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NCP1282

High Performance Active Clamp/Reset PWM Controller Featuring 500 V Startup

ON Semiconductor 

NCP1282

(VEA is not controlling the duty cycle)

VEA

VSS

FF Ramp

Figure 36. Soft−Stop Behavior After Soft−Start

is Complete and VEA is Open.

If the voltage on the VAUX pin reaches VAUX(off2), CSS is

immediately discharged and the outputs are disabled. VSS

should not be pulled up or down externally.

CURRENT LIMIT

The NCP1282 has two overcurrent modes,

cycle−by−cycle and cycle skip, providing the best

protection during momentary and continuous overcurrent

conditions.

Cycle−by−Cycle

In cycle−by−cycle, the conduction period ends once the

voltage on the CS pin reaches the current limit voltage

threshold (VILIM). The NCP1282 has a VILIM of 0.5 V.

Cycle Skip

Traditionally, a voltage on the CS higher than VILIM has

been used to trigger a cycle skip fault. Unfortunately, the

fast response time of modern controllers makes it hard to

reach a voltage on the CS pin higher than VILIM.

Instead of using a higher voltage threshold to detect a

cycle skip fault, the NCP1282 uses a timer. It monitors the

current limit comparator and if a continuous

cycle−by−cycle current limit condition exists the converter

is disabled. The time to disable the converter and the time

the converter is disabled are programmed by the capacitor

on the CSKIP pin, CCSKIP.

The cycle skip detection circuit charges CCSKIP with a

continuous 100 mA current once cycle−by−cycle current

limit fault is detected. If the current limit fault persists,

CCSKIP continues to charge until reaching the cycle skip

upper threshold (VCSKIP(peak)) of 3.0 V. Once reached, the

converter enters the soft–stop mode and CCSKIP is

discharged with a constant 10 mA current. A new soft−start

sequence commences once CCSKIP reaches the lower cycle

skip threshold (VCSKIP(valley)) of 0.5 V. If the overcurrent

condition is still present, the capacitor starts charging on

the next current limit event. Otherwise, CCSKIP is

discharged down to 0 V.

The cycle skip capacitor provides a means of

remembering previous overcurrent conditions. If a

continuous overcurrent condition is removed before

reaching VCSKIP(peak), CCSKIP starts a controlled

discharge. If the continuous overcurrent fault is once again

detected before CCSKIP is completely discharged, CCSKIP

charges from its existing voltage level, taking less time to

reach VCSKIP(peak). Figure 37 shows operating waveforms

during a continuous overcurrent condition. For optimal

operation, the cycle skip discharge time should be longer

than the soft−stop period.

VILIM

CS

VCSKIP(peak)

VCSKIP

VCSKIP(valley)

VSS

Figure 37. Cycle Skip Waveforms

In some instances it may be desired to latch (instead of auto

re−start) the NCP1282 after a cycle skip event is detected.

This can be easily achieved by adding an external latch.

Figures 35 and 36 show an implementation of an integrated

and a discrete latch, respectively. In general the circuits work

by pulling CSKIP to VREF, preventing it from reaching

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