LM5122: Unstable operation

Hi Anton,

Sorry to hear that you are still having trouble. With regards to the previous image, HO should still not have a forward drop of 10V when you are conducting through the body diode to the SW. Do the scope shots that you've provided before match what you are seeing now, or has anything changed?

As for the path- i think you should not have the 12V1 trace cut through the trace to VT1. Since that is an input rail, you could take a longer path around the FET. 

Thanks,

Richard 

SW (top) and HO (bottom) signal respect to the groung are presented on this picture. At first HO signal is normal, but then gaps are appeared on the top of pulses. May be this behavor is related to power integrity? (DC-DC was under 3 Ohm load conditiion)

The cause of unstable operation was Hiccup-Mode Overload Protection. i changed capacitor on Res pin by 0 ohm resistor and DC-DC started under 3 ohms load, but then VT2 was failure. I choose 2 mOhms current sense resistor. Is it correct choise for this application? I changed VD2 to jumper and gaps on the tops of HO pulses were remove. What do I make wrong?

Hi Anton,

Thanks for clarifying. How did VT2 fail? By connecting RES to AGND, you have removed hiccup-mode protection and are now in cycle-by-cycle current limit.

To check if you are in current limit, can you show a scope shot of inductor current? If you are in current limit, you may need to decrease the size of of your current sense resistor to add margin. 

For selecting current sense resistor, you need to make sure that the peak current value it can handle is equal to your peak inductor current +/- 30%. That should be a good starting point. 

As for VD2, whether you have a jumper or not should not be an issue, since having an external VCC bias reduces thermal dissipation in the IC. 

Thanks,

Richard

Hi, I have measured current signal on the current sense resistor before filter. It has strange form, but dc-dc works with off hiccup mode. Maybe it's related for EMC? (Load 4 Ohm) Are the current sense tracks not too long?)

I made some scope shots. First shot is HO and LO when DC-DC under 3 ohm load. Hiccup mode is enable.

I have solded 11 kOhm frequency setup resistor and 75 kOhm slope compensation resistor.But output voltage had spikes when HO was closing. And I redesigned current sense filter, 100 ohm resistor was replaced by 1,2 kOhm.

I think transistors have opening together sometimes, and short circuit is happened. May be IRF7480 is too slow for 800 kHz applications? It has tur on delay - 21 ns, turn on time - 70 ns, turn off delay - 68 ns and turn off time - 58 ns. How can I connect Pspice model of Lm5122 to my altium library and simulate it? I can't find any files with *.ckt file format for LM5122.

I have downloaded lm5122 SPICE models. I don't understand how to use it. What programm I need to use for simulation? I use Altium Designer for my designs, but I don't know how connect this models to my design. Archives aren't containe *.ckt or *.mdl files.

I have analyzed TI referecnce desingns (98% 135W Single Phase Boost Reference Design, 154W Synchronous Boost Converter Reference Design, 200W Synchronous Boost Audio Amplifier Reference Design). Transistors in this designs are greatly faster than IRF7480. For more current capability in this designs are used two transistors in parallel connection for more current capability.

Hi Richard, I have downloaded lm5122 SPICE models. I don't understand how to use it. What programm I need to use for simulation? I use Altium Designer for my designs, but I don't know how connect this models to my design. Archives aren't containe *.ckt or *.mdl files.

I have analyzed TI referecnce desingns (98% 135W Single Phase Boost Reference Design, 154W Synchronous Boost Converter Reference Design, 200W Synchronous Boost Audio Amplifier Reference Design). Transistors in this designs are greatly faster than IRF7480. For more current capability in this designs are used two transistors in parallel connection for more current capability.

Hello, how can I insert IRF7480 transistors in TINA simulation model? I don't understand how change transistor model in simulation project.LM5122.TSC

Hello, I try to simulate my design but I have a trouble. I attach simulation project and orginal spice model of IRF7480 by Infeneon (irf7480mtrpbf.spi) and model IRF7480 by me (irf7480mtrpbf.cir https://e2e.ti.com/cfs-file/__key/communityserver-discussions-components-files/196/IRF7480.7z), I corrected original file but It wasn't sucsessfull. Can you help me to starting simulation?4617.LM5122.TSC

Hello, great thanks for your answer, Richard. Modeling results looks ok (https://e2e.ti.com/cfs-file/__key/communityserver-discussions-components-files/196/LM5122.7z) . How can I investigate conductive electromagnetic interference levels with TINA-TI? I want to try add input and output EMC filters to my design.

Thanks for link, Richard. how can I use Signal Analyzer tool in transient analysis?

Hi Anton,

This will take me a bit of time to understand how to use the signal analyzer tool, so I will get back to you next week. Is there a specific reason you do not want to use the average model for the AC analysis as mentioned in the previous post?

Thanks,

Richard