LTC4267-1 POE 12V design Output issue

[moonsunshoot]

Hi all,

I have done the design as per the LTC4267-1 application circuit and I am using the PA1135NL 12V output transformer, But Getting 3.3V output only, Can anyone please help with this issue. I have attached the design here.

[fcb]

Immediate suspicion is that R23/R29 network is wrong.

Quick calculation shows that it should regulate VCC12V0 to about 6.6V, but this is not 3.3V as you have found.

Probably check all your rails (including PVCC), scope out ITH/RUN pin and check switching frequency assumptions.  Post pic of PCB, layout, etc..  Does the 3.3V hold up under load? Is it regulating correctly?

[moonsunshoot]

I'm getting constantly exact 3.3V, even output is connected to load. i have changed PA1135 to PA1133 transformer also, but still 3.3V!

[fcb]

Ha. Just looked up the datasheet for the exact D14 (ZR431LF02TA).  This version of the venerable TL431 is 1.24V.

So at regulation you'll get 1.24V (+/- 2.5%) across R29 (60.4K), so 20.53uA flows through R29.  Which means you'll get the same current through R23, which in-turn means the VCC12V0 rail must be sat at 3.29V (1.24V+(20.53uA*100K)).

So your circuit is acting correctly.

If you want 12V, you need to change the R23/R29 ratio.

So perhaps change the 60.4K to 11.5K (E48 preferred value) and you should get 12.02V

[moonsunshoot]

Thank You fcb,

Let me make the changes in R29 as 11.5K and check the output, I hope should get the output 12V now!