featured in Maxim's Engineering Journal A version of this application note was published in IntroductionThe final operating environment for a chip is often not well known durin g the IC's evaluation stage, and that environment can vary considerably from application to application. It is especially critical to understand thermal derating in portable and nonportable applications, since end systems oft en depend on forced airflow for cooling. This is why thermal-derating graphs are included in data sheets for puls e-width-modulated (PWM) controllers with internal MOSFETs. The derating curves show how much power can be dr awn from the chip under given airflow and ambient temperature conditions. To ensure that the application's thermal environment does not overload t he PWM controller, a thermal-derating graph can be used to select proper PWM ICs. A test system with a thermal -derating box provides a practical way of evaluating the thermal-derating performance of the PWM chip. This a MaximApp NotesPower-Supply CircuitsTemperature Sen