Part Number: LM5085
I am currently using the LM5085 DC/DC converter in order to convert 28V to 3,3V output :
We would like to improve the switching frequency thanks to the Rt resistance (R4002 in the schematic) and in the same time we would like to keep a good stability of the switching frequency.
I tried to change the switching frequency (resp 600kHz and 800kHz) but more it is big more the circuit is not stable ...
In the same time, I calculate the adjustement ripple circuit (R4010 + C4011 + C4016) thanks to this document : Transient response versus ripple - an analysis of ripple injection techniques used in hysteretic controllers, especially for the 600 kHz switching frequency, but the LM5085 seems to be unsteable.( We chose 100 mV output ripple for our application.)
Do you know if this integrated circuit has some metastability criteria above 400 kHz switching ?
Do you know some tips to fix Fsw ?
I would be grateful if you could answer this question as best as possible,
Would you be able to share your schematic again? I am unable to see your image.
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In reply to Katelyn Wiggenhorn:
Thanks for your answer. I put the schematic in attachement :
Can you see the image ?
In reply to Olivier DI BELLA:
Thanks a lot for your answer ! In fact the problem is the following : Our customer wants the CISPR25 class 5 standard and we have problems in radiated emission. I've recently opened a new topic on the forum where I describe the question with a better accuracy :
One idea is to increase the switching frequency of the LM5085 thanks to the Rt resistance.
I noticed that if I put 600kHz instead of 400kHz (our default value) the radiated spectrum seemed to be less big.
The problem is when I measure switching frequency just before the inductance, I see a very troubled signal with a variant Fsw.
We designed an EMC input filter from the voltage supply line :
We have chosen a pi filter which filters the 28V input voltage. Our requirements are the following :
- Input voltage : 18V to 32V dc
- output current : 1,4 A max
- output ripple : 100mV (but can be changed if necessary)
The problem is that we can not route the PCB again.
Finally, I downloaded the TINA-TI software and I tried to simulate the FFT response at the output node but I didn't manage to obtain the correct spectrum.
Thanks again for your future response,
We have problems especially in low frequencies, with switching harmonics (400k, 800, 1,2M, 1,6M ...)
You are right when you say that increasing Fsw obliges to check thermal and consumption performances again, and it could be bad for our application !
We have chosen several options to reduce harmonics :
- put a shielded self with the same inductance value (47µH);
- put a shielded hood above the Dc Dc converter area in order to limit radiated emissions;
- try to slow down commutation signal of the MOSFET transistor thanks to a gate/drain capacitor, but we don't know exactly how choosing a good value.
What do you think about these ideas ?
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