Continuous operation of a motor that is severely
over−loaded results in overheating and eventual failure.
This destructive condition can best be prevented with the use
of cycle−by−cycle current limiting. That is, each on−cycle
is treated as a separate event. Cycle−by−cycle current
limiting is accomplished by monitoring the stator current
build−up each time an output switch conducts, and upon
sensing an over current condition, immediately turning off
the switch and holding it off for the remaining duration of
oscillator ramp−up period. The stator current is converted to
a voltage by inserting a ground−referenced sense resistor RS
(Figure 35) in series with the three bottom switch transistors
(Q4, Q5, Q6). The voltage developed across the sense
resistor is monitored by the current sense input (Pin 12), and
compared to the internal 100 mV reference. If the current
sense threshold is exceeded, the comparator resets the lower
latch and terminates output switch conduction. The value for
the sense resistor is: