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Original question:

LM5176: LM5176 Output Failure – Buck-Boost Mode Issue (VIN < 20V)

LM5176-Q1: Power output failure for no reason - After recovered by external supply to VCC regulator

Soundhar B
Prodigy 50 points
Part Number: LM5176-Q1
Other Parts Discussed in Thread: LM25149-Q1, LM5176

Screenshot from 2025-07-11 12-33-16.png.pdf

Tool/software:

Hello everyone, We have designed a buck-boost power converter using LM5176-Q1. Specifications are below,

Vin = 12V and 24V

Vout = 24V 

O/P current = 8A

Switching frequency = 284 kHz

Hiccup mode enabled. 

Attached the schematic picture

For the past 4 months, daily we use this converter, we turn on and turn off multiple times, but today we turned on, but it did not turn on, the voltage output is 4V when 12V input is given, it should be 24V, when it was working it was giving stable 24V. But now it is giving 7V if i increase the input voltage gradually output voltage also increasing, it didnt work on boost mode, seems buck mode is working. Then i measured the VCC regulator output, it was 4V. After i gave the external 7V supply to the VCC and it started working. After we have not faced that no output issue. In 2 boards this problem happened. We still dont know that how this Controller ic is going into some sleep mode or some latched mode. After giving external VCC it started working. But need a permanent solution for this, why this happened and what can be the reason for this particular behaviour, the same i have seen in LM25149-Q1 also, that is a buck controller after giving external VCC it will work, but why this permanent latch is happening first of all. Why that VCC regulator is not coming up ? since there is no change in operation of our application.

Also in this LM5176-Q1 buck boost converter with the specs i mentioned, when we give 12V the inductor is heating up too much , but when i give 24V inductor is not heating. Need guides on this and the problem i mentioned.

Thanks

Soundhar B

  • 0
    TI__Guru 58641 points

    Hi Soundhar,

    first it would be good to identify the damage component on the PCB.

    I suggest to replace the LM5176 first and check if the boards working again.

    To further investigate on the root cause of the issue it would be good to have some measurement from a working PCB:

    • first let check the SWx for under and overshoots.
      probe that nodes (ensure a very short GND loop of the probe) and check the rising and falling edges in different load and input voltage conditions.
      please share the scope plots here.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 50 points

    Hello Stefan, There is no damage in any of the components. Everything is working fine once i give external VCC supply for a moment and taken it out. After that i dont need to do anything even after i turn off and turn on. But why this latched behaviour ? Since there is no damage in the component, it is working fine. Earlier we measured the plots with scope, everything was normal. Now I can probe and i can share those. Meanwhile need a solution for this peculiar issue.

    Thanks.

    Soundhar B

  • 0 in reply to Soundhar B
    TI__Guru 58641 points

    Hi Soundhar,

    when the LM5176 is supplied with > 8V VCC should be at ~ 7V as well. 

    If you do not get this, I still assume something in the LM5176 got damaged.

    Waiting for the scope plots.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 50 points

    Hello Stefan, If we give 12V input to the LM5176, still the voltage of the VCC is 3.9 to 4V only. 

    I hope you understood the trick which i did. I supplied externally 7V to VCC for a second and taken that out. After that, it is working perfectly alright. The VCC voltage is also came to 7.1V. Even after i turn it off and turn it on again and again. I dont need to supply externally anything to the VCC after that one time. I am wondering if the LM5176 failed then how it will work perfectly with all the voltage and current requirements, once i give external VCC for a second ? Right now 2 boards failed out of 6. I recovered them by giving external VCC once. Right now we dont have failed ones. That is why i came here. still i can share the working board scope plots .

    Thanks.

    Soundhar B

  • 0 in reply to Soundhar B
    TI__Guru 58641 points

    Hi Soundhar,

    Understood now =- this is really a strange behavior I have never seen before.

    So what happens to VCC if you keep EN/UVLO at 0.8V when powering up.

    Does it also work with an input voltage of 24V (device should then operate in BuckBoost mode with controlling all four switches)?

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 50 points

    Hello Stefan,

    Always we keep the EN/UVLO voltage more than its threshold. We kept the UVLO point to operate the buck-boost converter from 4.5V itself. So anyway at 12V input for sure it will be more than 0.8V. Also it will work at 24V input.

  • 0 in reply to Soundhar B
    TI__Guru 58641 points

    Hi Soundhar,

    Sure you have EN/UVLO above 0.8V - I would like to know 
    what happens to VCC if you keep EN/UVLO at 0.8V when powering up.

    Thanks,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 50 points

    Hello Stefan, I will do that test and let you know the results.

  • 0 in reply to Soundhar B
    Prodigy 50 points

    Hello Stefan, When EN voltage is at 0.8V, VCC is 2.8 to 3.4V. My input voltage is 4V. So anyway it wont effect right ? so VCC will be less than 4V.

  • 0 in reply to Soundhar B
    TI__Guru 58641 points

    Hi Soundhar,

    please use a supply of 12V for VIN but keep EN/UVLO at 0.8V e.g. by forcing this pin to that voltage by another supply.

    you can also adjust the resistor divider but ensure the VIN is well above 8V.

    Thanks and sorry for the confustion.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 155 points

    Hi Stefan, This is Malakondareddy - working with Soundhar B, We have kept Voltage at EN =0.8V exact and Vin = 12V, now VCC is 4.68V.

  • 0 in reply to Malakondareddy B
    TI__Guru 58641 points

    HI Malakondareddy,

    as you can see the VCC does not reach the expected output voltage level of ~ 7V.

    This can have two reasons:

    - the internal LDO got damaged

    - some external component supplied by VCC is adding more load then the LDO can supply.

    I suggest to replace the LM5176 by another device and check if the error is gone.

    If not some other component are giving the issue.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 155 points

    Hi Stefan schauer,

    Thanks for your reply,

    We have checked with the new device and even in the fresh piece of LM5176, we did not get the VCC of 7V, all were in the range of 4.5 to 5V only. Unable to identify where the issue is. We want to connect with you through an online meet, so that we can understand the issue more better, since the boards are in production level, we need to get the problem sorted quickly and it will be helpful to identify the solution faster. Please let me know the availability of yours. Thanks
    Soundhar B

  • 0 in reply to Malakondareddy B
    TI__Guru 58641 points

    HI Malakondareddy,

    as you just confirmed by replacing the LM5176 with still seeing the same issue the root cause of this issue seams to be outside the LM5176.

    So I suggest to remove part by part components which are connected or powered by VCC of the LM5176 and check with the same method once you get a proper output voltage on the VCC pin of ~ 7V.

    Unless you have any additional connection to VCC of the LM5176 this would be:

    - the 4 MOSFETs of the power stage

    - all components connected to HDRV1, HDRV2, BOOT1, BOOT2, SW1, SW2, VCC pin

    Best regards,

     Stefan