400EAFAB1 datasheet(42/47 Pages) ONSEMI

42 / 47 page

NCP12400

www.onsemi.com

FAULT Input

Figure 74. OVP/OTP Detection Schematic

The FAULT input pin is dedicated to the latch−off

function: it includes 2 levels of detection that define a

working window, between a high latch and a low latch:

within these 2 thresholds, the controller is allowed to run, but

as soon as either the low or the high threshold is crossed, the

controller is latched off. The controller can be released from

the latch mode by the autorecovery, but it depends on the

version of the product. The lower threshold is intended to be

used with an NTC thermistor, thanks to an internal current

source INTC.

An active clamp prevents the voltage from reaching the

high threshold if it is only pulled up by the INTC current. To

reach the high threshold, the pull−up current has to be higher

than the pull−down capability of the clamp (typically

1.5 mA at VOVP).

To avoid any false triggering, spikes shorter than 50

(for the high latch and 65 kHz version) or 350

ms (for the low

latch) are blanked and only longer signals can actually latch

the controller.

Reset occurs when a brown−out condition is detected or

the VCC is cycled down to a reset voltage, which in a real

application can only happen if the power supply is

unplugged from the ac line.

Upon startup, the internal references take some time

before being at their nominal values; so one of the

comparators could toggle even if it should not. Therefore the

internal logic does not take the latch signal into account

before the controller is ready to start: once VCC reaches

VCC(on), the latch pin High latch state is taken into account

and the DRV switching starts only if it is allowed; whereas

the Low latch (typically sensing an over temperature) is

taken into account only after the soft−start is finished. In

addition, the NTC current is doubled to INTC(SSTART) during

the soft−start period, to speed up the charging of the FAULT

pin capacitor The maximum value of FAULT pin capacitor

is given by the following formula (The standard start−up

condition is considered and the NTC current is neglected):

CFAULT max +

tSSTART min @ INTC(SSTART) min

VOTP max

3.2 @ 10−3 @ 60 @ 10−6

0.420

F + 457 nF

(eq. 4)

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