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

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!!