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LMZ14202: Enable Input used for Low Voltage output protect

Part Number: LMZ14202

We are performing a Failure Modes and Effects Analysis on a circuit for a device General Electric Transportation sells that performs remote control of locomotives and the device in question is the LMZ14202.  The datasheet covers output overvoltage protection. Okay, good for the failure mode of “output voltage drifts high” we have that failure mode covered.

The other failure mode of concern is “output voltage drifts low”.  Here is the text description text of the under voltage protection in the circuit design.

“EN signal generated from PWR signal or the microcontroller binary output UC_PWR.  Battery is rated for 9V to 12.6V. Assuming the battery is brand new and is 12.6V, the voltage divider will have 2.67V for EN.  This is at powerup time.  This enables U8 to generate 3.3V the microcontroller uses.  Once the microcontroller is powered up, it sends output UC_PWR which is a 3.3V signal.  From here on, U8 is using this signal for under voltage protection.” 

 

I pasted below some screen captures of parts of the schematic that pertain to this description

This does not quite make sense, because while true if the microcontroller drops UC_PWR, the voltage on EN will drop from 3.3 to 2.67, which does not meet the 1.09 V falling threshold per section 8.2.2.1.1 in the datasheet.  
 
So, my question is, will this scheme work for under voltage protect on the VOut from the LMZ14202 device?

Thanks and Best Regards,

Tom

Tom Woodbridge

GE Transportation

Digital Solutions – Train Performance

Senior Product System Safety Engineer

T: 321-435-7759

E: thomas.c.woodbridge@ge.com

 

1990 W. NASA Blvd.

Melbourne, FL 32904 USA

General Electric Company

 

  • Hi Tom,

    Can you reattach the files on this thread? I am having difficulties looking at your pasted schematics. 

    Regards,

    Jimmy 

  • In reply to Jimmy Hua:

    Hi Jimmy,

    Trying again with the attachments.  Trying fresh screen captures from the PDF file.

    Happy New Year!

    Tom

  • In reply to Tom Woodbridge:

    Hi Tom,

    After looking at your schematic, I got a better understanding of your system. Yes your assumption is correct. The falling threshold for the EN is 1.09V. Since EN is internally pulled to Vin, your system will see a voltage difference of 3.3- Vbat*(Renb/(Rent+Renb)). In your case, that is 3.3V - 12.6*(27/127)V = 0.62V and not trip the EN. You can increase the Rent which would decrease the EN voltage during power up. That way, after uC is powered to 3.3 the falling voltage difference would be greater than 1.09V when the uC turns off.

    Regards,
    Jimmy

  • In reply to Jimmy Hua:

    Thank you for confirming my assumption Jimmy.

    Best Regards,
    Tom

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