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TPS65131: Start-up problem

Kuramochi Tadahiko
Guru 28715 points
Part Number: TPS65131
Other Parts Discussed in Thread: LM2941

Hi,

My customer is evaluating TPS65131 on their board, however, positive-rail doesn't start-up properly when PSP/PSN pin is Low as below;

PSN/PSP: Low

PSN/PSP: High

VIN: +5.4V, VOUT=±13V

What is this caused by?

Best Regards,

Kuramochi

  • 0
    TI__Mastermind 18550 points

    Hello,

    This is very strange and does not make sense to me that tying PSP/PSN to GND causes the positive rail to not come up properly.

    Can you please share schematics of the customer design?

    Also, please check if the input supply of of 5.4V is somewhat current limited. I suggest to please scope probe input supply on a second scope channel at the same time as positive rail is coming up.

    Kind Regards,

    Liaqat

  • 0 in reply to Liaqat Khan
    Guru 28715 points

    Hi Liaqat-san,

    Thank you for your reply.

    I sent a friend request because I would like to send a schematic in a private message.

    Could you accept it?

    Best Regards,

    Kuramochi

  • 0 in reply to Kuramochi Tadahiko
    TI__Mastermind 18550 points

    FYI, I accepted it just now.

    Kind Regards,

    Liaqat

  • 0 in reply to Liaqat Khan
    Guru 28715 points

    Hi Liaqat-san,

    Thank you for your acceptance.

    I sent a message.

    Best Regards,

    Kuramochi

  • 0 in reply to Kuramochi Tadahiko
    TI__Mastermind 18550 points

    Hi Kuramochi-san,

    I reviewed the schematics and scope plots you sent and they look good. So I am puzzled as to why disabling the power save mode (tying PSP/PSN to GND) exhibits this issue. I feel like there is some system level sensitivity causing this issue but it is hard to say what it is. The one thing I noticed in the schematics is that the output capacitance is much bigger compared to typical application recommended value of 22uF. Can you please try it by removing three of the 22uF capacitors at the output to see if this changes the behavior in any way.

    Kind Regards,

    Liaqat

  • 0 in reply to Liaqat Khan
    Guru 28715 points

    Liaqat-san,

    Thank you for your support.

    I sent the block diagram via private message.

    The start-up problem is removed by eliminating 300uF capacitor.

    So I guess the cause of this problem is inrush current because LM2941 does not have soft start function.

    Why does inrush current occur this problem?

    Best Regards,

    Kuramochi

  • 0 in reply to Kuramochi Tadahiko
    TI__Mastermind 18550 points

    Kuramochi-san,

    I saw the block diagram and without soft-start of LM2941, TPS65131 sees a huge capacitance at its output. Recommended capacitance in typical applications is 22uF and even 44uF should not be a problem but your application has much bigger capacitance. I am thinking that perhaps TPS65131 output voltage is overshooting at startup causing the over voltage protection to kick in and shutdown the device.

    Kind Regards,

    Liaqat

  • 0 in reply to Liaqat Khan
    Guru 28715 points

    Hi Liaqat-san,

    Why does the problem occur only when PSP/PSN pin is Low?

    Best Regards,

    Kuramochi

  • +1 in reply to Kuramochi Tadahiko
    TI__Mastermind 18550 points

    Kuramochi-san,

    I am not able to find internal circuit information that explains why this happens when PSP/PSN is low but it is could be due to slightly faster ramp up when power save mode is disabled (PSP/PSN=low). I have a feeling that this can also happen when PSP/PSN is high under some other operating condition. Regardless, I think very high capacitance is causing bigger overshoot so that OVP kicks in and I would recommend to reduce output capacitance. Another experiment that customer can do is to increase compensation capacitors values on CP/CN pins and measure overshoot to see if it is reduced enough so as to not cause OVP to kick in.

    Kind Regards,

    Liaqat