NCP1237 datasheet(17/34 Pages) ONSEMI

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NCP1237

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Each time the controller is starting (i.e. the controller was

off and starts, or restarts, when VCC reaches VCC(on)), a

soft−start is applied: the current sense setpoint is linearly

increased from 0 (the minimum level can be higher than 0

because of the LEB and propagation delay) until it reaches

VILIM (after a duration of tSSTART), or until the FB loop

imposes a setpoint lower than the one imposed by the

soft−start (the 2 comparators outputs are OR’ed).

Figure 12. Soft−Start

Time

VFB

VFB(fault)

Time

Soft-start ramp

VILIM

tSSTART

Time

CS Setpoint

VILIMI

VFB takes

over soft-start

Under some conditions, like a winding short−circuit for

instance, not all the energy stored during the on time is

transferred to the output during the off time, even if the on

time duration is at its minimum (imposed by the propagation

delay of the detector added to the LEB duration). As a result,

the current sense voltage keeps on increasing above VILIM,

because the controller is blind during the LEB blanking

time. Dangerously high current can grow in the system if

nothing is done to stop the controller. In order to protect

against this, an additional comparator is included, that

senses when VCS reaches VCS(stop) ( = 1.5 x VILIM ). As soon

as this comparator toggles, the controller immediately enters

the protection mode (latched or autorecovery according to

the chosen option).

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