LS7631

A typical electronic dimmer may not operate properly with the inductive load encountered when driving a 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 cut-off. When a trigger pulse of LS7631/LS7632  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.
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2. Compensation for delayed triac turn-on. At the set conduction angle, a triac trigger pulse of 130.2s (60 Hz) is issued by the IC. If the triac fails to fire, a second trigger pulse of 260.4s 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 cut-off and delayed turn-on exceeds a preset threshold, a shutdown is initiated by turning off the triac trigger pulses. The safetyshutdown threshold value of LS7631/LS7632  is accumulated in a 4-bit Up/Down counter. The count increments for every occurrence of delayed cut-off or 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 safetyshut- down is effected.