UCC28610
www.ti.com ......................................................................................................................................... SLUS888C–JANUARY 2009–REVISED SEPTEMBER 2009
Maximum On-Time and Brown Out
The forced DCM feature provides protection against excessive primary currents in the event that the input
voltage becomes very low. The highest possible secondary currents can be described by Equation 13. The
UCC28610 adds further protection by allowing the user to program the maximum on-time.
The Maximum On-Time (MOT) function causes the converter to react as if there is an overload condition if the
load is sufficiently large during a line sag condition. During low line conditions the MOT function limits the on-time
of the primary switch which limits the peak current in the primary power stage. Figure 33 shows how the MOT
period, tMOT, is programmed over the range of 1.5 µs to 5 µs for either range of programming resistors. The
resistor range determines the controller’s response to a sustained overload fault – to either Latch-off or to
Shutdown/Retry, which is the same response for a line-sag, or brown out, condition.
External Shutdown Using the MOT Pin
Many applications require the ability to shutdown the power supply with external means. This feature is easily
implemented by connecting the collector and emitter of an NPN transistor between MOT and GND, respectively.
The NPN transistor can be the photo-transistor of an opto-isolator for isolated applications.
tMOT
RMOT
k
k
k
k
Figure 33. Programming MOT and Overload Fault Response
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