TOPSwitch-FX uses the proven TOPSwitch topology and cost-effectively integrates many new functions that reduce system cost and, at the same time, improve design flexibility, performance, and energy efficiency. Like TOPSwitch, the high voltage power MOSFET, PWM control, fault protection, and other control circuitry are all integrated onto a single CMOS chip, but with two added terminals. The first one is a MULTIFUNCTION (M) pin, which implements programmable line OV/UV shutdown and line feed-forward/DC(max) reduction with line voltage. The same pin can be used instead to externally set an accurate current limit. In either case, this pin can also be used for remote ON/OFF or to synchronize the oscillator to an external, lower-frequency signal. The second added terminal is the FREQUENCY (F) pin and is available only in the Y package. This pin provides the half-frequency option when connected to CONTROL (C) instead of SOURCE (S). The features on the new pins can be disabled by shorting them to the SOURCE, which allows the device to operate in a three-terminal TOPSwitch mode, but with the following new transparent features: soft-start, cycle skipping, 132 kHz switching frequency, frequency jittering, wider DC(max), hysteretic thermal shutdown and larger creepage. In addition, all critical parameters such as frequency, current limit, PWM gain, etc. have tighter temperature and absolute tolerances compared to the TOPSwitch-II family. Higher current limit accuracy and larger DC(max), when combined with other features allow for a 10% to 15% higher power capability on the TOPSwitch-FX devices compared to equivalent TOPSwitch-II devices for the same input/output conditions.
