LNK304PN help needed

Posted by: kn on 05/21/2013

Hi,

I am looking for help what would be the easiest way to test LNK304PN if it is fine or not. Currently I was replacing one of them on the hardware board as it was damaged, but on board regulator still does not work. I did measure 10.5V accross BP-S and 2.7V between FB and S in this application. I wonder if 10.5V I am observing is correct as my expactation would be to see 5.8V (or something close to it).

I took out a device from board and now want to test it to see if part is faulty or if it a problem with the board. Is it possible to check if internal regulator in LNK does operate correctly? Should I expect roughly 5.8V across Bp-S in the test circuit as per Figure 7 of datasheet when I will leave floating FB pin? Is there any other way to test LNK304PN?

Thank you

Comments

The BP pin voltage can not be as high as 10.5V. This part used a lot by diferent customer, so I d onot think it is caused by the part itself. Something is wrong around your external bias circuit. PLS open the external bias circuit for BP pin and test the BP pin voltage again.Without the external bias, the BP pin voltage should be around 5.8V.

PLS upload a schematic, so I will have more idea.

regards

Hi,

Thank you for a prompt reply.

I thought this is not correct. Anyway made my reference circuit where I did a test of part (rather than on failing board) and saw part not switching at all. After replacing it by the new one I saw switching of power mosfet and also about 5.9V at BP pin. So my conclusion is that first part was completely dead.

Looks like you solve the problem already. The key is why the part dead. I know the part could be dead since the BP pin voltage is too high.

regards

This is also interesting to me. 10.5V I did see across BP-S in the board I was fixing was due to external 10.5 Zener diode placed in BP-OUT path and used as part of a feedback loop (so I guess without this diode I would see even more). I am not sure if I can publish schematic as it is a "washing machine" board and I could not find "official" schematic of regulator in there so made it myself looking at board and components.

Behavior of part was like source having no connection from S to the external. I would not be surprised if part died because of ESD as when was buying it it came in regular non-conductive plastic bag ...