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LM2831: Power management forum

stevenh
Guru 19965 points
Part Number: LM2831
Other Parts Discussed in Thread: ADS8528, TLV62565, LMR10515

Hello,

We have been having FPGA I/Os go bad and we think it might be caused by the 3.3V LM2831 regulator supplying power to it.   The maximum FPGA's maximum allowed I/O power supply voltage is 3.6V.

After power is initially applied, the output of an LM2831 goes up to 3.860V (as shown in the image below, Nominal voltage is 3.3V). The input is fed by the 5V output of another LM2831.

What can we do to reduce the voltage overshoot?

Thanks,

Stephen

I am using an LM2831 to generate 3.3V.  The input of the LM2831 is supplied by 5V from the output of another LM2831.  

  • 0
    TI__Mastermind 20440 points

    Hi Stephen, 

    Could you please provide a schematic of the system. Also, could you please specify the input voltage of the first LM2831 and maximum load being used in both devices.

    Regards,

    Febin

  • 0 in reply to Febin
    Guru 19965 points

    Hello Febin,

    Please see power circuit below.  I was wrong about 5V being generated by an LM2831.  Vin is 8V to 10V AC.

    3.3V supplies a 28335 microcontroller, an XC7A35T FPGA , RTC, EEPROM, 3 ADS8528 and several other chips. 

    Stephen

  • 0 in reply to stevenh
    TI__Mastermind 20440 points

    Hi Stephen, 

    So the LM2831 is powered by 8V to 10V AC? LM2831 is a step-down DC-DC converter. Even if it was a typo, 8V to 10V is exceeding the input voltage range of the device.

    Regards,

    Febin

  • 0 in reply to Febin
    Guru 19965 points

    Hello Febin,

    The regulator used for P5 is not an LM2831.  It is a OKI-785R-5/1.5-W36-C.

    Below are the P3.3 regulator's Loaded startup voltage and current. and No-Load startup Voltage (P3.3 disconnected from rest of circuit)   

    The person performing the test used a five turn conductor to measure the current, so divide the current by 5 to get the actual value. As shown below, the current went up to about 2 Amps.  The steady state current is around 151mA.

    Stephen 

    P3.3 Loaded Startup Current (blue) and Voltage (Pink):

    P3.3 No-Load Startup Voltage:

    :

  • 0 in reply to stevenh
    TI__Mastermind 20440 points

    Hi Stephen,

    Thank you for the details. Please provide the complete part number of the device.

    Regards,

    Febin

  • 0 in reply to Febin
    Guru 19965 points

    LM2831XMF/NOPB 

  • 0 in reply to Febin
    Guru 19965 points

    Hello Febin,

    Is there a similar part (same footprint) that we could use?

    Stephen

  • 0 in reply to stevenh
    TI__Mastermind 20440 points

    Hi Stephen,

    Unfortunately this is a very old part. We do not have a drop-in replacement for LM2831XMF/NOPB.

    But we have some really good new devices like TLV62565/6 or TPS6282x. But the customer shall be responsible to carefully check the device & test it for its suitability in your complete application. Please let me know if you need any further assistance.

    Regards,

    Febin

  • 0 in reply to Febin
    Guru 19965 points

    Ok, we might try it.

    Do you know why the LM2831 is given such a high overshoot at startup?

  • 0 in reply to Febin
    Guru 19965 points

    LMR10515YMFX/NOPB  is a drop in replacement.  Do you think it will work for our application.

  • 0 in reply to stevenh
    TI__Mastermind 20440 points

    Hi Stephen,

    Could you please give the Iout used. I assume Vin is 5V and Vout is 3.3V

    Regards,

    Febin

  • 0 in reply to Febin
    Guru 19965 points

    On the schematic I previously posted, Vin was 8 to 10V AC and that supplied a OKI-7858-5/1.5-W36-C regulator. The output of that
    regulator supplied 5V to the LM2831 and the LM2831 was configured to output 3.3V.

    The LM2831's nominal output current is 151mA.  At startup, it jumped to almost 2A (see image in previous post).

  • 0 in reply to stevenh
    TI__Mastermind 20440 points

    Hi Stephen,

    Please be informed that I am discussing with my team regarding this. Unfortunately, I do not have the EVM and could not take the measurements for the device. I will get back to you early next week.

    Thank you for your understanding!

    Regards,

    Febin

  • 0 in reply to Febin
    TI__Mastermind 20440 points

    Hi Stephen,

    After a careful analysis, I think the problem is with the filter design.

    1. Referring to equation (7) in the datasheet, substituting values I get an inductance of 1.2 µH with 20% inductor ripple current. Please consider Iout as 1.5A as the design is based on the maximum output current of LM2831. Vin is 5V and Vout is 3.3V for LM2831 as per the schematic.

    (You may have considered Iout to be 151mA and hence arrived at a higher inductance value as seen from the schematic)

    2. Secondly, the output capacitor is one of the two external components that control the stability of the regulator control loop, most applications will require a minimum of 22 µF of output capacitance. As seen from the Design Example 3 in the datasheet, the output capacitance can go upto 2 x 22 µF if necessary.

    3. Inrush current at start-up will depend on the output capacitor selection.

    4. By reducing the filter size, the control loop will get faster and the voltage overshoot will also decrease.

    5. In any case, we recommend you to use our newer faster devices like TPS62825x for better performance.

    Please use the recommended values for inductor and capacitors and test the application again. Please let me know your feedback.

    Regards,

    Febin

  • 0 in reply to Febin
    TI__Mastermind 20440 points

    Hi Stephen,

    I would like to follow up on this. Do you have some updates?

    Regards,

    Febin

  • 0 in reply to Febin
    Guru 19965 points

    LMR10515YMFX/NOPB is a drop in replacement.  What do you think about it?

  • 0 in reply to stevenh
    TI__Mastermind 20440 points

    Hi Stephen,

    The problem seems to be with the filter design and this is applicable even if you choose LMR10515.

    Regards,

    Febin

  • 0 in reply to Febin
    TI__Mastermind 20440 points

    Hi Stephen,

    Could you change the filter design or try with a newer device?

    If you have the same problem with our newer device or if you may require any assistance on the alternate device suggested please open a new request.

    Would you then be okay to close this post now?

    Regards,

    Febin

  • 0 in reply to Febin
    Guru 19965 points

    We found out what was causing the original issue that caused me to post this question, i.e. I thought the high voltage overshoot was damaging a device, but it was not.

    I don't have time to look into this issue right now, so I think you can close this thread.