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TPS92520-Q1: CSN Protection Diode Recommendation

Jeff Yun
Expert 3005 points
Part Number: TPS92520-Q1

Hi, Team

For TPS92520 circuit design, datasheet recommends to use a Schottky diode to protect the CSP / CSN node.

“A Schottky diode with low forward voltage drop at room-temperature and nonrepetitive peak surge current rating of 10 A for duration of 5 μs is recommended.”

But in our EVM design, it selected "1N4148WS-7-F" which is not a Schottky diode and VF is 75V. This doesn't look like it fits the requirements in the datasheet.

Why did you selected this diode in the EVM design?

Could you please kindly help to recommend a suitable model for this diode?

Thanks & Best Regards

Jeff

  • 0
    TI__Mastermind 20695 points

    Hello Jeff, 

    In the datasheet we recommend a schottky because it offers more protection, but we also state the following:

    The key care about is that we bypass any energy from an the inductance inherent in long thin cables using the diode.  It can be done with both fast recovery or schottky diodes.  Schottky's are faster and also offer lower voltage clamping but they are more expensive.  They can use a fast diode, but they better test it to ensure it provides the protection needed.  I tested the 1N4148WS-7-F on my EVM with 3 meter, 28 AWG cables, and it offered the protected needed to avoid damaging the part and ensuring the devices doesn't see voltages that violate abs max of the part. 

    -fhoude  

  • 0 in reply to fhoude
    TI__Expert 3005 points

    Hi, fhoude

    Thank you for your explanation.

    Could you please provide the test conditions and test methods?

    My customer also wants to test the behavior of the diodes on their own boards.

    Thanks & Best Regards

    Jeff

  • 0 in reply to Jeff Yun
    TI__Mastermind 20695 points

    Hello Jeff, 

    We used 28 AWG, 3 meter cables between EVM and LED load board.  The output capacitor of the EVM was 0.22uF.  A hard mechanical short was applied at the LED load at the end of the 3 meters.  This is a very fast short and is a worst case.  You need to look at the output at the EVM (preferably at the device) when this happens to observe what voltage is seen by our device.  Without diodes the output could go negative to as much as 10V below ground.  Make sure they repeat this multiple times and try to see the best and worst case waveforms.    

    -fhoude

  • 0 in reply to fhoude
    TI__Expert 3005 points

    Hi, fhoude

    Could you please kindly provide a test report for customer reference?

    Thanks & Best Regards

    Jeff

  • +1 in reply to Jeff Yun
    TI__Mastermind 20695 points

    Hello Jeff,

    I am NOT going to provide our internal evaluation report to customers, this is proprietary and confidential.  I explained what they had to do.  It is quite simple, where is the confusion on how to do it?  

    Here is diagram of setup and where to probe.

    TPS92520EVM long lead short testing.pdf

    I also wanted to be clear that they should be looking at waveforms for both SHORT applied and SHORT removed.  

    Here is an example waveform when no diodes are used and there are long cables.  

    You see the VLED (output voltage) goes well below ground during ringing, which can cause damage to an IC because it violates the negative abs max of the pins.  

    -fhoude

  • 0 in reply to fhoude
    TI__Expert 3005 points

    Hi, fhoude

    Thanks for your detailed explanation~

    Jeff~