Rev Date: 06/04/2012

6

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CSS/CSSH Series

Ultra Precision Current Sensing Chip Resistor

Stackpole Electronics, Inc.

Resistive Product Solutions

High Power Chip Resistors and Thermal Management

Stackpole has developed several surface mount resistor series in addition to our current sense resistors,

which have had higher power ratings than standard resistor chips. This has caused some uncertainty and

even confusion by users as to how to reliably use these resistors at the higher power ratings in their designs.

The data sheets for the RHC, RMCP, RNCP, CSR, CSRN, CSRF, CSS, and CSSH state that the rated

power assumes an ambient temperature of no more than 100 degrees C for the CSS / CSSH series and 70

degrees C for all other high power resistor series. In addition, IPC and UL best practices dictate that the

combined temperature on any resistor due to power dissipated and ambient air shall be no more than 105C.

At first glance this wouldn’t seem too difficult, however the graph below shows typical heat rise for the CSR

½ 100 milliohm at full rated power. The heat rise for the RMCP and RNCP would be similar. The RHC with

its unique materials, design, and processes would have less heat rise and therefore would be easier to

implement for any given customer.

The 102 degrees C heat rise shown here would indicate there will be additional thermal reduction techniques

needed to keep this part under 105C total hot spot temperature if this part is to be used at 0.75 watts of

power. However, this same part at the usual power rating for this size would have a heat rise of around 72

degrees C. This additional heat rise may be dealt with using wider conductor traces, larger solder pads and

land patterns under the solder mask, heavier copper in the conductors, vias through PCB, air movement, and

heat sinks, among many other techniques. Because of the variety of methods customers can use to lower

the effective heat rise of the circuit, resistor manufacturers simply specify power ratings with the limitations

on ambient air temperature and total hot spot temperatures and leave the details of how to best accomplish

this to the design engineers. Design guidelines for products in various market segments can vary widely so it

would be unnecessarily constraining for a resistor manufacturer to recommend the use of any of these

methods over another.

Note:

The final resistance value can be affected by the board layout and assembly process, especially the size of the

mounting pads and the amount of solder used. This is especially notable for resistance values ≤ 50 mΩ.

This should be taken into account when designing.