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The higher density lithium-ion or lithium-polymer batteries used in modern wearable and mobile applications are potentially vulnerable to overcurrent and overtemperature during energy transfer. These conditions sometimes lead to thermal runaway, a chain reaction of overheating that is very difficult to stop, and can lead to injuries and damages. To prevent overheating, a polymer positive temperature coefficient (PPTC) fuse, such as the zeptoSMDC Series PolySwitch from Littelfuse, can effectively cut the current when it gets too high.
The tiny form factor (0201 package size) of the zeptoSMDC Series PPTCs makes them suitable for wearable and mobile applications. They are specifically designed to protect battery management ICs and fuel gauges, and feature a maximum electrical rating of 13 VDC and a short circuit current from 82 to 200mA. Although they behave in a similar manner to a typical PTC device, they have a faster trip time of 20ms at a lower trip current of 80mA.
Figure 1: zeptoSMDC Series PPTC Device (Image source: Littelfuse)
The fast trip time and low current of the zeptoSMDC PPTC makes it suitable for use as a limiting resistor to protect I/O ports, such as the I2C port on fuel gauge ICs. Figure 2 compares a current limiting circuit with a normal resistor versus a circuit with a zeptoSMDC PPTC.
Figure 2: Battery Fuel Gauge IC protection (Image Source: Littelfuse)
When used for I2C protection, standard reference designs recommend putting a resistor in front of the ESD protector (typically a Zener diode). The resistor reduces the energy of the ESD pulse, preventing a sustained overvoltage that can cause the failure of the ESD protector. Table 1 compares the performance of a standard 100 Ω resistor and a zeptoSMDC PPTC when used to protect a Zener diode (5V) from sustained overvoltage conditions of up to 20V. With the standard resistor, the Zener diode fails when IESD > 50 mA. Because the zeptoSMDC PPTC limits the current, the Zener diode remains operational.
Table 1: Test Results Comparing a Normal resistor with a zeptoSMDC PPTC (Source: Littelfuse)
Application Example: Multicell Battery Protection System
Figure 3 illustrates the use of zeptoSMDC PPTCs in a typical multicell battery protection reference system.
Figure 3: Multicell battery protection reference system (Source: Littelfuse)
The symbols in Figure 3 are described as follows:
I – PPTCs, such as the zeptoSMDC PPTC, provide overcurrent and overtemperature protection for the battery management ICs and fuel gauges.
II – TVS diode arrays provide ESD protection for the I2C input. The Littelfuse SP1006 is an example diode that works for this purpose.
III – Thermistors, such as the Littelfuse RA, RB, and KR series, provide ambient temperature measurement.
IV – A battery mini-breaker, such as the Littelfuse MHP-TAC, provides secondary overtemperature and overcurrent protection.
The zeptoSMDC Series PPTCs from Littelfuse provide effective overcurrent protection for a variety of circuits, helping to prevent conditions that lead to overheating and thermal runaway. Their small form factor makes them well-suited for use in modern portable devices, including smartphones, smartwatches, tablets, and wearables.
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