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IEEE1625-IEEE Datasheet(PDF) 5 Page - Littelfuse |
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IEEE1625-IEEE Datasheet(HTML) 5 Page - Littelfuse |
5 / 8 page 5 c n I , e s u fl e tt i L 12 0 2 © Application Note: Use of Low Resistivity Surface Mount PPTC in Li-ion Polymer Battery Packs Figure 6. The effect of temperature on the resistance of a PPTC PPTC resistors are over-current protection devices. Like fuses, they have two terminals and are placed in line with the circuit being protected (see Figure 5). Because they are ideal for situations where frequent over-current conditions occur or constant uptime is required, PPTCs are typically used in Li-ion battery pack applications. In order to limit unsafe currents while allowing constant safe current levels, their resistance will “reset” auto- matically when the fault is removed and temperature returns to a safe level. Under normal conditions, PPTCs act as a low value resistor – dissipating little power and barely warm. Under fault conditions, they heat up due to I2R (Ohmic heating; >100oC) and their resistance increases 1000X or more, limiting the current to a small value (see Figure 6). When the current is removed, the PPTC will return to normal temperature and resistance, restoring the circuit (see Figure 7). Temperature (°C) Trip Point PPTC trip times are inuenced by: ● Resistance of the device ● Ambient temperature and air currents ● PCB trace size and copper weight ● Proximity of other components Other items that inuence the effective heat transfer rate from the device to its surroundings can also impact performance. Figure 7. The effect of changing current levels on a PPTC’s temperature and resistance Time Current (TC) curves present the average values of the trip time at a given current for every part number (see Figure 8). PPTC trip times will be distrib- uted above and below the curve. Lower percentage overloads produce greater variations in trip time. Customer verication tests need to be done for actual applications to ensure proper component selection. Time Normal Operating Current Normal Operating Current Fault Current Leakage Current Power Down for Reset |
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