NCP1237 datasheet(28/34 Pages) ONSEMI

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NCP1237

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Latch−off Input

Figure 26. Latch Detection Schematic

−

Latch

VOVP

−

VOTP

tLatch(OVP)

blanking

VDD

Brown−out

Reset

Latch

Vclamp

INTC

tLatch(OTP)

blanking

1 kW

INTC

Soft−start

end

The Latch pin is dedicated to the latch−off function. It

includes two thresholds 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 lower threshold is intended to be used

with an NTC thermistor, with the internal current source

INTC providing the necessary bias current.

An active clamp prevents the voltage from reaching the

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

reach the high threshold, the pullup current has to be higher

than the pulldown capability of the clamp (typically 1.5 mA

at VOVP).

To avoid any false triggering, noise spikes shorter than

tLatch(OVP) or tLatch(OTP) respectively are blanked, and only

longer events can actually latch the controller.

Reset occurs when a brown−out condition is detected or

the VCC is cycled down to a VCC(reset), 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 reaching their nominal values; and one of the

comparators could toggle inadvertantly. Therefore the

internal logic ignores the latch signal before the controller is

ready to start. Once VCC reaches VCC(on), the latch pin High

latch state is enabled and the DRV switching starts only if it

is allowed; whereas the Low latch (typically sensing an

overtemperature) is taken into account only after the

soft−start is finished. In addition, the NTC current is doubled

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

Latch pin capacitor.

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