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ADM1278 Startup Problems

Question asked by ouradar2 on Mar 1, 2018
Latest reply on Mar 5, 2018 by Sal.Afzal

Hello,

 

I am currently working with the ADM1278-1 and I am having a very difficult time getting it to work.  I spent a lot of time working through the datasheet when I designed my board and I have checked just about all of my values and measured my current limits and so on.  I have attached my schematics in the hope that someone can take a look at them for me.  The behavior I am seeing is as follows:

 

-- The 12V rail never comes up.  The FAULTn pin stays LOW (fault) always.  Even if I provide a rising edge on the enable pin, the FAULTn pin stays low.  I can't even see it glitch high on my oscilloscope at any point.

-- I wired the RETRYn pin to ground to force the ADM1278 to attempt to retry the power-on sequence.

-- Each time the ADM1278 attempts to restart, I can see the voltage on the output side of my FET rise extremely quickly.  It gets to around 3.3V in 200us and then that's when the ADM1278 appears to initiate gate shutdown.  I also observed that the TIMER pin is active during this time.  At the exact same time that the FET starts to turn on, the TIMER pin starts to charge the capacitor (C251 in my schematic).  The timer pin reaches the 1V threshold in 200us and then the ADM1278 shuts down the FET.

 

Why is the TIMER pin active during the inrush period?

 

Any would be greatly appreciated.  Thank you

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