Littelfuse Expands its Line of PPTC Resettable Fuses
Littelfuse’s PolySwitch Low Rho SMD Series features hold currents from 0.75 to 9.0 Amps and are available in SMD sizes from 0402 to 2920
The company has announced the addition of 23 new ratings to its present large and varied product portfolio.
Littelfuse’s Low Rho SMD series of Polyswitch PPTC’s. Image courtesy of Low Rho PPTC series product page
What are PPTC Resettable Fuses?
PPTC stands for polymeric positive temperature coefficient. Under normal conditions, the device presents an extremely low electrical resistance. The new ratings within the Low Rho SMD Resettable PPTCs portfolio, for example, offer resistances as low as one milliohm.
When the current passing through becomes too high, the resistance across the PPTC increases by orders of magnitude. The high resistance allows very little current to pass through, protecting the rest of the circuit from an overload.
It should also be noted that PPTCs operate in a sort of “hiccup mode”. If the excessive current condition ends, they will “reset” themselves back into low resistance, and allow current to flow again.
Two very important terms are IHOLD and ITRIP:
- IHOLD is the current below which the device will not go to high resistance.
- ITRIP is the current above which the PPTC will definitely go to a high resistance.
What happens when the current is greater than IHOLD and less than ITRIP is undefined. For this reason, this class of devices may not be the right choice for some types of applications.
Another thing for designers to consider is that PPTCs handle relatively small voltages. Members of this series are specced at either 6, 12, or 24 volts.
Very Small Package Sizes
These units will take up very little board space. As per Littelfuse’s Datasheet:
- Length between 0.03 and 0.05 inches
- Width between 0.01 and 0.03 inches
- Length between 0.26 and 0.31 inches
- Width between 0.19 and 0.21 inches
As per Stephen Li, Global Product Manager at Littelfuse, "Our expanded Low Rho SMD Resettable PPTCs portfolio includes a much broader array of both ratings and sizes." He goes on to state that “This provides electronics engineers with greater design flexibility, especially when developing the next generation of battery protection and charging ports for consumer electronics applications."
Other Design Considerations
Members of this PPTC series are specced to operate over a temperature range of -40 to 85℃. However, there is considerable derating of rated current.
Temperature Derating Curve. Image courtesy of Littelfuse Datasheet
While actually 150% higher at -40℃, rated current is 50% down at 85℃.
Members of the expanded Low Rho SMD Resettable PPTCs portfolio will find application in:
- Servers and computing equipment
- Consumer electronics
- LCD and LED HDTV
- Tablet and notebook PCs
- Game consoles
- Computer peripherals
- Hard disk drives
- Digital cameras
- Mobile and wearables
- Lithium ion and lithium polymer battery packs
- Portable medical devices
- USB peripherals including USB 3.0 / 2.0 ports
Regulatory and Environmental Concerns
- UL recognized
- TUV approved
- RoHS compliant, lead free and halogen free.
- Solvent Resistance: MIL–STD–202, Method 215 No change
- Vibration: MIL–STD–883, Method 2007, Condition A No change
- Moisture Sensitivity Level: Level 1, J–STD–020