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LM5088MH-2EVAL: Device shutdown and restarts when load is removed

Part Number: LM5088MH-2EVAL

Tool/software:

Hi team,

This is a follow up of previous post LM5088MH-2EVAL: LM5088-2 Based DC-DC Buck Converter stuck in Hiccup mode with no load condition - Power management forum - Power management - TI E2E support forums. issue.

I’m encountering persistent problems related to output voltage fluctuations, load transients, and unexpected shutdowns.

To summarize System Specifications:

  • Vin = 36V
  • Vout = 32V (±1V)
  • Iout = 1.3A
  • Fsw = 250KHz
  • Cin, Cout = 390uF (Al-Electrolytic) + 22uF (Al-Electrolytic) + 10uF,0.47uF (ceramic x2)
  • L = 47uH

I’ve attached the latest modified schematic with test setup images for reference.

Latest 36V Sch.zipTest Setup.zip

Observed Issues & Troubleshooting Attempts:

  1. Continuous Voltage Swing (~143Hz) with 3V-6V Fluctuation
    1. This fluctuation persists until the device is turned off completely (Power Cycling)

    2. Occurs when:
      1. The load relay is switched ON.
      2. Sometimes randomly when the device is powered ON.
    3. Troubleshooting so far:

      1. Changed current sense resistor to 10mΩ → No improvement.
      2. Removed bulk 390uF capacitor and TVS diode → No improvement.
      3. Increased boot capacitor from 62nF to 109nF → No improvement
  2. Large Voltage Dip (~11.2V for 240µs) During Load Transient
  3. Momentary Shutdown (0V) When Load is Disconnected
    1. When the relay disconnects the load, the output voltage drops to 0V before recovering.
    2. Initially, the device recovered after 2 seconds.
    3. After reducing hiccup capacitor (Cres) from 2.2uF to 0.1uF, the device now recovers in ~100ms, but the momentary 0V shutdown still persists.
    4. Additional attempts:
      1. Reduced soft-start capacitor from 33nF to 10nF → No improvement.
      2. Swapped relay to rule out mechanical issues → No improvement.

Additional Observations:

  • Other LM5088-based converters (16V & 28V of same design) work fine in the same test setup.
  • To rule out test setup issues, we tested using a lab bench power supply (for 32V), and the setup worked fine.
  • The issue occurs only when using the LM5088-based 32V converter.

I would appreciate any insights or suggestions on Current Condition of these Issues:

  1. Load Transient issue, too much voltage dip need to minimize
    1. Scope Images Atached2.Load Transient.zip
  2. How to prevent Momentary device shutdown (0V) when load is disconnected, 
    1. Scope Images Atached3.Device Shutdown after disconnection of load -Initially.zip3.Device Shutdown after disconnection of load -Improved.zip
  3. How to eliminate Random Hiccups when power cycling (10V swings at output with ~143Hz rate)

    1. scope Images Atached1. Continios Voltage Swing at 143Hz.zip

Kindly let me know any suggestions, to improve these issues.

Thank you in Advance!!

  • Hello Arvind

    Continuous Voltage Swing :Looks like this is a normal operation at light load, or the system is unstable. The hiccup like waveform in 3. Device Shutdown after disconnection of load -Initially.zip is similar with this waveform. Please measure inductor current together with VOUT and check the issue is caused by instability or it is a normal operation at light load. 

    Large voltage drop during load transient : Looks like load is very inductive or load cable is long. Please use a resistive load with a short cable

    Unexpected shutdown : Please measure the EN, RES pin voltages together and confirm the issue is caused by hiccup mode protection or not. 

    Are you using EVM ? If not, my suggestion is to test your component selections on EVM. Also, please share the board photo. 

    -EL

  • Hi Eric

    Im still checking into the things you mentioned, i will get back to you as soon as i can.

    Im not using the EVM, but the design is taken reference from EVM, i have attached the BOM let me know what to look for particularly. 6712.BOM.pdf

    Board Photo attached here.Board Images.zip

  • Hi Arvind,

    Will check the schematic and layout and get back to you as soon as possible.

    Thanks,

    Nitya

  • Hi Arvind,

    Regarding the schematic : Webench is showing the design is unstable with these compensation values. Can you try the following

    Rcomp change 13.3k to around 36.5k

    Ccomp change 10 nF to around 3.3 nF

    Regarding layout : There are two critical loops, Cin->FET->Inductor->Cout and Cin->FET->Diode. It is recommended to minimize the size of these loops by placing the inductor, diode and FET close together. They are rather far away in the present layout. I would suggest following something similar to the example layout given in the datasheet.

    Regards,

    Niranjan

  • Hi Niranjan!

    Thanks for the heads-up. I'm currently reworking the layout as you suggested. I'll also take note of the compensation values, run some tests, and keep you posted.