POWER INTEGRATIONS, INC. v FAIRCHILD SEMICONDUCTOR, decided Sept. 20, 2108 after rehearing, Non-infringement

U.S. Patent Nos. 6,212,079 and 6,538,908 are directed to power supply controller chips (pages 2-3).   The claims recite a “fixed switching frequency for a first range of feedback signals” and “a multi-function circuit coupled to receive a signal at a multi-function terminal for adjusting a current limit of a power switch” (pages 3-4).  “Fixed switching frequency” has an ordinary meaning that does not preclude natural variation, and the term is used for a class of products that include minor variations in frequency, so infringement is appropriate (page 9).  “Current limit” is a value of current that can be used to turn off the power switch when the amount of current reaches a threshold (page 14).  The accused products use voltage (page 15).  Ohms law provides the known relationship between current and voltage, and prosecution history arguments about current verses the prior art voltage relate to other claim language for claims requiring both voltage and current, so infringement under doctrine of equivalents is proper (pages 16-17). 

Hindsight:  When using a absolute term, it may be best to disclose the absolute including a tolerance or to use a hedge term (e.g., substantially) noted in the specification as allowing tolerance.  Claiming a concept using different language may avoid having to rely on doctrine of equivalents, such as using “signal” instead of current.  While the specification could have indicated known interchangeability of current and voltage, this may risk being said to have disclaimed voltage when reciting current.