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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