designs. This pressure can be alleviated by a solution that can maintain better thermal performance during long exposure to high charge currents.
Performance comparisons To examine the thermal performance of both the traditional and proposed solutions, a 1-minute charge test was run at 270mA CHG_CC (normal CC current) and 75mA JEITA_CC (CC current above a JEITA_WARM temperature threshold). The goal was to show the thermal differences over this period and examine whether higher charging speed could be maintained for the two solutions without triggering JEITA protection. A battery simulator was used for the benefit of repeatability and isolating the thermal rise to that experienced by the IC alone. The thermal threshold used for this test was 45°C, a 21°C rise over the ambient temperature. The PCB for the reference design is a 6-layer board with copper thicknesses of 0.0014 inches, 0.0007 inches, 0.0007 inches, 0.0007 inches, 0.0007 inches, and 0.0014 inches. The conditions of the test are presented in Table 1, and the results are presented in Table 2.
Over the course of this test, the MAX77659 SIMO solution rose by 11.1°C within 1 minute,
Test Configuration CC
JEITA CC
JEITA Warm Batt Voltage
Table 1. JEITA charging test conditions
Solution A (Max77659 SIMO PMIC)
Time Status Temp (°C) Charge Current (mA) 0
15 30 45 60
Off CC
CC CC CC
24.3 29.1
32.2 33.2 35.4
Table 2. JEITA charging test results
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270 270
270 270 270
Off CC
Solution B (linear Charger) Status
Temp (°C) Charge Current (mA)
24.0 43.0
JEITA CC 47.1 JEITA CC 47.3 JEITA CC 44.2
270 270
75 75 75
Figure 5. Thermal imaging of charger solutions side-by-side.
and its rate of temperature increase slowed significantly after the first 30 seconds. The
270mA 75mA
45 C 3.0V
proposed solution did not enter JEITA mode at any point during the test. With the typical linear charging solution, the part rose by almost 20°C within just 15 seconds, and it triggered JEITA protection - throttling charge current within just 30 seconds.
Thermal imaging results Furthermore, to examine the thermal behaviour without JEITA protection enabled, a thermal camera was used in a separate test to measure the temperature of both the SIMO PMIC solution and the linear charger solution. The parameters were the same as in the first test, and only JEITA protection was disabled. The temperature of the linear solution rose to 58.1°C over the course of this 2-minute test, while the SIMO PMIC only rose to 37.5°C. Based on these results, the SIMO solution was able to reduce the temperature rise from the linear charging solution by about 72 per cent.
Conclusion
This article benchmarks the MAX77659 SIMO PMIC against traditional linear
charging solutions in an emulated TWS earbud application, showing the benefits of autonomous headroom tracking with a switching charger solution. The results presented show massive thermal improvements - a heat reduction of 72 per cent allowed the SIMO PMIC solution to safely maintain almost 4× the charge current of a traditional linear charging solution. This helps the system charge lightning fast while staying cool and comfortable, addressing key pain points for wearable devices.
The MAX77659 SIMO PMIC enables safe, reliable, and comfortable charging solutions for the next generation of wearable devices while also improving efficiency and reducing the necessary solution size and system BOM count. For more information, check out Analog Devices’ broad platform of SIMO PMICs and fuel gauges to see the best-in-class low power solutions for your next-generation wearable devices.
https://www.analog.com/en Components in Electronics July/August 2024 47
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