400EADBA0 datasheet(35/47 Pages) ONSEMI

35 / 47 page

NCP12400

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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. That’s what the additional comparator, that

senses when the current sense voltage on CS pin reaches

VCS(stop) ( = 1.5 x VILIM ), does: as soon as this comparator

toggles, the controller immediately enters the protection

mode.

In order to allow the NCP12400 to operate in CCM with

a duty−ratio above 50%, the fixed slope compensation is

internally applied to the current−mode control. The slope

appearing on the internal voltage setpoint for the PWM

comparator is −32.5 mV/

ms typical. The slope compensation

can be observable as a value of the peak current at CS pin.

The internal slope compensation circuitry uses a saw−tooth

signal synchronized with the internal oscillator is subtracted

from the FB voltage divided by KFB.

Figure 65. Slope Compensation Block Diagram

Figure 66. Slope Compensation Timing Diagram

Duty Cycle

Internal PWM setpoint

40%

100%

80%

VFB / KFB − 0.2 V

VFB / KFB

400EADBA0 Datenblatt(PDF) 35 Page - ON Semiconductor