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LM51581-Q1: IC fries when loading output with 24 Ohms

Jacob Christ
Prodigy 10 points
Part Number: LM51581-Q1
Other Parts Discussed in Thread: LM51581, LM5157

Ti, please help,

I'm trying to setup the LM51581 for 4-60V in 3.3V out but when I add a 24 Ohm load the LM51581 fries before I get to 30V in. Unloaded I seem to regulate nicely up to 60V in.


I'm tiered of replacing chips at $4 a test.  My current thinking is that the saturation point of my inductor is too low which causes an issue that leads to IC failure.  I'm going to try a 60 ohm load next (55mA) and see if the chip pops.  I'll report back here, but I'm about on my wits end.

Also, I can't seem to find the FB Internal Vref voltage in the datasheet.  Based on the equation and resistors chosen in the excel sheet it seems that is 1V

LM5157-58_Excel_Quickstart_Calculator_for_SEPIC_4-60Vin (version 1).xlsb.xlsxDC-DC 4.5V-60V IN 3.3V OUT.pdf

C25 is not installed.

D6 (or equivalent):

DIODES INC. B120-13-F

L2:

COILCRAFT LPD3015-224MRC

Thanks is advance,

Jacob

  • 0
    TI__Guru 67755 points

    Hello Jacob,

    I am sorry that you have issues with the LM51581.

    First I have a question to the design: You are using a SEPIC converter for a design that seem to request a buck converter. What is the reason that you are designing a SEPIC converter?

    I am asking because a SEPIC converter is a difficult topology, especially when we are taking about stability. In addition, the efficiency of the converter seems to be below 60% according to the calculation of the Excel sheet. (I cannot see an efficiency curve in the picture.

    You mention that the load causes the converter to get damaged. Can you check the device temperature when there is no load at the output?

    Then you need to add slowly load to check when the device starts to get hot.

    Please check if the problem gets better if you increase the value of PNP-000089 between the 2 windings.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    Prodigy 10 points
    Brigitte said:

    First I have a question to the design: You are using a SEPIC converter for a design that seem to request a buck converter. What is the reason that you are designing a SEPIC converter?

    I choose SEPIC topology because although this is specifically outputing 3.3V, future projects may require 5V or other higher output voltages and I was trying to create a general purpose configuration that could have different output voltages by adjusting the voltage divider on the FB pin.

    Brigitte said:
    I am asking because a SEPIC converter is a difficult topology, especially when we are taking about stability. In addition, the efficiency of the converter seems to be below 60% according to the calculation of the Excel sheet. (I cannot see an efficiency curve in the picture.

    I'm trying to replace an analog solutions that is less than 20% efficient, I would be very happy with better than 50% but if you think buck would be even better I may try it specifically for this project and move back to SEPIC for later projects.

    Brigitte said:

    You mention that the load causes the converter to get damaged. Can you check the device temperature when there is no load at the output?

    In my reply to the first e-mail I've since changed my mind and am thinking that the inductor is what is failing.  I have saved some the "blown" IC's and at some point I'm going to try to very that they are still operational.  But the I have three of the coupled inductors that each have one of the two coils open. So I'm pretty sure that its the inductor that is failing and not the IC.  Also, the last time the circuit failed I replaced the inductor and not the IC and brought it back to life.

    Brigitte said:

    Please check if the problem gets better if you increase the value of PNP-000089 between the 2 windings.

    Ah, using the equation in section 3.2.1 of app note ASNVAA43 this should be >= to 19,000pf. and its currently 10,000pf.  I'll try adding a second or third in parallel.  There doesn't seem to be any guidance on the upper value for Cac in this section, any suggestions.

    Also, I'm noticing in this same app note it looks like my PIV for D6 is too low and I should be > 63.3V.

    Jacob

  • 0 in reply to Jacob Christ
    TI__Guru 67755 points

    Hello Jacob,

    Buck is in general the most efficient converter topology.

    Thank you for the feedback about the inductor.

    I think you found the best app note you can use. For the voltage of the rectifier, please do not forget to add margin due to the negative spike, see the app note you mention above. So 100V rated rectifier should be good in your case.

    Best regards,
    Brigitte