Offline Flyback: NCP1239 datasheet interpretation?

[cupoftea]

Hi,
We are using the NCP1239P65 in a bog standard offline flyback as attached.
For the Fault pin, page 20 of the NCP1239 datasheet appears to suggest that the Fault pin draws in a
current of 1.2mA whenever the Fault pin is taken above 3V?. Would you say this is a correct interpretation
of the datasheet?

NCP1239 datasheet:

https://www.onsemi.com/pdf/datasheet/ncp1239-d.pdf

 

[KlausST]

Hi,

I expect the 1.2mA to be the nominal current that needs to be applied to the FAULT pin.
So, in order to SAFELY activate the OVP function I´d apply "1.2mA PLUS SOME MARGIN"

Let´s say your external circuit activates at about 36V.
Let the voltage across CE of Q4 be 0.5V
so the current through R32 is (36V - 0.5V - 3V) / 27k = 1.20mA
The extra current through R23 is 3V / 22k = 0.14mA
leaves a pin current of only 1.06mA

In my eyes not a safe level to activate OVP.

***
The max pin voltage is 5.5V ... thus you may apply (5.5V - 1.35V) / 1.35kOhm = 3.07mA (nom)

--> I see no problem in driving the PIN with 1.5mA...2.0mA (at least 20% margin)
If you still want to protect the pin you could add an external 4.7V zener.

****
Disclaimer: I did not read the complete datasheet. Thus I may have missed some informations.

Klaus

 

[FvM]

Wonder why are you using a zener, 5 resistors and 2 transistors where the datasheet application uses a zener and an optional series resistor for OVP fault detection?

Datasheet says:

To reach the upper threshold, the external pull-up current has to be higher than the pull-down capability of the clamp, i.e. 1.2 mA.

Unfortunately there's no maximum rating for current into fault pin, you may want to ask Onsemi for a specification.

 

[mtwieg]

Can't make sense of figure 39...
If there really is a 1.35V zener on that node, then it's impossible for the OVP comparator to ever trip (threshold is 3.0V nominal)
I think that Rfault(clamp) must actually be in series with the Vfault(clamp) zener, rather than in series with the Fault pin.