UL1642-2005 Datasheet, PDF (7/8 Pages) Littelfuse – Introduction to Li-ion Battery Technology

Application Note:
Use of Low Resistivity Surface Mount PPTC
in Li-ion Polymer Battery Packs

°
_{Holding the device in a tripped state for long}
periods of time will cause an increase in resistance.

°
_{Any hand soldering may have a signi cant effect if}
not performed properly and therefore is generally
not recommended (particularly for SMT
components)
4. PPTC devices have two distinct resistance ranges:

°
_{RMIN: the minimum resistance of un-soldered}
devices

°
_{R1MAX: the maximum resistance of a device at}
20°C, measured one hour after tripping or re ow
soldering at 260°C for 20 seconds.
When measuring resistance:

°
_{Always perform the measurement at room}
temperature.

°
_{Perform measurements at least one hour after}
any heating process to ensure that the device has
cooled thoroughly (soldering, testing, etc.).

°
_{Keep in mind that catalog speci cations (trip time,}
hold current, etc.) assume the parts have been
mounted on a PCB and the resistance shift has
already occurred.
Other critical considerations for component selection
include:

°
_{Maximum circuit voltage}

°
_{Maximum available short circuit current}

°
_{Desired trip current and trip time}

°
_{Form factor}

°
_{Maximum ambient operating temperature}

°
_{Normal operating current}

°
_{Maximum operating voltage}

°
_{Maximum interrupt current}
The use of a LoRho^TM SMT PPTC in a Li-ion
polymer battery pack
The SMT form factor is optimized for Li-ion polymer
cell pack construction. Li-ion polymer packs are used
for low pro le smartphone and tablet applications. The
SMT PPTC allows for more ef cient high volume
manufacturing because the PPTC can be surface
mounted directly on the PCM (Protection Circuit
Module). Also, in low pro le designs, the PCM board
is mounted at in the same plane as the polymer
pouch. This allows for the lowest pro le thickness of
the smartphone or tablet design. The low height
capability of the LoRho^TM SMT PPTC makes it a great
choice for this application. The PPTC can be added to
the PCM while maintaining a minimum height pro le
of the total assembly.
When using LoRho^TM SMT PPTCs, some design and
application testing aspects must be considered:
°
_{The overall space allocated for SMT PPTC needs to}
be balanced against the total hold current required.
The designer rst needs to determine the maximum
continuous current (this includes burst use or peak
current use) and the maximum temperature that the
PPTC can experience (ambient temperature in the
PPTC’s vicinity).
°
_{So, the rst step in selecting the appropriate PPTC is}
nding out how much current the device can hold at
the maximum temperature. The advantage of using a
LoRho^TM PPTC is that the device can hold a large
amount of current in a relatively small form factor
and low pro le device.
°
_{Application testing is suggested to verify that the}
selected PPTC can hold the required current at
temperature. Typically, the device is subjected to the
required current for 15 minutes, which is enough
time to reach thermal stability. If device can hold the
current for 15 minutes, then this is one data point
that can be used to verify the correct device has
been selected.
©2012 Littelfuse, Inc
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