Part Numbers

A typical electronic dimmer may not operate properly with the inductive load encountered when driving a magnetic transformer-coupled low voltage halogen lamp. The inductive load can cause a phenomenon called half-waving, wherein the triac fires in alternate half-cycles only, which may lead to the thermal destruction of the load transformer. The problems encountered in driving an inductive load are addressed by the LS7631/LS7632 CMOS ICs as follows:

1. Compensation for delayed triac cutoff When a trigger pulse is due to occur at a conduction angle which coincides with the on-state of the triac, the trigger pulse is delayed until the triac has turned off. This eliminates the underlying cause of half-waving.

2. Compensation for delayed triac turn-on At the set conduction angle, a triac trigger pulse of 130.2μs (60Hz) is issues by the IC. If the triac fails to fire, a second trigger pulse of 260.4μs width is issued 1ms later as a second attempt to fire the triac during the same half-cycle.

3. Safety Shutdown If the frequency of occurrences of the delayed turn-on exceeds a preset threshold, a shutdown is initiated by turning of the triac trigger pulses. The safety shutdown threshold value is accumulated in a 4-bit Up/Down counter. The count increments for every occurrence of delayed turn-on and decrements once every 8 SYNC pulses (AC line cycles). The counter will not decrement below zero. If the count reaches 15, the safety shutdown is effected.